Certain constructions in set theory go beyond the
finite. For example in the construction of the cumulative hierarchy
we defined and .
But we also said we would collect together
previous levels ever so often at
limit stages
. The idea of ordinal is a generalisation of counting
number that includes infinite values or limits, in which we can make these
constructions precise.
The formal definition is precise but not very instructive.
Definition.
An ordinal is (possibly empty) set which is transitive and on which the membership relation defines a linear order:
(We don't need the irreflexivity axiom as this holds for all sets .)
The relation restricted to an ordinal is often written . It is a well-order, meaning that it is a linear order such that for every nonempty subset the set has a -least element. (This is an easy application of foundation together with transitivity. By foundation, there is with and if also then by transitivity of and hence . Thus is the -least element of .)
A better view of ordinals, but one that at this stage one that has to be regarded as rather informal, is that they represent the well-orders in some canonical way. Indeed an ordinal is well-ordered as we have just seen, no two distinct ordinals are isomorphic as linear orderd, and also (as it turns out) any well-order is order-isomorphic to some ordinal.
Proposition.
is an ordinal.
Proof.
Easy
We denote by .
Proposition.
If is an ordinal then so is .
Proof.
If then either so or so by transitivity of . Hence is transitive.
If and then by checking all the cases ( or not) we find as required for transitivity of or else from which follows as is an ordinal.
If then by checking all the cases ( or not) we find or or as required for transitivity of , or else from which one of the three possibilities follows as is an ordinal.
We write for . We write for . We also use the relation for when looking at elements of an ordinal.
Proposition.
If are ordinals then is an ordinal.
Proof.
Easy.
Proposition.
If where is an ordinal then is an ordinal.
Proof.
Transitivity of follows from being transitive on and being transitive. The other properties are easy.
Proposition.
If are ordinals then either or .
Proof.
Assume first that is a proper subset of . Then there is a least in . If then and so by minimality of . So . Conversely if then with both being elements of we must have , , or . The second of these gives contradicting the choice of , and the third gives hence , likewise. Therefore and thus and these are equal. In particular from we have .
Similarly if is a proper subset of we deduce that . Thus we cannot have is both proper subset of and of else contradicting foundation.
Proposition.
If are ordinals then either or or .
Proof.
Suppose but that these are not the same. The by the proof of the preceding result the least is itself, so .
Proposition.
If is a nonzero ordinal then .
Proof.
By the previous proposition, either or , and of course the first of these is absurd.
Proposition.
If is a set of ordinals then is an ordinal.
Proof.
Exercise.
Proposition.
If is an ordinal and then or .
Proof.
If then is an ordinal and or or . We just have to show the last cannot happen. But if then or , both of which contradict foundation.
Proposition.
Any ordinal is exactly one of: zero, 0; a successor ordinal, for some ordinal ; a limit ordinal, a non-zero ordinal such that for each we have .
Proof.
If is neither zero nor a successor ordinal it must be a limit, by the previous proposition. It is easy to check that the three alternatives are mutually exclusive.