
 Ordinal number  LearnThis.Info Enclyclopedia (Site not responding. Last check: 20071016) 
  For instance, 2 is an element of 4 = {0, 1, 2, 3}, and 2 is equal to {0, 1} and so it is a subset of {0, 1, 2, 3}. 
  Whenever you have two ordinals S and T, S is an element of T if and only if S is a subset of T, and moreover, either S is an element of T, or T is an element of S, or they are equal. 
  An ordinal is finite if and only if the opposite order is also wellordered, which is the case if and only if each of its subsets has a greatest element. 
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