In mathematics, a partial order or total order < on a set is said to be dense if, for all and in for which , there is a in such that . That is, for any two elements, one less than the other, there is another element between them. For total orders this can be simplified to "for any two distinct elements, there is another element between them", since all elements of a total order are comparable.
For the element , due to the Archimedean property, if , there exists a largest integer with , and if , , and there exists a largest integer with . As a result, . For any two elements with , and . Therefore is dense.
On the other hand, the linear ordering on the integers is not dense.
Uniqueness for total dense orders without endpoints
None of them are necessary. For instance, there is a relation R that is not reflexive but dense. A non-empty and dense relation cannot be antitransitive.
A strict partial order < is a dense order if and only if < is a dense relation. A dense relation that is also transitive is said to be idempotent.
See also
Dense set — a subset of a topological space whose closure is the whole space
Dense-in-itself — a subset of a topological space such that does not contain an isolated point
Kripke semantics — a dense accessibility relation corresponds to the axiom
References
^Roitman, Judith (1990), "Theorem 27, p. 123", Introduction to Modern Set Theory, Pure and Applied Mathematics, vol. 8, John Wiley & Sons, ISBN9780471635192.
^Dasgupta, Abhijit (2013), Set Theory: With an Introduction to Real Point Sets, Springer-Verlag, p. 161, ISBN9781461488545.
dense, order, mathematics, partial, order, total, order, displaystyle, said, dense, displaystyle, displaystyle, displaystyle, which, displaystyle, there, displaystyle, displaystyle, such, that, displaystyle, that, elements, less, than, other, there, another, e. In mathematics a partial order or total order lt on a set X displaystyle X is said to be dense if for all x displaystyle x and y displaystyle y in X displaystyle X for which x lt y displaystyle x lt y there is a z displaystyle z in X displaystyle X such that x lt z lt y displaystyle x lt z lt y That is for any two elements one less than the other there is another element between them For total orders this can be simplified to for any two distinct elements there is another element between them since all elements of a total order are comparable Contents 1 Example 2 Uniqueness for total dense orders without endpoints 3 Generalizations 4 See also 5 References 6 Further readingExample EditThe rational numbers as a linearly ordered set are a densely ordered set in this sense as are the algebraic numbers the real numbers the dyadic rationals and the decimal fractions In fact every Archimedean ordered ring extension of the integers Z x displaystyle mathbb Z x is a densely ordered set Proof For the element x Z x displaystyle x in mathbb Z x due to the Archimedean property if x gt 0 displaystyle x gt 0 there exists a largest integer n lt x displaystyle n lt x with n lt x lt n 1 displaystyle n lt x lt n 1 and if x lt 0 displaystyle x lt 0 x gt 0 displaystyle x gt 0 and there exists a largest integer m n 1 lt x displaystyle m n 1 lt x with n 1 lt x lt n displaystyle n 1 lt x lt n As a result 0 lt x n lt 1 displaystyle 0 lt x n lt 1 For any two elements y z Z x displaystyle y z in mathbb Z x with z lt y displaystyle z lt y 0 lt x n y z lt y z displaystyle 0 lt x n y z lt y z and z lt x n y z z lt y displaystyle z lt x n y z z lt y Therefore Z x displaystyle mathbb Z x is dense On the other hand the linear ordering on the integers is not dense Uniqueness for total dense orders without endpoints EditMain article Cantor s isomorphism theorem Georg Cantor proved that every two non empty dense totally ordered countable sets without lower or upper bounds are order isomorphic 1 This makes the theory of dense linear orders without bounds an example of an w categorical theory where w is the smallest limit ordinal For example there exists an order isomorphism between the rational numbers and other densely ordered countable sets including the dyadic rationals and the algebraic numbers The proofs of these results use the back and forth method 2 Minkowski s question mark function can be used to determine the order isomorphisms between the quadratic algebraic numbers and the rational numbers and between the rationals and the dyadic rationals Generalizations EditAny binary relation R is said to be dense if for all R related x and y there is a z such that x and z and also z and y are R related Formally x y x R y z x R z z R y displaystyle forall x forall y xRy Rightarrow exists z xRz land zRy Alternatively in terms of composition of R with itself the dense condition may be expressed as R R R 3 Sufficient conditions for a binary relation R on a set X to be dense are R is reflexive R is coreflexive R is quasireflexive R is left or right Euclidean or R is symmetric and semi connex and X has at least 3 elements None of them are necessary For instance there is a relation R that is not reflexive but dense A non empty and dense relation cannot be antitransitive A strict partial order lt is a dense order if and only if lt is a dense relation A dense relation that is also transitive is said to be idempotent See also EditDense set a subset of a topological space whose closure is the whole space Dense in itself a subset A displaystyle A of a topological space such that A displaystyle A does not contain an isolated point Kripke semantics a dense accessibility relation corresponds to the axiom A A displaystyle Box Box A rightarrow Box A References Edit Roitman Judith 1990 Theorem 27 p 123 Introduction to Modern Set Theory Pure and Applied Mathematics vol 8 John Wiley amp Sons ISBN 9780471635192 Dasgupta Abhijit 2013 Set Theory With an Introduction to Real Point Sets Springer Verlag p 161 ISBN 9781461488545 Gunter Schmidt 2011 Relational Mathematics page 212 Cambridge University Press ISBN 978 0 521 76268 7Further reading EditDavid Harel Dexter Kozen Jerzy Tiuryn Dynamic logic MIT Press 2000 ISBN 0 262 08289 6 p 6ff Retrieved from https en wikipedia org w index php title Dense order amp oldid 1127631739, wikipedia, wiki, book, books, library,