
The following mapping theorem of Banach [I] is well known. It is the basis of most proofs of the Schroder-Bernstein equivalence theorem. If X and Y are sets and f: X-* Y and g: Y-*X are injective mappings, then there exists partitions2 X=X1+X2 and Y= Yi+ Y2 such that f (Xj) = Yi and g(Y2) =X2. The conclusion of this theorem can be rephrased in the following way. Let AC XX Y be the relation between X and Y defined by
combinatorics
combinatorics
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