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Empty product

In mathematics, an empty product, or nullary product or vacuous product, is the result of multiplying no factors. It is by convention equal to the multiplicative identity (assuming there is an identity for the multiplication operation in question), just as the empty sum—the result of adding no numbers—is by convention zero, or the additive identity.[1][2][3][4] When numbers are implied, the empty product becomes one.

The term empty product is most often used in the above sense when discussing arithmetic operations. However, the term is sometimes employed when discussing set-theoretic intersections, categorical products, and products in computer programming; these are discussed below.

Nullary arithmetic product

Definition

Let a1, a2, a3, ... be a sequence of numbers, and let

 

be the product of the first m elements of the sequence. Then

 

for all m = 1, 2, ... provided that we use the convention  . In other words, a "product" with no factors at all evaluates to 1. Allowing a "product" with zero factors reduces the number of cases to be considered in many mathematical formulas. Such a "product" is a natural starting point in induction proofs, as well as in algorithms. For these reasons, the "empty product is one" convention is common practice in mathematics and computer programming.

Relevance of defining empty products

The notion of an empty product is useful for the same reason that the number zero and the empty set are useful: while they seem to represent quite uninteresting notions, their existence allows for a much shorter mathematical presentation of many subjects.

For example, the empty products 0! = 1 (the factorial of zero) and x0 = 1 shorten Taylor series notation (see zero to the power of zero for a discussion of when x = 0). Likewise, if M is an n × n matrix, then M0 is the n × n identity matrix, reflecting the fact that applying a linear map zero times has the same effect as applying the identity map.

As another example, the fundamental theorem of arithmetic says that every positive integer greater than 1 can be written uniquely as a product of primes. However, if we do not allow products with only 0 or 1 factors, then the theorem (and its proof) become longer.[5][6]

More examples of the use of the empty product in mathematics may be found in the binomial theorem (which assumes and implies that x0 = 1 for all x), Stirling number, König's theorem, binomial type, binomial series, difference operator and Pochhammer symbol.

Logarithms and exponentials

Since logarithms map products to sums:

 

they map an empty product to an empty sum.

Conversely, the exponential function maps sums into products:

 

and maps an empty sum to an empty product.

Nullary Cartesian product

Consider the general definition of the Cartesian product:

 

If I is empty, the only such g is the empty function  , which is the unique subset of   that is a function  , namely the empty subset   (the only subset that   has):

 

Thus, the cardinality of the Cartesian product of no sets is 1.

Under the perhaps more familiar n-tuple interpretation,

 

that is, the singleton set containing the empty tuple. Note that in both representations the empty product has cardinality 1 – the number of all ways to produce 0 outputs from 0 inputs is 1.

Nullary categorical product

In any category, the product of an empty family is a terminal object of that category. This can be demonstrated by using the limit definition of the product. An n-fold categorical product can be defined as the limit with respect to a diagram given by the discrete category with n objects. An empty product is then given by the limit with respect to the empty category, which is the terminal object of the category if it exists. This definition specializes to give results as above. For example, in the category of sets the categorical product is the usual Cartesian product, and the terminal object is a singleton set. In the category of groups the categorical product is the Cartesian product of groups, and the terminal object is a trivial group with one element. To obtain the usual arithmetic definition of the empty product we must take the decategorification of the empty product in the category of finite sets.

Dually, the coproduct of an empty family is an initial object. Nullary categorical products or coproducts may not exist in a given category; e.g. in the category of fields, neither exists.

In logic

Classical logic defines the operation of conjunction, which is generalized to universal quantification in predicate calculus, and is widely known as logical multiplication because we intuitively identify true with 1 and false with 0 and our conjunction behaves as ordinary multiplier. Multipliers can have arbitrary number of inputs. In case of 0 inputs, we have empty conjunction, which is identically equal to true.

This is related to another concept in logic, vacuous truth, which tells us that empty set of objects can have any property. It can be explained the way that the conjunction (as part of logic in general) deals with values less or equal 1. This means that the longer the conjunction, the higher the probability of ending up with 0. Conjunction merely checks the propositions and returns 0 (or false) as soon as one of propositions evaluates to false. Reducing the number of conjoined propositions increases the chance to pass the check and stay with 1. Particularly, if there are 0 tests or members to check, none can fail, so by default we must always succeed regardless of which propositions or member properties were to be tested.

In computer programming

Many programming languages, such as Python, allow the direct expression of lists of numbers, and even functions that allow an arbitrary number of parameters. If such a language has a function that returns the product of all the numbers in a list, it usually works like this:

>>> math.prod([2, 3, 5]) 30 >>> math.prod([2, 3]) 6 >>> math.prod([2]) 2 >>> math.prod([]) 1 

(Please note: prod is not available in the math module prior to version 3.8.)

This convention helps avoid having to code special cases like "if length of list is 1" or "if length of list is zero" as special cases.

Multiplication is an infix operator and therefore a binary operator, complicating the notation of an empty product. Some programming languages handle this by implementing variadic functions. For example, the fully parenthesized prefix notation of Lisp languages gives rise to a natural notation for nullary functions:

(* 2 2 2)  ; evaluates to 8 (* 2 2)  ; evaluates to 4 (* 2)  ; evaluates to 2 (*)  ; evaluates to 1 

See also

References

  1. ^ Jaroslav Nešetřil, Jiří Matoušek (1998). Invitation to Discrete Mathematics. Oxford University Press. p. 12. ISBN 0-19-850207-9.
  2. ^ A.E. Ingham and R C Vaughan (1990). The Distribution of Prime Numbers. Cambridge University Press. p. 1. ISBN 0-521-39789-8.
  3. ^ Lang, Serge (2002), Algebra, Graduate Texts in Mathematics, vol. 211 (Revised third ed.), New York: Springer-Verlag, p. 9, ISBN 978-0-387-95385-4, MR 1878556, Zbl 0984.00001
  4. ^ David M. Bloom (1979). Linear Algebra and Geometry. pp. 45. ISBN 0521293243.
  5. ^ Edsger Wybe Dijkstra (1990-03-04). "How Computing Science created a new mathematical style". EWD. Retrieved 2010-01-20. Hardy and Wright: 'Every positive integer, except 1, is a product of primes', Harold M. Stark: 'If n is an integer greater than 1, then either n is prime or n is a finite product of primes'. These examples — which I owe to A. J. M. van Gasteren — both reject the empty product, the last one also rejects the product with a single factor.
  6. ^ Edsger Wybe Dijkstra (1986-11-14). "The nature of my research and why I do it". EWD. Archived from the original on 2012-07-15. Retrieved 2010-07-03. But also 0 is certainly finite and by defining the product of 0 factors — how else? — to be equal to 1 we can do away with the exception: 'If n is a positive integer, then n is a finite product of primes.'

External links

    empty, product, empty, product, that, equals, zero, zero, product, property, mathematics, empty, product, nullary, product, vacuous, product, result, multiplying, factors, convention, equal, multiplicative, identity, assuming, there, identity, multiplication, . For the non empty product that equals to zero see zero product property In mathematics an empty product or nullary product or vacuous product is the result of multiplying no factors It is by convention equal to the multiplicative identity assuming there is an identity for the multiplication operation in question just as the empty sum the result of adding no numbers is by convention zero or the additive identity 1 2 3 4 When numbers are implied the empty product becomes one The term empty product is most often used in the above sense when discussing arithmetic operations However the term is sometimes employed when discussing set theoretic intersections categorical products and products in computer programming these are discussed below Contents 1 Nullary arithmetic product 1 1 Definition 1 2 Relevance of defining empty products 1 3 Logarithms and exponentials 2 Nullary Cartesian product 3 Nullary categorical product 4 In logic 5 In computer programming 6 See also 7 References 8 External linksNullary arithmetic product EditDefinition Edit Let a1 a2 a3 be a sequence of numbers and let P m i 1 m a i a 1 a m displaystyle P m prod i 1 m a i a 1 cdots a m be the product of the first m elements of the sequence Then P m P m 1 a m displaystyle P m P m 1 a m for all m 1 2 provided that we use the convention P 0 1 displaystyle P 0 1 In other words a product with no factors at all evaluates to 1 Allowing a product with zero factors reduces the number of cases to be considered in many mathematical formulas Such a product is a natural starting point in induction proofs as well as in algorithms For these reasons the empty product is one convention is common practice in mathematics and computer programming Relevance of defining empty products Edit The notion of an empty product is useful for the same reason that the number zero and the empty set are useful while they seem to represent quite uninteresting notions their existence allows for a much shorter mathematical presentation of many subjects For example the empty products 0 1 the factorial of zero and x0 1 shorten Taylor series notation see zero to the power of zero for a discussion of when x 0 Likewise if M is an n n matrix then M0 is the n n identity matrix reflecting the fact that applying a linear map zero times has the same effect as applying the identity map As another example the fundamental theorem of arithmetic says that every positive integer greater than 1 can be written uniquely as a product of primes However if we do not allow products with only 0 or 1 factors then the theorem and its proof become longer 5 6 More examples of the use of the empty product in mathematics may be found in the binomial theorem which assumes and implies that x0 1 for all x Stirling number Konig s theorem binomial type binomial series difference operator and Pochhammer symbol Logarithms and exponentials Edit Since logarithms map products to sums ln i x i i ln x i displaystyle ln prod i x i sum i ln x i they map an empty product to an empty sum Conversely the exponential function maps sums into products e i x i i e x i displaystyle e sum i x i prod i e x i and maps an empty sum to an empty product Nullary Cartesian product EditConsider the general definition of the Cartesian product i I X i g I i I X i i g i X i displaystyle prod i in I X i left g I to bigcup i in I X i mid forall i g i in X i right If I is empty the only such g is the empty function f displaystyle f varnothing which is the unique subset of displaystyle varnothing times varnothing that is a function displaystyle varnothing to varnothing namely the empty subset displaystyle varnothing the only subset that displaystyle varnothing times varnothing varnothing has f displaystyle prod varnothing left f varnothing varnothing to varnothing right varnothing Thus the cardinality of the Cartesian product of no sets is 1 Under the perhaps more familiar n tuple interpretation displaystyle prod varnothing that is the singleton set containing the empty tuple Note that in both representations the empty product has cardinality 1 the number of all ways to produce 0 outputs from 0 inputs is 1 Nullary categorical product EditIn any category the product of an empty family is a terminal object of that category This can be demonstrated by using the limit definition of the product An n fold categorical product can be defined as the limit with respect to a diagram given by the discrete category with n objects An empty product is then given by the limit with respect to the empty category which is the terminal object of the category if it exists This definition specializes to give results as above For example in the category of sets the categorical product is the usual Cartesian product and the terminal object is a singleton set In the category of groups the categorical product is the Cartesian product of groups and the terminal object is a trivial group with one element To obtain the usual arithmetic definition of the empty product we must take the decategorification of the empty product in the category of finite sets Dually the coproduct of an empty family is an initial object Nullary categorical products or coproducts may not exist in a given category e g in the category of fields neither exists In logic EditClassical logic defines the operation of conjunction which is generalized to universal quantification in predicate calculus and is widely known as logical multiplication because we intuitively identify true with 1 and false with 0 and our conjunction behaves as ordinary multiplier Multipliers can have arbitrary number of inputs In case of 0 inputs we have empty conjunction which is identically equal to true This is related to another concept in logic vacuous truth which tells us that empty set of objects can have any property It can be explained the way that the conjunction as part of logic in general deals with values less or equal 1 This means that the longer the conjunction the higher the probability of ending up with 0 Conjunction merely checks the propositions and returns 0 or false as soon as one of propositions evaluates to false Reducing the number of conjoined propositions increases the chance to pass the check and stay with 1 Particularly if there are 0 tests or members to check none can fail so by default we must always succeed regardless of which propositions or member properties were to be tested In computer programming EditMany programming languages such as Python allow the direct expression of lists of numbers and even functions that allow an arbitrary number of parameters If such a language has a function that returns the product of all the numbers in a list it usually works like this gt gt gt math prod 2 3 5 30 gt gt gt math prod 2 3 6 gt gt gt math prod 2 2 gt gt gt math prod 1 Please note prod is not available in the math module prior to version 3 8 This convention helps avoid having to code special cases like if length of list is 1 or if length of list is zero as special cases Multiplication is an infix operator and therefore a binary operator complicating the notation of an empty product Some programming languages handle this by implementing variadic functions For example the fully parenthesized prefix notation of Lisp languages gives rise to a natural notation for nullary functions 2 2 2 evaluates to 8 2 2 evaluates to 4 2 evaluates to 2 evaluates to 1See also EditIterated binary operation Empty functionReferences Edit Jaroslav Nesetril Jiri Matousek 1998 Invitation to Discrete Mathematics Oxford University Press p 12 ISBN 0 19 850207 9 A E Ingham and R C Vaughan 1990 The Distribution of Prime Numbers Cambridge University Press p 1 ISBN 0 521 39789 8 Lang Serge 2002 Algebra Graduate Texts in Mathematics vol 211 Revised third ed New York Springer Verlag p 9 ISBN 978 0 387 95385 4 MR 1878556 Zbl 0984 00001 David M Bloom 1979 Linear Algebra and Geometry pp 45 ISBN 0521293243 Edsger Wybe Dijkstra 1990 03 04 How Computing Science created a new mathematical style EWD Retrieved 2010 01 20 Hardy and Wright Every positive integer except 1 is a product of primes Harold M Stark If n is an integer greater than 1 then either n is prime or n is a finite product of primes These examples which I owe to A J M van Gasteren both reject the empty product the last one also rejects the product with a single factor Edsger Wybe Dijkstra 1986 11 14 The nature of my research and why I do it EWD Archived from the original on 2012 07 15 Retrieved 2010 07 03 But also 0 is certainly finite and by defining the product of 0 factors how else to be equal to 1 we can do away with the exception If n is a positive integer then n is a finite product of primes External links EditPlanetMath article on the empty product Retrieved from https en wikipedia org w index php title Empty product amp oldid 1121785825, wikipedia, wiki, book, books, library,

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