Transitive binary relations  

 
indicates that the column's property is always true the row's term (at the very left), while ✗ indicates that the property is not guaranteed in general (it might, or might not, hold). For example, that every equivalence relation is symmetric, but not necessarily antisymmetric, is indicated by in the "Symmetric" column and ✗ in the "Antisymmetric" column, respectively. All definitions tacitly require the homogeneous relation be transitive: for all if and then 
In mathematics, a wellorder (or wellordering or wellorder relation) on a set S is a total ordering on S with the property that every nonempty subset of S has a least element in this ordering. The set S together with the ordering is then called a wellordered 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 nonempty wellordered set has a least element. Every element s of a wellordered 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, that have no predecessor (see § Natural numbers below for an example). A wellordered 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 nonstrict well ordering, then < is a strict well ordering. A relation is a strict well ordering if and only if it is a wellfounded strict total order. The distinction between strict and nonstrict well orders is often ignored since they are easily interconvertible.
Every wellordered set is uniquely order isomorphic to a unique ordinal number, called the order type of the wellordered set. The wellordering 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 wellfounded 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 lessthan relation is commonly called the wellordering principle (for natural numbers).
Every wellordered set is uniquely order isomorphic to a unique ordinal number, called the order type of the wellordered set. The position of each element within the ordered set is also given by an ordinal number. In the case of a finite set, the basic operation of counting, to find the ordinal number of a particular object, or to find the object with a particular ordinal number, corresponds to assigning ordinal numbers one by one to the objects. The size (number of elements, cardinal number) of a finite set is equal to the order type.^{[1]} Counting in the everyday sense typically starts from one, so it assigns to each object the size of the initial segment with that object as last element. Note that these numbers are one more than the formal ordinal numbers according to the isomorphic order, because these are equal to the number of earlier objects (which corresponds to counting from zero). Thus for finite n, the expression "nth element" of a wellordered set requires context to know whether this counts from zero or one. In a notation "βth element" where β can also be an infinite ordinal, it will typically count from zero.
For an infinite set the order type determines the cardinality, but not conversely: wellordered sets of a particular cardinality can have many different order types (see § Natural numbers, below, for an example). For a countably infinite set, the set of possible order types is uncountable.
The standard ordering ≤ of the natural numbers is a well ordering and has the additional property that every nonzero natural number has a unique predecessor.
Another well ordering of the natural numbers is given by defining that all even numbers are less than all odd numbers, and the usual ordering applies within the evens and the odds:
This is a wellordered set of order type ω + ω. Every element has a successor (there is no largest element). Two elements lack a predecessor: 0 and 1.
Unlike the standard ordering ≤ of the natural numbers, the standard ordering ≤ of the integers is not a well ordering, since, for example, the set of negative integers does not contain a least element.
The following binary relation R is an example of well ordering of the integers: x R y if and only if one of the following conditions holds:
This relation R can be visualized as follows:
R is isomorphic to the ordinal number ω + ω.
Another relation for well ordering the integers is the following definition: if and only if
This well order can be visualized as follows:
This has the order type ω.
The standard ordering ≤ of any real interval is not a well ordering, since, for example, the open interval does not contain a least element. From the ZFC axioms of set theory (including the axiom of choice) one can show that there is a well order of the reals. Also Wacław Sierpiński proved that ZF + GCH (the generalized continuum hypothesis) imply the axiom of choice and hence a well order of the reals. Nonetheless, it is possible to show that the ZFC+GCH axioms alone are not sufficient to prove the existence of a definable (by a formula) well order of the reals.^{[2]} However it is consistent with ZFC that a definable well ordering of the reals exists—for example, it is consistent with ZFC that V=L, and it follows from ZFC+V=L that a particular formula well orders the reals, or indeed any set.
An uncountable subset of the real numbers with the standard ordering ≤ cannot be a well order: Suppose X is a subset of well ordered by ≤. For each x in X, let s(x) be the successor of x in ≤ ordering on X (unless x is the last element of X). Let whose elements are nonempty and disjoint intervals. Each such interval contains at least one rational number, so there is an injective function from A to There is an injection from X to A (except possibly for a last element of X, which could be mapped to zero later). And it is well known that there is an injection from to the natural numbers (which could be chosen to avoid hitting zero). Thus there is an injection from X to the natural numbers, which means that X is countable. On the other hand, a countably infinite subset of the reals may or may not be a well order with the standard ≤. For example,
Examples of well orders:
If a set is totally ordered, then the following are equivalent to each other:
Every wellordered set can be made into a topological space by endowing it with the order topology.
With respect to this topology there can be two kinds of elements:
For subsets we can distinguish:
A subset is cofinal in the whole set if and only if it is unbounded in the whole set or it has a maximum that is also maximum of the whole set.
A wellordered set as topological space is a firstcountable space if and only if it has order type less than or equal to ω_{1} (omegaone), that is, if and only if the set is countable or has the smallest uncountable order type.
Key concepts  

Results  
Properties & Types (list) 

Constructions  
Topology & Orders  
Related 