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Membership in Inverse Relations, Preimages and Images of Classes, and Enlarging the Codomain of a Map

Membership in the inverse of a class relation and in the preimage of a class, the preimage and the image under a map between classes, and enlarging the codomain of a map.

Statement

In the setting of Class Theory NBG: the Axioms, Standing Conventions and Basic Notation, let RR, CC, FF, AA, BB and DD be classes, not necessarily sets. R−1R^{-1} is the inverse of RR, F[A]F[A] is the image of AA under FF, and R−1[C]R^{-1}[C] is the preimage of CC under RR.

For all sets xx and yy: (x,y)∈R−1(x,y)\in R^{-1} if and only if (y,x)∈R(y,x)\in R.

For every set xx: x∈R−1[C]x\in R^{-1}[C] if and only if there is a set ww with w∈Cw\in C and (x,w)∈R(x,w)\in R.

If F:A→BF:A\to B is a map, then for every set xx: x∈F−1[C]x\in F^{-1}[C] if and only if x∈Ax\in A and F(x)∈CF(x)\in C, where F(x)F(x) is the value of FF at xx.

If F:A→BF:A\to B is a map, then F[A]⊆BF[A]\subseteq B.

If F:A→BF:A\to B is a map and F(u)∈DF(u)\in D for every u∈Au\in A, then F:A→DF:A\to D is a map.

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