well-ordered set - meaning and definition. What is well-ordered set
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What (who) is well-ordered set - definition

TOTAL ORDER SUCH THAT EVERY NONEMPTY SUBSET OF THE DOMAIN HAS A LEAST ELEMENT
Well-ordered set; Well-ordered; Well-ordering; Well ordered; Well ordering; Well-ordering property; Wellorder; Wellordering; Well ordered set; Wellordered; Well ordering theory; Well ordering property; Well-Ordering; Well-Ordered; Well-orderable set; Well order

well-ordered set         
<mathematics> A set with a total ordering and no infinite descending chains. A total ordering "<=" satisfies x <= x x <= y <= z => x <= z x <= y <= x => x = y for all x, y: x <= y or y <= x In addition, if a set W is well-ordered then all non-empty subsets A of W have a least element, i.e. there exists x in A such that for all y in A, x <= y. Ordinals are isomorphism classes of well-ordered sets, just as integers are isomorphism classes of finite sets. (1995-04-19)
partially ordered set         
  • '''Fig. 3''' Graph of the divisibility of numbers from 1 to 4. This set is partially, but not totally, ordered because there is a relationship from 1 to every other number, but there is no relationship from 2 to 3 or 3 to 4
  • least}} element.
  • '''Fig.6''' Nonnegative integers, ordered by divisibility
  • '''Fig.2''' [[Commutative diagram]] about the connections between strict/non-strict relations and their duals, via the operations of reflexive closure (''cls''), irreflexive kernel (''ker''), and converse relation (''cnv''). Each relation is depicted by its [[logical matrix]] for the poset whose [[Hasse diagram]] is depicted in the center. For example <math>3 \not\leq 4</math> so row 3, column 4 of the bottom left matrix is empty.
SET ORDERED BY A TRANSITIVE, ANTISYMMETRIC, AND REFLEXIVE BINARY RELATION
PartialOrderedSet; PartialOrder; Partial order; Poset; Partial ordering relation; Partial ordering; Partially ordered; Strict order; Partially ordered sets; Ordered n-tuple; Strict partial ordering; Strict partial order; Poset category; Ordered collection; Non-strict order; Ordered set; Strict ordering; Interval (partial order); Ordinal sum; Partial Order; Partially-ordered set
A set with a partial ordering.
partial ordering         
  • '''Fig. 3''' Graph of the divisibility of numbers from 1 to 4. This set is partially, but not totally, ordered because there is a relationship from 1 to every other number, but there is no relationship from 2 to 3 or 3 to 4
  • least}} element.
  • '''Fig.6''' Nonnegative integers, ordered by divisibility
  • '''Fig.2''' [[Commutative diagram]] about the connections between strict/non-strict relations and their duals, via the operations of reflexive closure (''cls''), irreflexive kernel (''ker''), and converse relation (''cnv''). Each relation is depicted by its [[logical matrix]] for the poset whose [[Hasse diagram]] is depicted in the center. For example <math>3 \not\leq 4</math> so row 3, column 4 of the bottom left matrix is empty.
SET ORDERED BY A TRANSITIVE, ANTISYMMETRIC, AND REFLEXIVE BINARY RELATION
PartialOrderedSet; PartialOrder; Partial order; Poset; Partial ordering relation; Partial ordering; Partially ordered; Strict order; Partially ordered sets; Ordered n-tuple; Strict partial ordering; Strict partial order; Poset category; Ordered collection; Non-strict order; Ordered set; Strict ordering; Interval (partial order); Ordinal sum; Partial Order; Partially-ordered set
A relation R is a partial ordering if it is a pre-order (i.e. it is reflexive (x R x) and transitive (x R y R z => x R z)) and it is also antisymmetric (x R y R x => x = y). The ordering is partial, rather than total, because there may exist elements x and y for which neither x R y nor y R x. In domain theory, if D is a set of values including the undefined value (bottom) then we can define a partial ordering relation <= on D by x <= y if x = bottom or x = y. The constructed set D x D contains the very undefined element, (bottom, bottom) and the not so undefined elements, (x, bottom) and (bottom, x). The partial ordering on D x D is then (x1,y1) <= (x2,y2) if x1 <= x2 and y1 <= y2. The partial ordering on D -> D is defined by f <= g if f(x) <= g(x) for all x in D. (No f x is more defined than g x.) A lattice is a partial ordering where all finite subsets have a least upper bound and a greatest lower bound. ("<=" is written in LaTeX as sqsubseteq). (1995-02-03)

Wikipedia

Well-order

In mathematics, a well-order (or well-ordering or well-order relation) on a set S is a total order on S with the property that every non-empty subset of S has a least element in this ordering. The set S together with the well-order relation is then called a well-ordered set. In some academic articles and textbooks these terms are instead written as wellorder, wellordered, and wellordering or well order, well ordered, and well ordering.

Every non-empty well-ordered set has a least element. Every element s of a well-ordered set, except a possible greatest element, has a unique successor (next element), namely the least element of the subset of all elements greater than s. There may be elements besides the least element which have no predecessor (see § Natural numbers below for an example). A well-ordered set S contains for every subset T with an upper bound a least upper bound, namely the least element of the subset of all upper bounds of T in S.

If ≤ is a non-strict well ordering, then < is a strict well ordering. A relation is a strict well ordering if and only if it is a well-founded strict total order. The distinction between strict and non-strict well orders is often ignored since they are easily interconvertible.

Every well-ordered set is uniquely order isomorphic to a unique ordinal number, called the order type of the well-ordered set. The well-ordering theorem, which is equivalent to the axiom of choice, states that every set can be well ordered. If a set is well ordered (or even if it merely admits a well-founded relation), the proof technique of transfinite induction can be used to prove that a given statement is true for all elements of the set.

The observation that the natural numbers are well ordered by the usual less-than relation is commonly called the well-ordering principle (for natural numbers).