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Wikipedia

Markov chain

A Markov chain or Markov process is a stochastic model describing a sequence of possible events in which the probability of each event depends only on the state attained in the previous event.[1][2][3] Informally, this may be thought of as, "What happens next depends only on the state of affairs now." A countably infinite sequence, in which the chain moves state at discrete time steps, gives a discrete-time Markov chain (DTMC). A continuous-time process is called a continuous-time Markov chain (CTMC). It is named after the Russian mathematician Andrey Markov.

A diagram representing a two-state Markov process. The numbers are the probability of changing from one state to another state.

Markov chains have many applications as statistical models of real-world processes,[1][4][5][6] such as studying cruise control systems in motor vehicles, queues or lines of customers arriving at an airport, currency exchange rates and animal population dynamics.[7]

Markov processes are the basis for general stochastic simulation methods known as Markov chain Monte Carlo, which are used for simulating sampling from complex probability distributions, and have found application in Bayesian statistics, thermodynamics, statistical mechanics, physics, chemistry, economics, finance, signal processing, information theory and speech processing.[7][8][9]

The adjectives Markovian and Markov are used to describe something that is related to a Markov process.[1][10][11]

Principles edit

 
Russian mathematician Andrey Markov

Definition edit

A Markov process is a stochastic process that satisfies the Markov property[1] (sometimes characterized as "memorylessness"). In simpler terms, it is a process for which predictions can be made regarding future outcomes based solely on its present state and—most importantly—such predictions are just as good as the ones that could be made knowing the process's full history.[12] In other words, conditional on the present state of the system, its future and past states are independent.

A Markov chain is a type of Markov process that has either a discrete state space or a discrete index set (often representing time), but the precise definition of a Markov chain varies.[13] For example, it is common to define a Markov chain as a Markov process in either discrete or continuous time with a countable state space (thus regardless of the nature of time),[14][15][16][17] but it is also common to define a Markov chain as having discrete time in either countable or continuous state space (thus regardless of the state space).[13]

Types of Markov chains edit

The system's state space and time parameter index need to be specified. The following table gives an overview of the different instances of Markov processes for different levels of state space generality and for discrete time v. continuous time:

Countable state space Continuous or general state space
Discrete-time (discrete-time) Markov chain on a countable or finite state space Markov chain on a measurable state space (for example, Harris chain)
Continuous-time Continuous-time Markov process or Markov jump process Any continuous stochastic process with the Markov property (for example, the Wiener process)

Note that there is no definitive agreement in the literature on the use of some of the terms that signify special cases of Markov processes. Usually the term "Markov chain" is reserved for a process with a discrete set of times, that is, a discrete-time Markov chain (DTMC),[1][18] but a few authors use the term "Markov process" to refer to a continuous-time Markov chain (CTMC) without explicit mention.[19][20][21] In addition, there are other extensions of Markov processes that are referred to as such but do not necessarily fall within any of these four categories (see Markov model). Moreover, the time index need not necessarily be real-valued; like with the state space, there are conceivable processes that move through index sets with other mathematical constructs. Notice that the general state space continuous-time Markov chain is general to such a degree that it has no designated term.

While the time parameter is usually discrete, the state space of a Markov chain does not have any generally agreed-on restrictions: the term may refer to a process on an arbitrary state space.[22] However, many applications of Markov chains employ finite or countably infinite state spaces, which have a more straightforward statistical analysis. Besides time-index and state-space parameters, there are many other variations, extensions and generalizations (see Variations). For simplicity, most of this article concentrates on the discrete-time, discrete state-space case, unless mentioned otherwise.

Transitions edit

The changes of state of the system are called transitions.[1] The probabilities associated with various state changes are called transition probabilities. The process is characterized by a state space, a transition matrix describing the probabilities of particular transitions, and an initial state (or initial distribution) across the state space. By convention, we assume all possible states and transitions have been included in the definition of the process, so there is always a next state, and the process does not terminate.

A discrete-time random process involves a system which is in a certain state at each step, with the state changing randomly between steps.[1] The steps are often thought of as moments in time, but they can equally well refer to physical distance or any other discrete measurement. Formally, the steps are the integers or natural numbers, and the random process is a mapping of these to states.[23] The Markov property states that the conditional probability distribution for the system at the next step (and in fact at all future steps) depends only on the current state of the system, and not additionally on the state of the system at previous steps.

Since the system changes randomly, it is generally impossible to predict with certainty the state of a Markov chain at a given point in the future.[23] However, the statistical properties of the system's future can be predicted.[23] In many applications, it is these statistical properties that are important.

History edit

Andrey Markov studied Markov processes in the early 20th century, publishing his first paper on the topic in 1906.[24][25][26][27] Markov Processes in continuous time were discovered long before his work in the early 20th century[1] in the form of the Poisson process.[28][29][30] Markov was interested in studying an extension of independent random sequences, motivated by a disagreement with Pavel Nekrasov who claimed independence was necessary for the weak law of large numbers to hold.[1][31] In his first paper on Markov chains, published in 1906, Markov showed that under certain conditions the average outcomes of the Markov chain would converge to a fixed vector of values, so proving a weak law of large numbers without the independence assumption,[1][25][26][27] which had been commonly regarded as a requirement for such mathematical laws to hold.[27] Markov later used Markov chains to study the distribution of vowels in Eugene Onegin, written by Alexander Pushkin, and proved a central limit theorem for such chains.[1][25]

In 1912 Henri Poincaré studied Markov chains on finite groups with an aim to study card shuffling. Other early uses of Markov chains include a diffusion model, introduced by Paul and Tatyana Ehrenfest in 1907, and a branching process, introduced by Francis Galton and Henry William Watson in 1873, preceding the work of Markov.[25][26] After the work of Galton and Watson, it was later revealed that their branching process had been independently discovered and studied around three decades earlier by Irénée-Jules Bienaymé.[32] Starting in 1928, Maurice Fréchet became interested in Markov chains, eventually resulting in him publishing in 1938 a detailed study on Markov chains.[25][33]

Andrey Kolmogorov developed in a 1931 paper a large part of the early theory of continuous-time Markov processes.[34][35] Kolmogorov was partly inspired by Louis Bachelier's 1900 work on fluctuations in the stock market as well as Norbert Wiener's work on Einstein's model of Brownian movement.[34][36] He introduced and studied a particular set of Markov processes known as diffusion processes, where he derived a set of differential equations describing the processes.[34][37] Independent of Kolmogorov's work, Sydney Chapman derived in a 1928 paper an equation, now called the Chapman–Kolmogorov equation, in a less mathematically rigorous way than Kolmogorov, while studying Brownian movement.[38] The differential equations are now called the Kolmogorov equations[39] or the Kolmogorov–Chapman equations.[40] Other mathematicians who contributed significantly to the foundations of Markov processes include William Feller, starting in 1930s, and then later Eugene Dynkin, starting in the 1950s.[35]

Examples edit

  • Random walks based on integers and the gambler's ruin problem are examples of Markov processes.[41][42] Some variations of these processes were studied hundreds of years earlier in the context of independent variables.[43][44][45] Two important examples of Markov processes are the Wiener process, also known as the Brownian motion process, and the Poisson process,[28] which are considered the most important and central stochastic processes in the theory of stochastic processes.[46][47][48] These two processes are Markov processes in continuous time, while random walks on the integers and the gambler's ruin problem are examples of Markov processes in discrete time.[41][42]
  • A famous Markov chain is the so-called "drunkard's walk", a random walk on the number line where, at each step, the position may change by +1 or −1 with equal probability. From any position there are two possible transitions, to the next or previous integer. The transition probabilities depend only on the current position, not on the manner in which the position was reached. For example, the transition probabilities from 5 to 4 and 5 to 6 are both 0.5, and all other transition probabilities from 5 are 0. These probabilities are independent of whether the system was previously in 4 or 6.
  • A series of independent states (for example, a series of coin flips) satisfies the formal definition of a Markov chain. However, the theory is usually applied only when the probability distribution of the next state depends on the current one.

A non-Markov example edit

Suppose that there is a coin purse containing five quarters (each worth 25¢), five dimes (each worth 10¢), and five nickels (each worth 5¢), and one by one, coins are randomly drawn from the purse and are set on a table. If   represents the total value of the coins set on the table after n draws, with  , then the sequence   is not a Markov process.

To see why this is the case, suppose that in the first six draws, all five nickels and a quarter are drawn. Thus  . If we know not just  , but the earlier values as well, then we can determine which coins have been drawn, and we know that the next coin will not be a nickel; so we can determine that   with probability 1. But if we do not know the earlier values, then based only on the value   we might guess that we had drawn four dimes and two nickels, in which case it would certainly be possible to draw another nickel next. Thus, our guesses about   are impacted by our knowledge of values prior to  .

However, it is possible to model this scenario as a Markov process. Instead of defining   to represent the total value of the coins on the table, we could define   to represent the count of the various coin types on the table. For instance,   could be defined to represent the state where there is one quarter, zero dimes, and five nickels on the table after 6 one-by-one draws. This new model could be represented by   possible states, where each state represents the number of coins of each type (from 0 to 5) that are on the table. (Not all of these states are reachable within 6 draws.) Suppose that the first draw results in state  . The probability of achieving   now depends on  ; for example, the state   is not possible. After the second draw, the third draw depends on which coins have so far been drawn, but no longer only on the coins that were drawn for the first state (since probabilistically important information has since been added to the scenario). In this way, the likelihood of the   state depends exclusively on the outcome of the   state.

Formal definition edit

Discrete-time Markov chain edit

A discrete-time Markov chain is a sequence of random variables X1, X2, X3, ... with the Markov property, namely that the probability of moving to the next state depends only on the present state and not on the previous states:

  if both conditional probabilities are well defined, that is, if  

The possible values of Xi form a countable set S called the state space of the chain.

Variations edit

  • Time-homogeneous Markov chains are processes where
     
    for all n. The probability of the transition is independent of n.
  • Stationary Markov chains are processes where
     
    for all n and k. Every stationary chain can be proved to be time-homogeneous by Bayes' rule.
    A necessary and sufficient condition for a time-homogeneous Markov chain to be stationary is that the distribution of   is a stationary distribution of the Markov chain.
  • A Markov chain with memory (or a Markov chain of order m) where m is finite, is a process satisfying
     
    In other words, the future state depends on the past m states. It is possible to construct a chain   from   which has the 'classical' Markov property by taking as state space the ordered m-tuples of X values, i.e.,  .

Continuous-time Markov chain edit

A continuous-time Markov chain (Xt)t ≥ 0 is defined by a finite or countable state space S, a transition rate matrix Q with dimensions equal to that of the state space and initial probability distribution defined on the state space. For i ≠ j, the elements qij are non-negative and describe the rate of the process transitions from state i to state j. The elements qii are chosen such that each row of the transition rate matrix sums to zero, while the row-sums of a probability transition matrix in a (discrete) Markov chain are all equal to one.

There are three equivalent definitions of the process.[49]

Infinitesimal definition edit

 
The continuous time Markov chain is characterized by the transition rates, the derivatives with respect to time of the transition probabilities between states i and j.

Let   be the random variable describing the state of the process at time t, and assume the process is in a state i at time t. Then, knowing  ,   is independent of previous values  , and as h → 0 for all j and for all t,

 
where   is the Kronecker delta, using the little-o notation. The   can be seen as measuring how quickly the transition from i to j happens.

Jump chain/holding time definition edit

Define a discrete-time Markov chain Yn to describe the nth jump of the process and variables S1, S2, S3, ... to describe holding times in each of the states where Si follows the exponential distribution with rate parameter −qYiYi.

Transition probability definition edit

For any value n = 0, 1, 2, 3, ... and times indexed up to this value of n: t0, t1, t2, ... and all states recorded at these times i0, i1, i2, i3, ... it holds that

 

where pij is the solution of the forward equation (a first-order differential equation)

 

with initial condition P(0) is the identity matrix.

Finite state space edit

If the state space is finite, the transition probability distribution can be represented by a matrix, called the transition matrix, with the (i, j)th element of P equal to

 

Since each row of P sums to one and all elements are non-negative, P is a right stochastic matrix.

Stationary distribution relation to eigenvectors and simplices edit

A stationary distribution π is a (row) vector, whose entries are non-negative and sum to 1, is unchanged by the operation of transition matrix P on it and so is defined by

 

By comparing this definition with that of an eigenvector we see that the two concepts are related and that

 

is a normalized ( ) multiple of a left eigenvector e of the transition matrix P with an eigenvalue of 1. If there is more than one unit eigenvector then a weighted sum of the corresponding stationary states is also a stationary state. But for a Markov chain one is usually more interested in a stationary state that is the limit of the sequence of distributions for some initial distribution.

The values of a stationary distribution   are associated with the state space of P and its eigenvectors have their relative proportions preserved. Since the components of π are positive and the constraint that their sum is unity can be rewritten as   we see that the dot product of π with a vector whose components are all 1 is unity and that π lies on a simplex.

Time-homogeneous Markov chain with a finite state space edit

If the Markov chain is time-homogeneous, then the transition matrix P is the same after each step, so the k-step transition probability can be computed as the k-th power of the transition matrix, Pk.

If the Markov chain is irreducible and aperiodic, then there is a unique stationary distribution π.[50] Additionally, in this case Pk converges to a rank-one matrix in which each row is the stationary distribution π:

 

where 1 is the column vector with all entries equal to 1. This is stated by the Perron–Frobenius theorem. If, by whatever means,   is found, then the stationary distribution of the Markov chain in question can be easily determined for any starting distribution, as will be explained below.

For some stochastic matrices P, the limit   does not exist while the stationary distribution does, as shown by this example:

 
 

(This example illustrates a periodic Markov chain.)

Because there are a number of different special cases to consider, the process of finding this limit if it exists can be a lengthy task. However, there are many techniques that can assist in finding this limit. Let P be an n×n matrix, and define  

It is always true that

 

Subtracting Q from both sides and factoring then yields

 

where In is the identity matrix of size n, and 0n,n is the zero matrix of size n×n. Multiplying together stochastic matrices always yields another stochastic matrix, so Q must be a stochastic matrix (see the definition above). It is sometimes sufficient to use the matrix equation above and the fact that Q is a stochastic matrix to solve for Q. Including the fact that the sum of each the rows in P is 1, there are n+1 equations for determining n unknowns, so it is computationally easier if on the one hand one selects one row in Q and substitutes each of its elements by one, and on the other one substitutes the corresponding element (the one in the same column) in the vector 0, and next left-multiplies this latter vector by the inverse of transformed former matrix to find Q.

Here is one method for doing so: first, define the function f(A) to return the matrix A with its right-most column replaced with all 1's. If [f(PIn)]−1 exists then[51][50]

 
Explain: The original matrix equation is equivalent to a system of n×n linear equations in n×n variables. And there are n more linear equations from the fact that Q is a right stochastic matrix whose each row sums to 1. So it needs any n×n independent linear equations of the (n×n+n) equations to solve for the n×n variables. In this example, the n equations from “Q multiplied by the right-most column of (P-In)” have been replaced by the n stochastic ones.

One thing to notice is that if P has an element Pi,i on its main diagonal that is equal to 1 and the ith row or column is otherwise filled with 0's, then that row or column will remain unchanged in all of the subsequent powers Pk. Hence, the ith row or column of Q will have the 1 and the 0's in the same positions as in P.

Convergence speed to the stationary distribution edit

As stated earlier, from the equation   (if exists) the stationary (or steady state) distribution π is a left eigenvector of row stochastic matrix P. Then assuming that P is diagonalizable or equivalently that P has n linearly independent eigenvectors, speed of convergence is elaborated as follows. (For non-diagonalizable, that is, defective matrices, one may start with the Jordan normal form of P and proceed with a bit more involved set of arguments in a similar way.[52]

Let U be the matrix of eigenvectors (each normalized to having an L2 norm equal to 1) where each column is a left eigenvector of P and let Σ be the diagonal matrix of left eigenvalues of P, that is, Σ = diag(λ1,λ2,λ3,...,λn). Then by eigendecomposition

 

Let the eigenvalues be enumerated such that:

 

Since P is a row stochastic matrix, its largest left eigenvalue is 1. If there is a unique stationary distribution, then the largest eigenvalue and the corresponding eigenvector is unique too (because there is no other π which solves the stationary distribution equation above). Let ui be the i-th column of U matrix, that is, ui is the left eigenvector of P corresponding to λi. Also let x be a length n row vector that represents a valid probability distribution; since the eigenvectors ui span   we can write

 

If we multiply x with P from right and continue this operation with the results, in the end we get the stationary distribution π. In other words, π = a1 u1xPP...P = xPk as k → ∞. That means

 

Since π is parallel to u1(normalized by L2 norm) and π(k) is a probability vector, π(k) approaches to a1 u1 = π as k → ∞ with a speed in the order of λ2/λ1 exponentially. This follows because   hence λ2/λ1 is the dominant term. The smaller the ratio is, the faster the convergence is.[53] Random noise in the state distribution π can also speed up this convergence to the stationary distribution.[54]

General state space edit

Harris chains edit

Many results for Markov chains with finite state space can be generalized to chains with uncountable state space through Harris chains.

The use of Markov chains in Markov chain Monte Carlo methods covers cases where the process follows a continuous state space.

Locally interacting Markov chains edit

Considering a collection of Markov chains whose evolution takes in account the state of other Markov chains, is related to the notion of locally interacting Markov chains. This corresponds to the situation when the state space has a (Cartesian-) product form. See interacting particle system and stochastic cellular automata (probabilistic cellular automata). See for instance Interaction of Markov Processes[55] or.[56]

Properties edit

Two states are said to communicate with each other if both are reachable from one another by a sequence of transitions that have positive probability. This is an equivalence relation which yields a set of communicating classes. A class is closed if the probability of leaving the class is zero. A Markov chain is irreducible if there is one communicating class, the state space.

A state i has period k if k is the greatest common divisor of the number of transitions by which i can be reached, starting from i. That is:

 

Although the state as a whole is only described as periodic if  . If   then the state is described as being aperiodic.

A state i is said to be transient if, starting from i, there is a non-zero probability that the chain will never return to i. It is called recurrent (or persistent) otherwise.[57] For a recurrent state i, the mean hitting time is defined as:

 

State i is positive recurrent if   is finite and null recurrent otherwise. Periodicity, transience, recurrence and positive and null recurrence are class properties — that is, if one state has the property then all states in its communicating class have the property.[58]

A state i is called absorbing if there are no outgoing transitions from the state.

Irreducibility edit

Since periodicity is a class property, if a Markov chain is irreducible, then all its states have the same period. In particular, if one state is aperiodic, then the whole Markov chain is aperiodic.

If a finite Markov chain is irreducible, then all states are positive recurrent, and it has a unique stationary distribution given by  .

Ergodicity edit

A state i is said to be ergodic if it is aperiodic and positive recurrent. In other words, a state i is ergodic if it is recurrent, has a period of 1, and has finite mean recurrence time.

If all states in an irreducible Markov chain are ergodic, then the chain is said to be ergodic. Equivalently, there exists some integer   such that all entries of   are positive.

It can be shown that a finite state irreducible Markov chain is ergodic if it has an aperiodic state. More generally, a Markov chain is ergodic if there is a number N such that any state can be reached from any other state in any number of steps less or equal to a number N. In case of a fully connected transition matrix, where all transitions have a non-zero probability, this condition is fulfilled with N = 1.

A Markov chain with more than one state and just one out-going transition per state is either not irreducible or not aperiodic, hence cannot be ergodic.

Terminology edit

Some authors call any irreducible, positive recurrent Markov chains ergodic, even periodic ones.[59] In fact, merely irreducible Markov chains correspond to ergodic processes, defined according to ergodic theory.[60]

Some authors call a matrix primitive iff there exists some integer   such that all entries of   are positive.[61] Some authors call it regular.[62]

Index of primitivity edit

The index of primitivity, or exponent, of a regular matrix, is the smallest   such that all entries of   are positive. The exponent is purely a graph-theoretic property, since it depends only on whether each entry of   is zero or positive, and therefore can be found on a directed graph with   as its adjacency matrix.

There are several combinatorial results about the exponent when there are finitely many states. Let   be the number of states, then[63]

  • The exponent is  . The only case where it is an equality is when the graph of   goes like  .
  • If   has   diagonal entries, then its exponent is  .
  • If   is symmetric, then   has positive diagonal entries, which by previous proposition means its exponent is  .
  • (Dulmage-Mendelsohn theorem) The exponent is   where   is the girth of the graph. It can be improved to  , where   is the diameter of the graph.[64]

Measure-preserving dynamical system edit

If a Markov chain has a stationary distribution, then it can be converted to a measure-preserving dynamical system: Let the probability space be  , where   is the set of all states for the Markov chain. Let the sigma-algebra on the probability space be generated by the cylinder sets. Let the probability measure be generated by the stationary distribution, and the Markov chain transition. Let   be the shift operator:  . Similarly we can construct such a dynamical system with   instead.[65]

Since irreducible Markov chains with finite state spaces have a unique stationary distribution, the above construction is unambiguous for irreducible Markov chains.

In ergodic theory, a measure-preserving dynamical system is called "ergodic" iff any measurable subset   such that   implies   or   (up to a null set).

The terminology is inconsistent. Given a Markov chain with a stationary distribution that is strictly positive on all states, the Markov chain is irreducible iff its corresponding measure-preserving dynamical system is ergodic.[60]

Markovian representations edit

In some cases, apparently non-Markovian processes may still have Markovian representations, constructed by expanding the concept of the "current" and "future" states. For example, let X be a non-Markovian process. Then define a process Y, such that each state of Y represents a time-interval of states of X. Mathematically, this takes the form:

 

If Y has the Markov property, then it is a Markovian representation of X.

An example of a non-Markovian process with a Markovian representation is an autoregressive time series of order greater than one.[66]

Hitting times edit

The hitting time is the time, starting in a given set of states until the chain arrives in a given state or set of states. The distribution of such a time period has a phase type distribution. The simplest such distribution is that of a single exponentially distributed transition.

Expected hitting times edit

For a subset of states A ⊆ S, the vector kA of hitting times (where element   represents the expected value, starting in state i that the chain enters one of the states in the set A) is the minimal non-negative solution to[67]

 

Time reversal edit

For a CTMC Xt, the time-reversed process is defined to be  . By Kelly's lemma this process has the same stationary distribution as the forward process.

A chain is said to be reversible if the reversed process is the same as the forward process. Kolmogorov's criterion states that the necessary and sufficient condition for a process to be reversible is that the product of transition rates around a closed loop must be the same in both directions.

Embedded Markov chain edit

One method of finding the stationary probability distribution, π, of an ergodic continuous-time Markov chain, Q, is by first finding its embedded Markov chain (EMC). Strictly speaking, the EMC is a regular discrete-time Markov chain, sometimes referred to as a jump process. Each element of the one-step transition probability matrix of the EMC, S, is denoted by sij, and represents the conditional probability of transitioning from state i into state j. These conditional probabilities may be found by

 

From this, S may be written as

 

where I is the identity matrix and diag(Q) is the diagonal matrix formed by selecting the main diagonal from the matrix Q and setting all other elements to zero.

To find the stationary probability distribution vector, we must next find   such that

 

with   being a row vector, such that all elements in   are greater than 0 and   = 1. From this, π may be found as

 

(S may be periodic, even if Q is not. Once π is found, it must be normalized to a unit vector.)

Another discrete-time process that may be derived from a continuous-time Markov chain is a δ-skeleton—the (discrete-time) Markov chain formed by observing X(t) at intervals of δ units of time. The random variables X(0), X(δ), X(2δ), ... give the sequence of states visited by the δ-skeleton.

Special types of Markov chains edit

Markov model edit

Markov models are used to model changing systems. There are 4 main types of models, that generalize Markov chains depending on whether every sequential state is observable or not, and whether the system is to be adjusted on the basis of observations made:

System state is fully observable System state is partially observable
System is autonomous Markov chain Hidden Markov model
System is controlled Markov decision process Partially observable Markov decision process

Bernoulli scheme edit

A Bernoulli scheme is a special case of a Markov chain where the transition probability matrix has identical rows, which means that the next state is independent of even the current state (in addition to being independent of the past states). A Bernoulli scheme with only two possible states is known as a Bernoulli process.

Note, however, by the Ornstein isomorphism theorem, that every aperiodic and irreducible Markov chain is isomorphic to a Bernoulli scheme;[68] thus, one might equally claim that Markov chains are a "special case" of Bernoulli schemes. The isomorphism generally requires a complicated recoding. The isomorphism theorem is even a bit stronger: it states that any stationary stochastic process is isomorphic to a Bernoulli scheme; the Markov chain is just one such example.

Subshift of finite type edit

When the Markov matrix is replaced by the adjacency matrix of a finite graph, the resulting shift is termed a topological Markov chain or a subshift of finite type.[68] A Markov matrix that is compatible with the adjacency matrix can then provide a measure on the subshift. Many chaotic dynamical systems are isomorphic to topological Markov chains; examples include diffeomorphisms of closed manifolds, the Prouhet–Thue–Morse system, the Chacon system, sofic systems, context-free systems and block-coding systems.[68]

Applications edit

Research has reported the application and usefulness of Markov chains in a wide range of topics such as physics, chemistry, biology, medicine, music, game theory and sports.

Physics edit

Markovian systems appear extensively in thermodynamics and statistical mechanics, whenever probabilities are used to represent unknown or unmodelled details of the system, if it can be assumed that the dynamics are time-invariant, and that no relevant history need be considered which is not already included in the state description.[69][70] For example, a thermodynamic state operates under a probability distribution that is difficult or expensive to acquire. Therefore, Markov Chain Monte Carlo method can be used to draw samples randomly from a black-box to approximate the probability distribution of attributes over a range of objects.[70]

The paths, in the path integral formulation of quantum mechanics, are Markov chains.[71]

Markov chains are used in lattice QCD simulations.[72]

Chemistry edit

 
Michaelis-Menten kinetics. The enzyme (E) binds a substrate (S) and produces a product (P). Each reaction is a state transition in a Markov chain.

A reaction network is a chemical system involving multiple reactions and chemical species. The simplest stochastic models of such networks treat the system as a continuous time Markov chain with the state being the number of molecules of each species and with reactions modeled as possible transitions of the chain.[73] Markov chains and continuous-time Markov processes are useful in chemistry when physical systems closely approximate the Markov property. For example, imagine a large number n of molecules in solution in state A, each of which can undergo a chemical reaction to state B with a certain average rate. Perhaps the molecule is an enzyme, and the states refer to how it is folded. The state of any single enzyme follows a Markov chain, and since the molecules are essentially independent of each other, the number of molecules in state A or B at a time is n times the probability a given molecule is in that state.

The classical model of enzyme activity, Michaelis–Menten kinetics, can be viewed as a Markov chain, where at each time step the reaction proceeds in some direction. While Michaelis-Menten is fairly straightforward, far more complicated reaction networks can also be modeled with Markov chains.[74]

An algorithm based on a Markov chain was also used to focus the fragment-based growth of chemicals in silico towards a desired class of compounds such as drugs or natural products.[75] As a molecule is grown, a fragment is selected from the nascent molecule as the "current" state. It is not aware of its past (that is, it is not aware of what is already bonded to it). It then transitions to the next state when a fragment is attached to it. The transition probabilities are trained on databases of authentic classes of compounds.[76]

Also, the growth (and composition) of copolymers may be modeled using Markov chains. Based on the reactivity ratios of the monomers that make up the growing polymer chain, the chain's composition may be calculated (for example, whether monomers tend to add in alternating fashion or in long runs of the same monomer). Due to steric effects, second-order Markov effects may also play a role in the growth of some polymer chains.

Similarly, it has been suggested that the crystallization and growth of some epitaxial superlattice oxide materials can be accurately described by Markov chains.[77]

Biology edit

Markov chains are used in various areas of biology. Notable examples include:

Testing edit

Several theorists have proposed the idea of the Markov chain statistical test (MCST), a method of conjoining Markov chains to form a "Markov blanket", arranging these chains in several recursive layers ("wafering") and producing more efficient test sets—samples—as a replacement for exhaustive testing. MCSTs also have uses in temporal state-based networks; Chilukuri et al.'s paper entitled "Temporal Uncertainty Reasoning Networks for Evidence Fusion with Applications to Object Detection and Tracking" (ScienceDirect) gives a background and case study for applying MCSTs to a wider range of applications.

Solar irradiance variability edit

Solar irradiance variability assessments are useful for solar power applications. Solar irradiance variability at any location over time is mainly a consequence of the deterministic variability of the sun's path across the sky dome and the variability in cloudiness. The variability of accessible solar irradiance on Earth's surface has been modeled using Markov chains,[80][81][82][83] also including modeling the two states of clear and cloudiness as a two-state Markov chain.[84][85]

Speech recognition edit

Hidden Markov models are the basis for most modern automatic speech recognition systems.

Information theory edit

Markov chains are used throughout information processing. Claude Shannon's famous 1948 paper A Mathematical Theory of Communication, which in a single step created the field of information theory, opens by introducing the concept of entropy through Markov modeling of the English language. Such idealized models can capture many of the statistical regularities of systems. Even without describing the full structure of the system perfectly, such signal models can make possible very effective data compression through entropy encoding techniques such as arithmetic coding. They also allow effective state estimation and pattern recognition. Markov chains also play an important role in reinforcement learning.

Markov chains are also the basis for hidden Markov models, which are an important tool in such diverse fields as telephone networks (which use the Viterbi algorithm for error correction), speech recognition and bioinformatics (such as in rearrangements detection[86]).

The LZMA lossless data compression algorithm combines Markov chains with Lempel-Ziv compression to achieve very high compression ratios.

Queueing theory edit

Markov chains are the basis for the analytical treatment of queues (queueing theory). Agner Krarup Erlang initiated the subject in 1917.[87] This makes them critical for optimizing the performance of telecommunications networks, where messages must often compete for limited resources (such as bandwidth).[88]

Numerous queueing models use continuous-time Markov chains. For example, an M/M/1 queue is a CTMC on the non-negative integers where upward transitions from i to i + 1 occur at rate λ according to a Poisson process and describe job arrivals, while transitions from i to i – 1 (for i > 1) occur at rate μ (job service times are exponentially distributed) and describe completed services (departures) from the queue.

Internet applications edit

 
A state diagram that represents the PageRank algorithm with a transitional probability of M, or  .

The PageRank of a webpage as used by Google is defined by a Markov chain.[89][90][91] It is the probability to be at page   in the stationary distribution on the following Markov chain on all (known) webpages. If   is the number of known webpages, and a page   has   links to it then it has transition probability   for all pages that are linked to and   for all pages that are not linked to. The parameter   is taken to be about 0.15.[92]

Markov models have also been used to analyze web navigation behavior of users. A user's web link transition on a particular website can be modeled using first- or second-order Markov models and can be used to make predictions regarding future navigation and to personalize the web page for an individual user.

Statistics edit

Markov chain methods have also become very important for generating sequences of random numbers to accurately reflect very complicated desired probability distributions, via a process called Markov chain Monte Carlo (MCMC). In recent years this has revolutionized the practicability of Bayesian inference methods, allowing a wide range of posterior distributions to be simulated and their parameters found numerically.

Economics and finance edit

Markov chains are used in finance and economics to model a variety of different phenomena, including the distribution of income, the size distribution of firms, asset prices and market crashes. D. G. Champernowne built a Markov chain model of the distribution of income in 1953.[93] Herbert A. Simon and co-author Charles Bonini used a Markov chain model to derive a stationary Yule distribution of firm sizes.[94] Louis Bachelier was the first to observe that stock prices followed a random walk.[95] The random walk was later seen as evidence in favor of the efficient-market hypothesis and random walk models were popular in the literature of the 1960s.[96] Regime-switching models of business cycles were popularized by James D. Hamilton (1989), who used a Markov chain to model switches between periods of high and low GDP growth (or, alternatively, economic expansions and recessions).[97] A more recent example is the Markov switching multifractal model of Laurent E. Calvet and Adlai J. Fisher, which builds upon the convenience of earlier regime-switching models.[98][99] It uses an arbitrarily large Markov chain to drive the level of volatility of asset returns.

Dynamic macroeconomics makes heavy use of Markov chains. An example is using Markov chains to exogenously model prices of equity (stock) in a general equilibrium setting.[100]

Credit rating agencies produce annual tables of the transition probabilities for bonds of different credit ratings.[101]

Social sciences edit

Markov chains are generally used in describing path-dependent arguments, where current structural configurations condition future outcomes. An example is the reformulation of the idea, originally due to Karl Marx's Das Kapital, tying economic development to the rise of capitalism. In current research, it is common to use a Markov chain to model how once a country reaches a specific level of economic development, the configuration of structural factors, such as size of the middle class, the ratio of urban to rural residence, the rate of political mobilization, etc., will generate a higher probability of transitioning from authoritarian to democratic regime.[102]

Games edit

Markov chains can be used to model many games of chance.[1] The children's games Snakes and Ladders and "Hi Ho! Cherry-O", for example, are represented exactly by Markov chains. At each turn, the player starts in a given state (on a given square) and from there has fixed odds of moving to certain other states (squares).

Music edit

Markov chains are employed in algorithmic music composition, particularly in software such as Csound, Max, and SuperCollider. In a first-order chain, the states of the system become note or pitch values, and a probability vector for each note is constructed, completing a transition probability matrix (see below). An algorithm is constructed to produce output note values based on the transition matrix weightings, which could be MIDI note values, frequency (Hz), or any other desirable metric.[103]

1st-order matrix
Note A C E
A 0.1 0.6 0.3
C 0.25 0.05 0.7
E 0.7 0.3 0
2nd-order matrix
Notes A D G
AA 0.18 0.6 0.22
AD 0.5 0.5 0
AG 0.15 0.75 0.1
DD 0 0 1
DA 0.25 0 0.75
DG 0.9 0.1 0
GG 0.4 0.4 0.2
GA 0.5 0.25 0.25
GD 1 0 0

A second-order Markov chain can be introduced by considering the current state and also the previous state, as indicated in the second table. Higher, nth-order chains tend to "group" particular notes together, while 'breaking off' into other patterns and sequences occasionally. These higher-order chains tend to generate results with a sense of phrasal structure, rather than the 'aimless wandering' produced by a first-order system.[104]

Markov chains can be used structurally, as in Xenakis's Analogique A and B.[105] Markov chains are also used in systems which use a Markov model to react interactively to music input.[106]

Usually musical systems need to enforce specific control constraints on the finite-length sequences they generate, but control constraints are not compatible with Markov models, since they induce long-range dependencies that violate the Markov hypothesis of limited memory. In order to overcome this limitation, a new approach has been proposed.[107]

Baseball edit

Markov chain models have been used in advanced baseball analysis since 1960, although their use is still rare. Each half-inning of a baseball game fits the Markov chain state when the number of runners and outs are considered. During any at-bat, there are 24 possible combinations of number of outs and position of the runners. Mark Pankin shows that Markov chain models can be used to evaluate runs created for both individual players as well as a team.[108] He also discusses various kinds of strategies and play conditions: how Markov chain models have been used to analyze statistics for game situations such as bunting and base stealing and differences when playing on grass vs. AstroTurf.[109]

Markov text generators edit

Markov processes can also be used to generate superficially real-looking text given a sample document. Markov processes are used in a variety of recreational "parody generator" software (see dissociated press, Jeff Harrison,[110] Mark V. Shaney,[111][112] and Academias Neutronium). Several open-source text generation libraries using Markov chains exist.

Probabilistic forecasting edit

Markov chains have been used for forecasting in several areas: for example, price trends,[113] wind power,[114] stochastic terrorism,[115][116] and solar irradiance.[117] The Markov chain forecasting models utilize a variety of settings, from discretizing the time series,[114] to hidden Markov models combined with wavelets,[113] and the Markov chain mixture distribution model (MCM).[117]

See also edit

Notes edit

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References edit

  • A. A. Markov (1906) "Rasprostranenie zakona bol'shih chisel na velichiny, zavisyaschie drug ot druga". Izvestiya Fiziko-matematicheskogo obschestva pri Kazanskom universitete, 2-ya seriya, tom 15, pp. 135–156.
  • A. A. Markov (1971). "Extension of the limit theorems of probability theory to a sum of variables connected in a chain". reprinted in Appendix B of: R. Howard. Dynamic Probabilistic Systems, volume 1: Markov Chains. John Wiley and Sons.
  • Classical Text in Translation: Markov, A. A. (2006). "An Example of Statistical Investigation of the Text Eugene Onegin Concerning the Connection of Samples in Chains". Science in Context. 19 (4). Translated by Link, David: 591–600. doi:10.1017/s0269889706001074. S2CID 144854176.
  • Leo Breiman (1992) [1968] Probability. Original edition published by Addison-Wesley; reprinted by Society for Industrial and Applied Mathematics ISBN 0-89871-296-3. (See Chapter 7)
  • J. L. Doob (1953) Stochastic Processes. New York: John Wiley and Sons ISBN 0-471-52369-0.
  • S. P. Meyn and R. L. Tweedie (1993) Markov Chains and Stochastic Stability. London: Springer-Verlag ISBN 0-387-19832-6. online: . Second edition to appear, Cambridge University Press, 2009.
  • Dynkin, Eugene Borisovich (1965) [1963-03-10, 1962-03-31]. Written at University of Moscow, Moscow, Russia. Markov Processes-I. Die Grundlehren der mathematischen Wissenschaften in Einzeldarstellungen mit besonderer Berücksichtigung der Anwendungsgebiete. Vol. I (121). Translated by Fabius, Jaap [at Wikidata]; Greenberg, Vida Lazarus [at Wikidata]; Maitra, Ashok Prasad [at Wikidata]; Majone, Giandomenico (1 ed.). New York, USA / Berlin, Germany: Springer-Verlag (Academic Press, Inc., Publishers). doi:10.1007/978-3-662-00031-1. ISBN 978-3-662-00033-5. ISSN 0072-7830. LCCN 64-24812. S2CID 251691119. Title-No. 5104. Retrieved 2023-09-02. Markov Processes: Volume I (xii+365+1 pages); Dynkin, Eugene Borisovich (1965) [1963-03-10, 1962-03-31]. Written at University of Moscow, Moscow, Russia. Markov Processes-II. Die Grundlehren der mathematischen Wissenschaften in Einzeldarstellungen mit besonderer Berücksichtigung der Anwendungsgebiete. Vol. II (122). Translated by Fabius, Jaap [at Wikidata]; Greenberg, Vida Lazarus [at Wikidata]; Maitra, Ashok Prasad [at Wikidata]; Majone, Giandomenico (1 ed.). New York, USA / Berlin, Germany: Springer-Verlag. doi:10.1007/978-3-662-25360-1. ISBN 978-3-662-23320-7. ISSN 0072-7830. LCCN 64-24812. Title-No. 5105. Retrieved 2023-09-02. (viii+274+2 pages) (NB. This was originally published in Russian as "Markovskie prot︠s︡essy" (Марковские процессы) by Fizmatgiz (Физматгиз) in 1963 and translated to English with the assistance of the author.)
  • S. P. Meyn. Control Techniques for Complex Networks. Cambridge University Press, 2007. ISBN 978-0-521-88441-9. Appendix contains abridged Meyn & Tweedie. online:
  • Booth, Taylor L. (1967). Sequential Machines and Automata Theory (1st ed.). New York, NY: John Wiley and Sons, Inc. Library of Congress Card Catalog Number 67-25924. ] Extensive, wide-ranging book meant for specialists, written for both theoretical computer scientists as well as electrical engineers. With detailed explanations of state minimization techniques, FSMs, Turing machines, Markov processes, and undecidability. Excellent treatment of Markov processes pp. 449ff. Discusses Z-transforms, D transforms in their context.
  • Kemeny, John G.; Hazleton Mirkil; J. Laurie Snell; Gerald L. Thompson (1959). Finite Mathematical Structures (1st ed.). Englewood Cliffs, NJ: Prentice-Hall, Inc. Library of Congress Card Catalog Number 59-12841. Classical text. cf Chapter 6 Finite Markov Chains pp. 384ff.
  • John G. Kemeny & J. Laurie Snell (1960) Finite Markov Chains, D. van Nostrand Company ISBN 0-442-04328-7
  • E. Nummelin. "General irreducible Markov chains and non-negative operators". Cambridge University Press, 1984, 2004. ISBN 0-521-60494-X
  • Seneta, E. Non-negative matrices and Markov chains. 2nd rev. ed., 1981, XVI, 288 p., Softcover Springer Series in Statistics. (Originally published by Allen & Unwin Ltd., London, 1973) ISBN 978-0-387-29765-1
  • Kishor S. Trivedi, Probability and Statistics with Reliability, Queueing, and Computer Science Applications, John Wiley & Sons, Inc. New York, 2002. ISBN 0-471-33341-7.
  • K. S. Trivedi and R.A.Sahner, SHARPE at the age of twenty-two, vol. 36, no. 4, pp. 52–57, ACM SIGMETRICS Performance Evaluation Review, 2009.
  • R. A. Sahner, K. S. Trivedi and A. Puliafito, Performance and reliability analysis of computer systems: an example-based approach using the SHARPE software package, Kluwer Academic Publishers, 1996. ISBN 0-7923-9650-2.
  • G. Bolch, S. Greiner, H. de Meer and K. S. Trivedi, Queueing Networks and Markov Chains, John Wiley, 2nd edition, 2006. ISBN 978-0-7923-9650-5.

External links edit

  • Introduction to Markov Chains on YouTube
  • "Markov chain", Encyclopedia of Mathematics, EMS Press, 2001 [1994]
  • Techniques to Understand Computer Simulations: Markov Chain Analysis
  • Markov Chains chapter in American Mathematical Society's introductory probability book 2008-05-22 at the Wayback Machine(pdf)
  • A beautiful visual explanation of Markov Chains
  • Making Sense and Nonsense of Markov Chains 2020-02-04 at the Wayback Machine
  • Original paper by A.A Markov(1913): An Example of Statistical Investigation of the Text Eugene Onegin Concerning the Connection of Samples in Chains (translated from Russian)

markov, chain, markov, process, stochastic, model, describing, sequence, possible, events, which, probability, each, event, depends, only, state, attained, previous, event, informally, this, thought, what, happens, next, depends, only, state, affairs, countabl. A Markov chain or Markov process is a stochastic model describing a sequence of possible events in which the probability of each event depends only on the state attained in the previous event 1 2 3 Informally this may be thought of as What happens next depends only on the state of affairs now A countably infinite sequence in which the chain moves state at discrete time steps gives a discrete time Markov chain DTMC A continuous time process is called a continuous time Markov chain CTMC It is named after the Russian mathematician Andrey Markov A diagram representing a two state Markov process The numbers are the probability of changing from one state to another state Markov chains have many applications as statistical models of real world processes 1 4 5 6 such as studying cruise control systems in motor vehicles queues or lines of customers arriving at an airport currency exchange rates and animal population dynamics 7 Markov processes are the basis for general stochastic simulation methods known as Markov chain Monte Carlo which are used for simulating sampling from complex probability distributions and have found application in Bayesian statistics thermodynamics statistical mechanics physics chemistry economics finance signal processing information theory and speech processing 7 8 9 The adjectives Markovian and Markov are used to describe something that is related to a Markov process 1 10 11 Contents 1 Principles 1 1 Definition 1 2 Types of Markov chains 1 3 Transitions 2 History 3 Examples 3 1 A non Markov example 4 Formal definition 4 1 Discrete time Markov chain 4 1 1 Variations 4 2 Continuous time Markov chain 4 2 1 Infinitesimal definition 4 2 2 Jump chain holding time definition 4 2 3 Transition probability definition 4 3 Finite state space 4 3 1 Stationary distribution relation to eigenvectors and simplices 4 3 2 Time homogeneous Markov chain with a finite state space 4 3 3 Convergence speed to the stationary distribution 4 4 General state space 4 4 1 Harris chains 4 4 2 Locally interacting Markov chains 5 Properties 5 1 Irreducibility 5 2 Ergodicity 5 2 1 Terminology 5 2 2 Index of primitivity 5 3 Measure preserving dynamical system 5 4 Markovian representations 5 5 Hitting times 5 5 1 Expected hitting times 5 6 Time reversal 5 7 Embedded Markov chain 6 Special types of Markov chains 6 1 Markov model 6 2 Bernoulli scheme 6 3 Subshift of finite type 7 Applications 7 1 Physics 7 2 Chemistry 7 3 Biology 7 4 Testing 7 5 Solar irradiance variability 7 6 Speech recognition 7 7 Information theory 7 8 Queueing theory 7 9 Internet applications 7 10 Statistics 7 11 Economics and finance 7 12 Social sciences 7 13 Games 7 14 Music 7 15 Baseball 7 16 Markov text generators 7 17 Probabilistic forecasting 8 See also 9 Notes 10 References 11 External linksPrinciples edit nbsp Russian mathematician Andrey Markov Definition edit A Markov process is a stochastic process that satisfies the Markov property 1 sometimes characterized as memorylessness In simpler terms it is a process for which predictions can be made regarding future outcomes based solely on its present state and most importantly such predictions are just as good as the ones that could be made knowing the process s full history 12 In other words conditional on the present state of the system its future and past states are independent A Markov chain is a type of Markov process that has either a discrete state space or a discrete index set often representing time but the precise definition of a Markov chain varies 13 For example it is common to define a Markov chain as a Markov process in either discrete or continuous time with a countable state space thus regardless of the nature of time 14 15 16 17 but it is also common to define a Markov chain as having discrete time in either countable or continuous state space thus regardless of the state space 13 Types of Markov chains edit The system s state space and time parameter index need to be specified The following table gives an overview of the different instances of Markov processes for different levels of state space generality and for discrete time v continuous time Countable state space Continuous or general state space Discrete time discrete time Markov chain on a countable or finite state space Markov chain on a measurable state space for example Harris chain Continuous time Continuous time Markov process or Markov jump process Any continuous stochastic process with the Markov property for example the Wiener process Note that there is no definitive agreement in the literature on the use of some of the terms that signify special cases of Markov processes Usually the term Markov chain is reserved for a process with a discrete set of times that is a discrete time Markov chain DTMC 1 18 but a few authors use the term Markov process to refer to a continuous time Markov chain CTMC without explicit mention 19 20 21 In addition there are other extensions of Markov processes that are referred to as such but do not necessarily fall within any of these four categories see Markov model Moreover the time index need not necessarily be real valued like with the state space there are conceivable processes that move through index sets with other mathematical constructs Notice that the general state space continuous time Markov chain is general to such a degree that it has no designated term While the time parameter is usually discrete the state space of a Markov chain does not have any generally agreed on restrictions the term may refer to a process on an arbitrary state space 22 However many applications of Markov chains employ finite or countably infinite state spaces which have a more straightforward statistical analysis Besides time index and state space parameters there are many other variations extensions and generalizations see Variations For simplicity most of this article concentrates on the discrete time discrete state space case unless mentioned otherwise Transitions edit The changes of state of the system are called transitions 1 The probabilities associated with various state changes are called transition probabilities The process is characterized by a state space a transition matrix describing the probabilities of particular transitions and an initial state or initial distribution across the state space By convention we assume all possible states and transitions have been included in the definition of the process so there is always a next state and the process does not terminate A discrete time random process involves a system which is in a certain state at each step with the state changing randomly between steps 1 The steps are often thought of as moments in time but they can equally well refer to physical distance or any other discrete measurement Formally the steps are the integers or natural numbers and the random process is a mapping of these to states 23 The Markov property states that the conditional probability distribution for the system at the next step and in fact at all future steps depends only on the current state of the system and not additionally on the state of the system at previous steps Since the system changes randomly it is generally impossible to predict with certainty the state of a Markov chain at a given point in the future 23 However the statistical properties of the system s future can be predicted 23 In many applications it is these statistical properties that are important History editAndrey Markov studied Markov processes in the early 20th century publishing his first paper on the topic in 1906 24 25 26 27 Markov Processes in continuous time were discovered long before his work in the early 20th century 1 in the form of the Poisson process 28 29 30 Markov was interested in studying an extension of independent random sequences motivated by a disagreement with Pavel Nekrasov who claimed independence was necessary for the weak law of large numbers to hold 1 31 In his first paper on Markov chains published in 1906 Markov showed that under certain conditions the average outcomes of the Markov chain would converge to a fixed vector of values so proving a weak law of large numbers without the independence assumption 1 25 26 27 which had been commonly regarded as a requirement for such mathematical laws to hold 27 Markov later used Markov chains to study the distribution of vowels in Eugene Onegin written by Alexander Pushkin and proved a central limit theorem for such chains 1 25 In 1912 Henri Poincare studied Markov chains on finite groups with an aim to study card shuffling Other early uses of Markov chains include a diffusion model introduced by Paul and Tatyana Ehrenfest in 1907 and a branching process introduced by Francis Galton and Henry William Watson in 1873 preceding the work of Markov 25 26 After the work of Galton and Watson it was later revealed that their branching process had been independently discovered and studied around three decades earlier by Irenee Jules Bienayme 32 Starting in 1928 Maurice Frechet became interested in Markov chains eventually resulting in him publishing in 1938 a detailed study on Markov chains 25 33 Andrey Kolmogorov developed in a 1931 paper a large part of the early theory of continuous time Markov processes 34 35 Kolmogorov was partly inspired by Louis Bachelier s 1900 work on fluctuations in the stock market as well as Norbert Wiener s work on Einstein s model of Brownian movement 34 36 He introduced and studied a particular set of Markov processes known as diffusion processes where he derived a set of differential equations describing the processes 34 37 Independent of Kolmogorov s work Sydney Chapman derived in a 1928 paper an equation now called the Chapman Kolmogorov equation in a less mathematically rigorous way than Kolmogorov while studying Brownian movement 38 The differential equations are now called the Kolmogorov equations 39 or the Kolmogorov Chapman equations 40 Other mathematicians who contributed significantly to the foundations of Markov processes include William Feller starting in 1930s and then later Eugene Dynkin starting in the 1950s 35 Examples editMain article Examples of Markov chains Random walks based on integers and the gambler s ruin problem are examples of Markov processes 41 42 Some variations of these processes were studied hundreds of years earlier in the context of independent variables 43 44 45 Two important examples of Markov processes are the Wiener process also known as the Brownian motion process and the Poisson process 28 which are considered the most important and central stochastic processes in the theory of stochastic processes 46 47 48 These two processes are Markov processes in continuous time while random walks on the integers and the gambler s ruin problem are examples of Markov processes in discrete time 41 42 A famous Markov chain is the so called drunkard s walk a random walk on the number line where at each step the position may change by 1 or 1 with equal probability From any position there are two possible transitions to the next or previous integer The transition probabilities depend only on the current position not on the manner in which the position was reached For example the transition probabilities from 5 to 4 and 5 to 6 are both 0 5 and all other transition probabilities from 5 are 0 These probabilities are independent of whether the system was previously in 4 or 6 A series of independent states for example a series of coin flips satisfies the formal definition of a Markov chain However the theory is usually applied only when the probability distribution of the next state depends on the current one A non Markov example edit Suppose that there is a coin purse containing five quarters each worth 25 five dimes each worth 10 and five nickels each worth 5 and one by one coins are randomly drawn from the purse and are set on a table If X n displaystyle X n nbsp represents the total value of the coins set on the table after n draws with X 0 0 displaystyle X 0 0 nbsp then the sequence X n n N displaystyle X n n in mathbb N nbsp is not a Markov process To see why this is the case suppose that in the first six draws all five nickels and a quarter are drawn Thus X 6 0 50 displaystyle X 6 0 50 nbsp If we know not just X 6 displaystyle X 6 nbsp but the earlier values as well then we can determine which coins have been drawn and we know that the next coin will not be a nickel so we can determine that X 7 0 60 displaystyle X 7 geq 0 60 nbsp with probability 1 But if we do not know the earlier values then based only on the value X 6 displaystyle X 6 nbsp we might guess that we had drawn four dimes and two nickels in which case it would certainly be possible to draw another nickel next Thus our guesses about X 7 displaystyle X 7 nbsp are impacted by our knowledge of values prior to X 6 displaystyle X 6 nbsp However it is possible to model this scenario as a Markov process Instead of defining X n displaystyle X n nbsp to represent the total value of the coins on the table we could define X n displaystyle X n nbsp to represent the count of the various coin types on the table For instance X 6 1 0 5 displaystyle X 6 1 0 5 nbsp could be defined to represent the state where there is one quarter zero dimes and five nickels on the table after 6 one by one draws This new model could be represented by 6 6 6 216 displaystyle 6 times 6 times 6 216 nbsp possible states where each state represents the number of coins of each type from 0 to 5 that are on the table Not all of these states are reachable within 6 draws Suppose that the first draw results in state X 1 0 1 0 displaystyle X 1 0 1 0 nbsp The probability of achieving X 2 displaystyle X 2 nbsp now depends on X 1 displaystyle X 1 nbsp for example the state X 2 1 0 1 displaystyle X 2 1 0 1 nbsp is not possible After the second draw the third draw depends on which coins have so far been drawn but no longer only on the coins that were drawn for the first state since probabilistically important information has since been added to the scenario In this way the likelihood of the X n i j k displaystyle X n i j k nbsp state depends exclusively on the outcome of the X n 1 ℓ m p displaystyle X n 1 ell m p nbsp state Formal definition editDiscrete time Markov chain edit Main article Discrete time Markov chain A discrete time Markov chain is a sequence of random variables X1 X2 X3 with the Markov property namely that the probability of moving to the next state depends only on the present state and not on the previous states Pr X n 1 x X 1 x 1 X 2 x 2 X n x n Pr X n 1 x X n x n displaystyle Pr X n 1 x mid X 1 x 1 X 2 x 2 ldots X n x n Pr X n 1 x mid X n x n nbsp if both conditional probabilities are well defined that is if Pr X 1 x 1 X n x n gt 0 displaystyle Pr X 1 x 1 ldots X n x n gt 0 nbsp The possible values of Xi form a countable set S called the state space of the chain Variations edit Time homogeneous Markov chains are processes where Pr X n 1 x X n y Pr X n x X n 1 y displaystyle Pr X n 1 x mid X n y Pr X n x mid X n 1 y nbsp for all n The probability of the transition is independent of n Stationary Markov chains are processes where Pr X 0 x 0 X 1 x 1 X k x k Pr X n x 0 X n 1 x 1 X n k x k displaystyle Pr X 0 x 0 X 1 x 1 ldots X k x k Pr X n x 0 X n 1 x 1 ldots X n k x k nbsp for all n and k Every stationary chain can be proved to be time homogeneous by Bayes rule A necessary and sufficient condition for a time homogeneous Markov chain to be stationary is that the distribution of X 0 displaystyle X 0 nbsp is a stationary distribution of the Markov chain A Markov chain with memory or a Markov chain of order m where m is finite is a process satisfying Pr X n x n X n 1 x n 1 X n 2 x n 2 X 1 x 1 Pr X n x n X n 1 x n 1 X n 2 x n 2 X n m x n m for n gt m displaystyle begin aligned amp Pr X n x n mid X n 1 x n 1 X n 2 x n 2 dots X 1 x 1 amp Pr X n x n mid X n 1 x n 1 X n 2 x n 2 dots X n m x n m text for n gt m end aligned nbsp In other words the future state depends on the past m states It is possible to construct a chain Y n displaystyle Y n nbsp from X n displaystyle X n nbsp which has the classical Markov property by taking as state space the ordered m tuples of X values i e Y n X n X n 1 X n m 1 displaystyle Y n left X n X n 1 ldots X n m 1 right nbsp Continuous time Markov chain edit Main article Continuous time Markov chain A continuous time Markov chain Xt t 0 is defined by a finite or countable state space S a transition rate matrix Q with dimensions equal to that of the state space and initial probability distribution defined on the state space For i j the elements qij are non negative and describe the rate of the process transitions from state i to state j The elements qii are chosen such that each row of the transition rate matrix sums to zero while the row sums of a probability transition matrix in a discrete Markov chain are all equal to one There are three equivalent definitions of the process 49 Infinitesimal definition edit nbsp The continuous time Markov chain is characterized by the transition rates the derivatives with respect to time of the transition probabilities between states i and j Let X t displaystyle X t nbsp be the random variable describing the state of the process at time t and assume the process is in a state i at time t Then knowing X t i displaystyle X t i nbsp X t h j displaystyle X t h j nbsp is independent of previous values X s s lt t displaystyle left X s s lt t right nbsp and as h 0 for all j and for all t Pr X t h j X t i d i j q i j h o h displaystyle Pr X t h j mid X t i delta ij q ij h o h nbsp where d i j displaystyle delta ij nbsp is the Kronecker delta using the little o notation The q i j displaystyle q ij nbsp can be seen as measuring how quickly the transition from i to j happens Jump chain holding time definition edit Define a discrete time Markov chain Yn to describe the nth jump of the process and variables S1 S2 S3 to describe holding times in each of the states where Si follows the exponential distribution with rate parameter qYiYi Transition probability definition edit For any value n 0 1 2 3 and times indexed up to this value of n t0 t1 t2 and all states recorded at these times i0 i1 i2 i3 it holds that Pr X t n 1 i n 1 X t 0 i 0 X t 1 i 1 X t n i n p i n i n 1 t n 1 t n displaystyle Pr X t n 1 i n 1 mid X t 0 i 0 X t 1 i 1 ldots X t n i n p i n i n 1 t n 1 t n nbsp where pij is the solution of the forward equation a first order differential equation P t P t Q displaystyle P t P t Q nbsp with initial condition P 0 is the identity matrix Finite state space edit If the state space is finite the transition probability distribution can be represented by a matrix called the transition matrix with the i j th element of P equal to p i j Pr X n 1 j X n i displaystyle p ij Pr X n 1 j mid X n i nbsp Since each row of P sums to one and all elements are non negative P is a right stochastic matrix Stationary distribution relation to eigenvectors and simplices edit A stationary distribution p is a row vector whose entries are non negative and sum to 1 is unchanged by the operation of transition matrix P on it and so is defined by p P p displaystyle pi mathbf P pi nbsp By comparing this definition with that of an eigenvector we see that the two concepts are related and that p e i e i displaystyle pi frac e sum i e i nbsp is a normalized i p i 1 textstyle sum i pi i 1 nbsp multiple of a left eigenvector e of the transition matrix P with an eigenvalue of 1 If there is more than one unit eigenvector then a weighted sum of the corresponding stationary states is also a stationary state But for a Markov chain one is usually more interested in a stationary state that is the limit of the sequence of distributions for some initial distribution The values of a stationary distribution p i displaystyle textstyle pi i nbsp are associated with the state space of P and its eigenvectors have their relative proportions preserved Since the components of p are positive and the constraint that their sum is unity can be rewritten as i 1 p i 1 textstyle sum i 1 cdot pi i 1 nbsp we see that the dot product of p with a vector whose components are all 1 is unity and that p lies on a simplex Time homogeneous Markov chain with a finite state space edit This section includes a list of references related reading or external links but its sources remain unclear because it lacks inline citations Please help improve this section by introducing more precise citations February 2012 Learn how and when to remove this message If the Markov chain is time homogeneous then the transition matrix P is the same after each step so the k step transition probability can be computed as the k th power of the transition matrix Pk If the Markov chain is irreducible and aperiodic then there is a unique stationary distribution p 50 Additionally in this case Pk converges to a rank one matrix in which each row is the stationary distribution p lim k P k 1 p displaystyle lim k to infty mathbf P k mathbf 1 pi nbsp where 1 is the column vector with all entries equal to 1 This is stated by the Perron Frobenius theorem If by whatever means lim k P k textstyle lim k to infty mathbf P k nbsp is found then the stationary distribution of the Markov chain in question can be easily determined for any starting distribution as will be explained below For some stochastic matrices P the limit lim k P k textstyle lim k to infty mathbf P k nbsp does not exist while the stationary distribution does as shown by this example P 0 1 1 0 P 2 k I P 2 k 1 P displaystyle mathbf P begin pmatrix 0 amp 1 1 amp 0 end pmatrix qquad mathbf P 2k I qquad mathbf P 2k 1 mathbf P nbsp 1 2 1 2 0 1 1 0 1 2 1 2 displaystyle begin pmatrix frac 1 2 amp frac 1 2 end pmatrix begin pmatrix 0 amp 1 1 amp 0 end pmatrix begin pmatrix frac 1 2 amp frac 1 2 end pmatrix nbsp This example illustrates a periodic Markov chain Because there are a number of different special cases to consider the process of finding this limit if it exists can be a lengthy task However there are many techniques that can assist in finding this limit Let P be an n n matrix and define Q lim k P k textstyle mathbf Q lim k to infty mathbf P k nbsp It is always true that Q P Q displaystyle mathbf QP mathbf Q nbsp Subtracting Q from both sides and factoring then yields Q P I n 0 n n displaystyle mathbf Q mathbf P mathbf I n mathbf 0 n n nbsp where In is the identity matrix of size n and 0n n is the zero matrix of size n n Multiplying together stochastic matrices always yields another stochastic matrix so Q must be a stochastic matrix see the definition above It is sometimes sufficient to use the matrix equation above and the fact that Q is a stochastic matrix to solve for Q Including the fact that the sum of each the rows in P is 1 there are n 1 equations for determining n unknowns so it is computationally easier if on the one hand one selects one row in Q and substitutes each of its elements by one and on the other one substitutes the corresponding element the one in the same column in the vector 0 and next left multiplies this latter vector by the inverse of transformed former matrix to find Q Here is one method for doing so first define the function f A to return the matrix A with its right most column replaced with all 1 s If f P In 1 exists then 51 50 Q f 0 n n f P I n 1 displaystyle mathbf Q f mathbf 0 n n f mathbf P mathbf I n 1 nbsp Explain The original matrix equation is equivalent to a system of n n linear equations in n n variables And there are n more linear equations from the fact that Q is a right stochastic matrix whose each row sums to 1 So it needs any n n independent linear equations of the n n n equations to solve for the n n variables In this example the n equations from Q multiplied by the right most column of P In have been replaced by the n stochastic ones One thing to notice is that if P has an element Pi i on its main diagonal that is equal to 1 and the ith row or column is otherwise filled with 0 s then that row or column will remain unchanged in all of the subsequent powers Pk Hence the ith row or column of Q will have the 1 and the 0 s in the same positions as in P Convergence speed to the stationary distribution edit This section includes a list of references related reading or external links but its sources remain unclear because it lacks inline citations Please help improve this section by introducing more precise citations February 2012 Learn how and when to remove this message As stated earlier from the equation p p P displaystyle boldsymbol pi boldsymbol pi mathbf P nbsp if exists the stationary or steady state distribution p is a left eigenvector of row stochastic matrix P Then assuming that P is diagonalizable or equivalently that P has n linearly independent eigenvectors speed of convergence is elaborated as follows For non diagonalizable that is defective matrices one may start with the Jordan normal form of P and proceed with a bit more involved set of arguments in a similar way 52 Let U be the matrix of eigenvectors each normalized to having an L2 norm equal to 1 where each column is a left eigenvector of P and let S be the diagonal matrix of left eigenvalues of P that is S diag l1 l2 l3 ln Then by eigendecomposition P U S U 1 displaystyle mathbf P mathbf U Sigma U 1 nbsp Let the eigenvalues be enumerated such that 1 l 1 gt l 2 l 3 l n displaystyle 1 lambda 1 gt lambda 2 geq lambda 3 geq cdots geq lambda n nbsp Since P is a row stochastic matrix its largest left eigenvalue is 1 If there is a unique stationary distribution then the largest eigenvalue and the corresponding eigenvector is unique too because there is no other p which solves the stationary distribution equation above Let ui be the i th column of U matrix that is ui is the left eigenvector of P corresponding to li Also let x be a length n row vector that represents a valid probability distribution since the eigenvectors ui span R n displaystyle mathbb R n nbsp we can write x T i 1 n a i u i a i R displaystyle mathbf x mathsf T sum i 1 n a i mathbf u i qquad a i in mathbb R nbsp If we multiply x with P from right and continue this operation with the results in the end we get the stationary distribution p In other words p a1 u1 xPP P xPk as k That means p k x U S U 1 U S U 1 U S U 1 x U S k U 1 a 1 u 1 T a 2 u 2 T a n u n T U S k U 1 a 1 l 1 k u 1 T a 2 l 2 k u 2 T a n l n k u n T u i u j for i j l 1 k a 1 u 1 T a 2 l 2 l 1 k u 2 T a 3 l 3 l 1 k u 3 T a n l n l 1 k u n T displaystyle begin aligned boldsymbol pi k amp mathbf x left mathbf U Sigma U 1 right left mathbf U Sigma U 1 right cdots left mathbf U Sigma U 1 right amp mathbf xU Sigma k mathbf U 1 amp left a 1 mathbf u 1 mathsf T a 2 mathbf u 2 mathsf T cdots a n mathbf u n mathsf T right mathbf U Sigma k mathbf U 1 amp a 1 lambda 1 k mathbf u 1 mathsf T a 2 lambda 2 k mathbf u 2 mathsf T cdots a n lambda n k mathbf u n mathsf T amp amp u i bot u j text for i neq j amp lambda 1 k left a 1 mathbf u 1 mathsf T a 2 left frac lambda 2 lambda 1 right k mathbf u 2 mathsf T a 3 left frac lambda 3 lambda 1 right k mathbf u 3 mathsf T cdots a n left frac lambda n lambda 1 right k mathbf u n mathsf T right end aligned nbsp Since p is parallel to u1 normalized by L2 norm and p k is a probability vector p k approaches to a1 u1 p as k with a speed in the order of l2 l1 exponentially This follows because l 2 l n displaystyle lambda 2 geq cdots geq lambda n nbsp hence l2 l1 is the dominant term The smaller the ratio is the faster the convergence is 53 Random noise in the state distribution p can also speed up this convergence to the stationary distribution 54 General state space edit Main article Markov chains on a measurable state space Harris chains edit Many results for Markov chains with finite state space can be generalized to chains with uncountable state space through Harris chains The use of Markov chains in Markov chain Monte Carlo methods covers cases where the process follows a continuous state space Locally interacting Markov chains edit Considering a collection of Markov chains whose evolution takes in account the state of other Markov chains is related to the notion of locally interacting Markov chains This corresponds to the situation when the state space has a Cartesian product form See interacting particle system and stochastic cellular automata probabilistic cellular automata See for instance Interaction of Markov Processes 55 or 56 Properties editTwo states are said to communicate with each other if both are reachable from one another by a sequence of transitions that have positive probability This is an equivalence relation which yields a set of communicating classes A class is closed if the probability of leaving the class is zero A Markov chain is irreducible if there is one communicating class the state space A state i has period k if k is the greatest common divisor of the number of transitions by which i can be reached starting from i That is k gcd n gt 0 Pr X n i X 0 i gt 0 displaystyle k gcd n gt 0 Pr X n i mid X 0 i gt 0 nbsp Although the state as a whole is only described as periodic if k gt 1 displaystyle k gt 1 nbsp If k 1 displaystyle k 1 nbsp then the state is described as being aperiodic A state i is said to be transient if starting from i there is a non zero probability that the chain will never return to i It is called recurrent or persistent otherwise 57 For a recurrent state i the mean hitting time is defined as M i E T i n 1 n f i i n displaystyle M i E T i sum n 1 infty n cdot f ii n nbsp State i is positive recurrent if M i displaystyle M i nbsp is finite and null recurrent otherwise Periodicity transience recurrence and positive and null recurrence are class properties that is if one state has the property then all states in its communicating class have the property 58 A state i is called absorbing if there are no outgoing transitions from the state Irreducibility edit Since periodicity is a class property if a Markov chain is irreducible then all its states have the same period In particular if one state is aperiodic then the whole Markov chain is aperiodic If a finite Markov chain is irreducible then all states are positive recurrent and it has a unique stationary distribution given by p i 1 E T i displaystyle pi i 1 E T i nbsp Ergodicity edit A state i is said to be ergodic if it is aperiodic and positive recurrent In other words a state i is ergodic if it is recurrent has a period of 1 and has finite mean recurrence time If all states in an irreducible Markov chain are ergodic then the chain is said to be ergodic Equivalently there exists some integer k displaystyle k nbsp such that all entries of M k displaystyle M k nbsp are positive It can be shown that a finite state irreducible Markov chain is ergodic if it has an aperiodic state More generally a Markov chain is ergodic if there is a number N such that any state can be reached from any other state in any number of steps less or equal to a number N In case of a fully connected transition matrix where all transitions have a non zero probability this condition is fulfilled with N 1 A Markov chain with more than one state and just one out going transition per state is either not irreducible or not aperiodic hence cannot be ergodic Terminology edit Some authors call any irreducible positive recurrent Markov chains ergodic even periodic ones 59 In fact merely irreducible Markov chains correspond to ergodic processes defined according to ergodic theory 60 Some authors call a matrix primitive iff there exists some integer k displaystyle k nbsp such that all entries of M k displaystyle M k nbsp are positive 61 Some authors call it regular 62 Index of primitivity edit The index of primitivity or exponent of a regular matrix is the smallest k displaystyle k nbsp such that all entries of M k displaystyle M k nbsp are positive The exponent is purely a graph theoretic property since it depends only on whether each entry of M displaystyle M nbsp is zero or positive and therefore can be found on a directed graph with s i g n M displaystyle mathrm sign M nbsp as its adjacency matrix There are several combinatorial results about the exponent when there are finitely many states Let n displaystyle n nbsp be the number of states then 63 The exponent is n 1 2 1 displaystyle leq n 1 2 1 nbsp The only case where it is an equality is when the graph of M displaystyle M nbsp goes like 1 2 n 1 and 2 displaystyle 1 to 2 to dots to n to 1 text and 2 nbsp If M displaystyle M nbsp has k 1 displaystyle k geq 1 nbsp diagonal entries then its exponent is 2 n k 1 displaystyle leq 2n k 1 nbsp If s i g n M displaystyle mathrm sign M nbsp is symmetric then M 2 displaystyle M 2 nbsp has positive diagonal entries which by previous proposition means its exponent is 2 n 2 displaystyle leq 2n 2 nbsp Dulmage Mendelsohn theorem The exponent is n s n 2 displaystyle leq n s n 2 nbsp where s displaystyle s nbsp is the girth of the graph It can be improved to d 1 s d 1 2 displaystyle leq d 1 s d 1 2 nbsp where d displaystyle d nbsp is the diameter of the graph 64 Measure preserving dynamical system edit If a Markov chain has a stationary distribution then it can be converted to a measure preserving dynamical system Let the probability space be W S N displaystyle Omega Sigma mathbb N nbsp where S displaystyle Sigma nbsp is the set of all states for the Markov chain Let the sigma algebra on the probability space be generated by the cylinder sets Let the probability measure be generated by the stationary distribution and the Markov chain transition Let T W W displaystyle T Omega to Omega nbsp be the shift operator T X 0 X 1 X 1 displaystyle T X 0 X 1 dots X 1 dots nbsp Similarly we can construct such a dynamical system with W S Z displaystyle Omega Sigma mathbb Z nbsp instead 65 Since irreducible Markov chains with finite state spaces have a unique stationary distribution the above construction is unambiguous for irreducible Markov chains In ergodic theory a measure preserving dynamical system is called ergodic iff any measurable subset S displaystyle S nbsp such that T 1 S S displaystyle T 1 S S nbsp implies S displaystyle S emptyset nbsp or W displaystyle Omega nbsp up to a null set The terminology is inconsistent Given a Markov chain with a stationary distribution that is strictly positive on all states the Markov chain is irreducible iff its corresponding measure preserving dynamical system is ergodic 60 Markovian representations edit In some cases apparently non Markovian processes may still have Markovian representations constructed by expanding the concept of the current and future states For example let X be a non Markovian process Then define a process Y such that each state of Y represents a time interval of states of X Mathematically this takes the form Y t X s s a t b t displaystyle Y t big X s s in a t b t big nbsp If Y has the Markov property then it is a Markovian representation of X An example of a non Markovian process with a Markovian representation is an autoregressive time series of order greater than one 66 Hitting times edit Main article Phase type distributionThe hitting time is the time starting in a given set of states until the chain arrives in a given state or set of states The distribution of such a time period has a phase type distribution The simplest such distribution is that of a single exponentially distributed transition Expected hitting times edit For a subset of states A S the vector kA of hitting times where element k i A displaystyle k i A nbsp represents the expected value starting in state i that the chain enters one of the states in the set A is the minimal non negative solution to 67 k i A 0 for i A j S q i j k j A 1 for i A displaystyle begin aligned k i A 0 amp text for i in A sum j in S q ij k j A 1 amp text for i notin A end aligned nbsp Time reversal edit For a CTMC Xt the time reversed process is defined to be X t X T t displaystyle hat X t X T t nbsp By Kelly s lemma this process has the same stationary distribution as the forward process A chain is said to be reversible if the reversed process is the same as the forward process Kolmogorov s criterion states that the necessary and sufficient condition for a process to be reversible is that the product of transition rates around a closed loop must be the same in both directions Embedded Markov chain edit One method of finding the stationary probability distribution p of an ergodic continuous time Markov chain Q is by first finding its embedded Markov chain EMC Strictly speaking the EMC is a regular discrete time Markov chain sometimes referred to as a jump process Each element of the one step transition probability matrix of the EMC S is denoted by sij and represents the conditional probability of transitioning from state i into state j These conditional probabilities may be found by s i j q i j k i q i k if i j 0 otherwise displaystyle s ij begin cases frac q ij sum k neq i q ik amp text if i neq j 0 amp text otherwise end cases nbsp From this S may be written as S I diag Q 1 Q displaystyle S I left operatorname diag Q right 1 Q nbsp where I is the identity matrix and diag Q is the diagonal matrix formed by selecting the main diagonal from the matrix Q and setting all other elements to zero To find the stationary probability distribution vector we must next find f displaystyle varphi nbsp such that f S f displaystyle varphi S varphi nbsp with f displaystyle varphi nbsp being a row vector such that all elements in f displaystyle varphi nbsp are greater than 0 and f 1 displaystyle varphi 1 nbsp 1 From this p may be found as p f diag Q 1 f diag Q 1 1 displaystyle pi varphi operatorname diag Q 1 over left varphi operatorname diag Q 1 right 1 nbsp S may be periodic even if Q is not Once p is found it must be normalized to a unit vector Another discrete time process that may be derived from a continuous time Markov chain is a d skeleton the discrete time Markov chain formed by observing X t at intervals of d units of time The random variables X 0 X d X 2d give the sequence of states visited by the d skeleton Special types of Markov chains editMarkov model edit Main article Markov model Markov models are used to model changing systems There are 4 main types of models that generalize Markov chains depending on whether every sequential state is observable or not and whether the system is to be adjusted on the basis of observations made System state is fully observable System state is partially observable System is autonomous Markov chain Hidden Markov model System is controlled Markov decision process Partially observable Markov decision process Bernoulli scheme edit Main article Bernoulli scheme A Bernoulli scheme is a special case of a Markov chain where the transition probability matrix has identical rows which means that the next state is independent of even the current state in addition to being independent of the past states A Bernoulli scheme with only two possible states is known as a Bernoulli process Note however by the Ornstein isomorphism theorem that every aperiodic and irreducible Markov chain is isomorphic to a Bernoulli scheme 68 thus one might equally claim that Markov chains are a special case of Bernoulli schemes The isomorphism generally requires a complicated recoding The isomorphism theorem is even a bit stronger it states that any stationary stochastic process is isomorphic to a Bernoulli scheme the Markov chain is just one such example Subshift of finite type edit Main article Subshift of finite type When the Markov matrix is replaced by the adjacency matrix of a finite graph the resulting shift is termed a topological Markov chain or a subshift of finite type 68 A Markov matrix that is compatible with the adjacency matrix can then provide a measure on the subshift Many chaotic dynamical systems are isomorphic to topological Markov chains examples include diffeomorphisms of closed manifolds the Prouhet Thue Morse system the Chacon system sofic systems context free systems and block coding systems 68 Applications editResearch has reported the application and usefulness of Markov chains in a wide range of topics such as physics chemistry biology medicine music game theory and sports Physics edit Markovian systems appear extensively in thermodynamics and statistical mechanics whenever probabilities are used to represent unknown or unmodelled details of the system if it can be assumed that the dynamics are time invariant and that no relevant history need be considered which is not already included in the state description 69 70 For example a thermodynamic state operates under a probability distribution that is difficult or expensive to acquire Therefore Markov Chain Monte Carlo method can be used to draw samples randomly from a black box to approximate the probability distribution of attributes over a range of objects 70 The paths in the path integral formulation of quantum mechanics are Markov chains 71 Markov chains are used in lattice QCD simulations 72 Chemistry edit E S E Substrate binding S E Catalytic step P displaystyle ce E underset Substrate atop binding S lt gt E overset Catalytic atop step S gt E P nbsp Michaelis Menten kinetics The enzyme E binds a substrate S and produces a product P Each reaction is a state transition in a Markov chain A reaction network is a chemical system involving multiple reactions and chemical species The simplest stochastic models of such networks treat the system as a continuous time Markov chain with the state being the number of molecules of each species and with reactions modeled as possible transitions of the chain 73 Markov chains and continuous time Markov processes are useful in chemistry when physical systems closely approximate the Markov property For example imagine a large number n of molecules in solution in state A each of which can undergo a chemical reaction to state B with a certain average rate Perhaps the molecule is an enzyme and the states refer to how it is folded The state of any single enzyme follows a Markov chain and since the molecules are essentially independent of each other the number of molecules in state A or B at a time is n times the probability a given molecule is in that state The classical model of enzyme activity Michaelis Menten kinetics can be viewed as a Markov chain where at each time step the reaction proceeds in some direction While Michaelis Menten is fairly straightforward far more complicated reaction networks can also be modeled with Markov chains 74 An algorithm based on a Markov chain was also used to focus the fragment based growth of chemicals in silico towards a desired class of compounds such as drugs or natural products 75 As a molecule is grown a fragment is selected from the nascent molecule as the current state It is not aware of its past that is it is not aware of what is already bonded to it It then transitions to the next state when a fragment is attached to it The transition probabilities are trained on databases of authentic classes of compounds 76 Also the growth and composition of copolymers may be modeled using Markov chains Based on the reactivity ratios of the monomers that make up the growing polymer chain the chain s composition may be calculated for example whether monomers tend to add in alternating fashion or in long runs of the same monomer Due to steric effects second order Markov effects may also play a role in the growth of some polymer chains Similarly it has been suggested that the crystallization and growth of some epitaxial superlattice oxide materials can be accurately described by Markov chains 77 Biology edit Markov chains are used in various areas of biology Notable examples include Phylogenetics and bioinformatics where most models of DNA evolution use continuous time Markov chains to describe the nucleotide present at a given site in the genome Population dynamics where Markov chains are in particular a central tool in the theoretical study of matrix population models Neurobiology where Markov chains have been used e g to simulate the mammalian neocortex 78 Systems biology for instance with the modeling of viral infection of single cells 79 Compartmental models for disease outbreak and epidemic modeling Testing edit Several theorists have proposed the idea of the Markov chain statistical test MCST a method of conjoining Markov chains to form a Markov blanket arranging these chains in several recursive layers wafering and producing more efficient test sets samples as a replacement for exhaustive testing MCSTs also have uses in temporal state based networks Chilukuri et al s paper entitled Temporal Uncertainty Reasoning Networks for Evidence Fusion with Applications to Object Detection and Tracking ScienceDirect gives a background and case study for applying MCSTs to a wider range of applications Solar irradiance variability edit Solar irradiance variability assessments are useful for solar power applications Solar irradiance variability at any location over time is mainly a consequence of the deterministic variability of the sun s path across the sky dome and the variability in cloudiness The variability of accessible solar irradiance on Earth s surface has been modeled using Markov chains 80 81 82 83 also including modeling the two states of clear and cloudiness as a two state Markov chain 84 85 Speech recognition edit Hidden Markov models are the basis for most modern automatic speech recognition systems Information theory edit Markov chains are used throughout information processing Claude Shannon s famous 1948 paper A Mathematical Theory of Communication which in a single step created the field of information theory opens by introducing the concept of entropy through Markov modeling of the English language Such idealized models can capture many of the statistical regularities of systems Even without describing the full structure of the system perfectly such signal models can make possible very effective data compression through entropy encoding techniques such as arithmetic coding They also allow effective state estimation and pattern recognition Markov chains also play an important role in reinforcement learning Markov chains are also the basis for hidden Markov models which are an important tool in such diverse fields as telephone networks which use the Viterbi algorithm for error correction speech recognition and bioinformatics such as in rearrangements detection 86 The LZMA lossless data compression algorithm combines Markov chains with Lempel Ziv compression to achieve very high compression ratios Queueing theory edit Main article Queueing theoryMarkov chains are the basis for the analytical treatment of queues queueing theory Agner Krarup Erlang initiated the subject in 1917 87 This makes them critical for optimizing the performance of telecommunications networks where messages must often compete for limited resources such as bandwidth 88 Numerous queueing models use continuous time Markov chains For example an M M 1 queue is a CTMC on the non negative integers where upward transitions from i to i 1 occur at rate l according to a Poisson process and describe job arrivals while transitions from i to i 1 for i gt 1 occur at rate m job service times are exponentially distributed and describe completed services departures from the queue Internet applications edit nbsp A state diagram that represents the PageRank algorithm with a transitional probability of M or a k i 1 a N displaystyle frac alpha k i frac 1 alpha N nbsp The PageRank of a webpage as used by Google is defined by a Markov chain 89 90 91 It is the probability to be at page i displaystyle i nbsp in the stationary distribution on the following Markov chain on all known webpages If N displaystyle N nbsp is the number of known webpages and a page i displaystyle i nbsp has k i displaystyle k i nbsp links to it then it has transition probability a k i 1 a N displaystyle frac alpha k i frac 1 alpha N nbsp for all pages that are linked to and 1 a N displaystyle frac 1 alpha N nbsp for all pages that are not linked to The parameter a displaystyle alpha nbsp is taken to be about 0 15 92 Markov models have also been used to analyze web navigation behavior of users A user s web link transition on a particular website can be modeled using first or second order Markov models and can be used to make predictions regarding future navigation and to personalize the web page for an individual user Statistics edit Markov chain methods have also become very important for generating sequences of random numbers to accurately reflect very complicated desired probability distributions via a process called Markov chain Monte Carlo MCMC In recent years this has revolutionized the practicability of Bayesian inference methods allowing a wide range of posterior distributions to be simulated and their parameters found numerically Economics and finance edit Markov chains are used in finance and economics to model a variety of different phenomena including the distribution of income the size distribution of firms asset prices and market crashes D G Champernowne built a Markov chain model of the distribution of income in 1953 93 Herbert A Simon and co author Charles Bonini used a Markov chain model to derive a stationary Yule distribution of firm sizes 94 Louis Bachelier was the first to observe that stock prices followed a random walk 95 The random walk was later seen as evidence in favor of the efficient market hypothesis and random walk models were popular in the literature of the 1960s 96 Regime switching models of business cycles were popularized by James D Hamilton 1989 who used a Markov chain to model switches between periods of high and low GDP growth or alternatively economic expansions and recessions 97 A more recent example is the Markov switching multifractal model of Laurent E Calvet and Adlai J Fisher which builds upon the convenience of earlier regime switching models 98 99 It uses an arbitrarily large Markov chain to drive the level of volatility of asset returns Dynamic macroeconomics makes heavy use of Markov chains An example is using Markov chains to exogenously model prices of equity stock in a general equilibrium setting 100 Credit rating agencies produce annual tables of the transition probabilities for bonds of different credit ratings 101 Social sciences edit Markov chains are generally used in describing path dependent arguments where current structural configurations condition future outcomes An example is the reformulation of the idea originally due to Karl Marx s Das Kapital tying economic development to the rise of capitalism In current research it is common to use a Markov chain to model how once a country reaches a specific level of economic development the configuration of structural factors such as size of the middle class the ratio of urban to rural residence the rate of political mobilization etc will generate a higher probability of transitioning from authoritarian to democratic regime 102 Games edit Markov chains can be used to model many games of chance 1 The children s games Snakes and Ladders and Hi Ho Cherry O for example are represented exactly by Markov chains At each turn the player starts in a given state on a given square and from there has fixed odds of moving to certain other states squares Music edit Markov chains are employed in algorithmic music composition particularly in software such as Csound Max and SuperCollider In a first order chain the states of the system become note or pitch values and a probability vector for each note is constructed completing a transition probability matrix see below An algorithm is constructed to produce output note values based on the transition matrix weightings which could be MIDI note values frequency Hz or any other desirable metric 103 1st order matrix Note A C E A 0 1 0 6 0 3 C 0 25 0 05 0 7 E 0 7 0 3 0 2nd order matrix Notes A D G AA 0 18 0 6 0 22 AD 0 5 0 5 0 AG 0 15 0 75 0 1 DD 0 0 1 DA 0 25 0 0 75 DG 0 9 0 1 0 GG 0 4 0 4 0 2 GA 0 5 0 25 0 25 GD 1 0 0 A second order Markov chain can be introduced by considering the current state and also the previous state as indicated in the second table Higher nth order chains tend to group particular notes together while breaking off into other patterns and sequences occasionally These higher order chains tend to generate results with a sense of phrasal structure rather than the aimless wandering produced by a first order system 104 Markov chains can be used structurally as in Xenakis s Analogique A and B 105 Markov chains are also used in systems which use a Markov model to react interactively to music input 106 Usually musical systems need to enforce specific control constraints on the finite length sequences they generate but control constraints are not compatible with Markov models since they induce long range dependencies that violate the Markov hypothesis of limited memory In order to overcome this limitation a new approach has been proposed 107 Baseball edit Markov chain models have been used in advanced baseball analysis since 1960 although their use is still rare Each half inning of a baseball game fits the Markov chain state when the number of runners and outs are considered During any at bat there are 24 possible combinations of number of outs and position of the runners Mark Pankin shows that Markov chain models can be used to evaluate runs created for both individual players as well as a team 108 He also discusses various kinds of strategies and play conditions how Markov chain models have been used to analyze statistics for game situations such as bunting and base stealing and differences when playing on grass vs AstroTurf 109 Markov text generators edit Markov processes can also be used to generate superficially real looking text given a sample document Markov processes are used in a variety of recreational parody generator software see dissociated press Jeff Harrison 110 Mark V Shaney 111 112 and Academias Neutronium Several open source text generation libraries using Markov chains exist Probabilistic forecasting edit Markov chains have been used for forecasting in several areas for example price trends 113 wind power 114 stochastic terrorism 115 116 and solar irradiance 117 The Markov chain forecasting models utilize a variety of settings from discretizing the time series 114 to hidden Markov models combined with wavelets 113 and the Markov chain mixture distribution model MCM 117 See also editDynamics of Markovian particles Gauss Markov process Markov chain approximation method Markov chain geostatistics Markov chain mixing time Markov chain tree theorem Markov decision process Markov information source Markov odometer Markov operator Markov random field Master equation Quantum Markov chain Semi Markov process Stochastic cellular automaton Telescoping Markov chain Variable order Markov modelNotes edit a b c d e f g h i j k l Gagniuc Paul A 2017 Markov Chains From Theory to Implementation and Experimentation USA NJ John Wiley amp Sons pp 1 235 ISBN 978 1 119 38755 8 Markov chain Definition of Markov chain in US English by Oxford Dictionaries Oxford Dictionaries Archived from the original on December 15 2017 Retrieved 2017 12 14 Definition at Brilliant org Brilliant Math and Science Wiki Retrieved on 12 May 2019 Samuel Karlin Howard E Taylor 2 December 2012 A First Course in Stochastic Processes Academic Press p 47 ISBN 978 0 08 057041 9 Archived from the original on 23 March 2017 Bruce Hajek 12 March 2015 Random Processes for Engineers Cambridge University Press ISBN 978 1 316 24124 0 Archived from the original on 23 March 2017 G Latouche V Ramaswami 1 January 1999 Introduction to Matrix Analytic Methods in Stochastic Modeling SIAM pp 4 ISBN 978 0 89871 425 8 Archived from the original on 23 March 2017 a b Sean Meyn Richard L Tweedie 2 April 2009 Markov Chains and Stochastic Stability Cambridge University Press p 3 ISBN 978 0 521 73182 9 Archived from the original on 23 March 2017 Reuven Y Rubinstein Dirk P Kroese 20 September 2011 Simulation and the Monte Carlo Method John Wiley amp Sons p 225 ISBN 978 1 118 21052 9 Archived from the original on 23 March 2017 Dani Gamerman Hedibert F Lopes 10 May 2006 Markov Chain Monte Carlo Stochastic Simulation for Bayesian Inference Second Edition CRC Press ISBN 978 1 58488 587 0 Archived from the original on 23 March 2017 Markovian Oxford English Dictionary Online ed Oxford University Press Subscription or participating institution membership required Model Based Signal Processing John Wiley amp Sons 27 October 2005 ISBN 9780471732662 Oksendal B K Bernt Karsten 2003 Stochastic differential equations an introduction with applications 6th ed Berlin Springer ISBN 3540047581 OCLC 52203046 a b Soren Asmussen 15 May 2003 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probability theory to a sum of variables connected in a chain reprinted in Appendix B of R Howard Dynamic Probabilistic Systems volume 1 Markov Chains John Wiley and Sons Classical Text in Translation Markov A A 2006 An Example of Statistical Investigation of the Text Eugene Onegin Concerning the Connection of Samples in Chains Science in Context 19 4 Translated by Link David 591 600 doi 10 1017 s0269889706001074 S2CID 144854176 Leo Breiman 1992 1968 Probability Original edition published by Addison Wesley reprinted by Society for Industrial and Applied Mathematics ISBN 0 89871 296 3 See Chapter 7 J L Doob 1953 Stochastic Processes New York John Wiley and Sons ISBN 0 471 52369 0 S P Meyn and R L Tweedie 1993 Markov Chains and Stochastic Stability London Springer Verlag ISBN 0 387 19832 6 online MCSS Second edition to appear Cambridge University Press 2009 Dynkin Eugene Borisovich 1965 1963 03 10 1962 03 31 Written at University of Moscow Moscow Russia Markov Processes I Die Grundlehren der mathematischen Wissenschaften in Einzeldarstellungen mit besonderer Berucksichtigung der Anwendungsgebiete Vol I 121 Translated by Fabius Jaap at Wikidata Greenberg Vida Lazarus at Wikidata Maitra Ashok Prasad at Wikidata Majone Giandomenico 1 ed New York USA Berlin Germany Springer Verlag Academic Press Inc Publishers doi 10 1007 978 3 662 00031 1 ISBN 978 3 662 00033 5 ISSN 0072 7830 LCCN 64 24812 S2CID 251691119 Title No 5104 Retrieved 2023 09 02 Markov Processes Volume I xii 365 1 pages Dynkin Eugene Borisovich 1965 1963 03 10 1962 03 31 Written at University of Moscow Moscow Russia Markov Processes II Die Grundlehren der mathematischen Wissenschaften in Einzeldarstellungen mit besonderer Berucksichtigung der Anwendungsgebiete Vol II 122 Translated by Fabius Jaap at Wikidata Greenberg Vida Lazarus at Wikidata Maitra Ashok Prasad at Wikidata Majone Giandomenico 1 ed New York USA Berlin Germany Springer Verlag doi 10 1007 978 3 662 25360 1 ISBN 978 3 662 23320 7 ISSN 0072 7830 LCCN 64 24812 Title No 5105 Retrieved 2023 09 02 viii 274 2 pages NB This was originally published in Russian as Markovskie prot s essy Markovskie processy by Fizmatgiz Fizmatgiz in 1963 and translated to English with the assistance of the author S P Meyn Control Techniques for Complex Networks Cambridge University Press 2007 ISBN 978 0 521 88441 9 Appendix contains abridged Meyn amp Tweedie online CTCN Booth Taylor L 1967 Sequential Machines and Automata Theory 1st ed New York NY John Wiley and Sons Inc Library of Congress Card Catalog Number 67 25924 Extensive wide ranging book meant for specialists written for both theoretical computer scientists as well as electrical engineers With detailed explanations of state minimization techniques FSMs Turing machines Markov processes and undecidability Excellent treatment of Markov processes pp 449ff Discusses Z transforms D transforms in their context Kemeny John G Hazleton Mirkil J Laurie Snell Gerald L Thompson 1959 Finite Mathematical Structures 1st ed Englewood Cliffs NJ Prentice Hall Inc Library of Congress Card Catalog Number 59 12841 Classical text cf Chapter 6 Finite Markov Chains pp 384ff John G Kemeny amp J Laurie Snell 1960 Finite Markov Chains D van Nostrand Company ISBN 0 442 04328 7 E Nummelin General irreducible Markov chains and non negative operators Cambridge University Press 1984 2004 ISBN 0 521 60494 X Seneta E Non negative matrices and Markov chains 2nd rev ed 1981 XVI 288 p Softcover Springer Series in Statistics Originally published by Allen amp Unwin Ltd London 1973 ISBN 978 0 387 29765 1 Kishor S Trivedi Probability and Statistics with Reliability Queueing and Computer Science Applications John Wiley amp Sons Inc New York 2002 ISBN 0 471 33341 7 K S Trivedi and R A Sahner SHARPE at the age of twenty two vol 36 no 4 pp 52 57 ACM SIGMETRICS Performance Evaluation Review 2009 R A Sahner K S Trivedi and A Puliafito Performance and reliability analysis of computer systems an example based approach using the SHARPE software package Kluwer Academic Publishers 1996 ISBN 0 7923 9650 2 G Bolch S Greiner H de Meer and K S Trivedi Queueing Networks and Markov Chains John Wiley 2nd edition 2006 ISBN 978 0 7923 9650 5 External links editIntroduction to Markov Chains on YouTube Markov chain Encyclopedia of Mathematics EMS Press 2001 1994 Techniques to Understand Computer Simulations Markov Chain Analysis Markov Chains chapter in American Mathematical Society s introductory probability book Archived 2008 05 22 at the Wayback Machine pdf A beautiful visual explanation of Markov Chains Making Sense and Nonsense of Markov Chains Archived 2020 02 04 at the Wayback Machine Original paper by A A Markov 1913 An Example of Statistical Investigation of the Text Eugene Onegin Concerning the Connection of Samples in Chains translated from Russian Retrieved from https en wikipedia org w index php title Markov chain amp oldid 1222130447 Properties, wikipedia, wiki, book, books, library,

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