In all these cases, all inductive numbers occur in the top row, and only some in the bottom row.
Cases of this sort, where the converse domain is a "proper part" of the domain (i.e. a part not the whole), will occupy us again when we come to deal with infinity. For the present, we wish only to note that they exist and demand consideration.
Another class of correlations which are often important is the class called "permutations," where the domain and converse domain are identical. Consider, for example, the six possible arrangements of three letters:
Each of these can be obtained from any one of the others by means of a correlation. Take, for example, the first and last,
and
. Here