TheoremBase

Images and Preimages under a Bijection and Its Inverse

For a bijection f from X to Y with inverse g and a subset E of X, the image f[E] is the preimage of E under g, and the preimage of f[E] under f is E.

Statement

In the setting of Routine Facts Used Without Citation: Arithmetic, Maps, Predicativity and Indices, let XX and YY be sets, let f:X→Yf:X\to Y be a bijection, let g=f−1:Y→Xg=f^{-1}:Y\to X be its inverse, and let E⊆XE\subseteq X. Images and preimages are as in The Image and the Preimage of a Class under a Class (i) and The Image and the Preimage of a Class under a Class (ii).

The image under ff is the preimage under gg.

f[E]=g−1[E].f[E]=g^{-1}[E].

The preimage of the image is the set itself.

f−1[f[E]]=E.f^{-1}\bigl[f[E]\bigr]=E.

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