TheoremBase

Restriction of a Continuous Map, and Continuous Images of Compact Subsets

Statement

Let (X,TX)(X,\mathcal{T}_{X}) and (Y,TY)(Y,\mathcal{T}_{Y}) be topological spaces, let f:X→Yf:X\to Y be a continuous map, and let A⊆XA\subseteq X be equipped with the subspace topology TA\mathcal{T}_{A}. Let f∣A:A→Yf|_{A}:A\to Y denote the map with f∣A(x)=f(x)f|_{A}(x)=f(x) for every x∈Ax\in A, and write f(A)={f(x):x∈A}f(A)=\{f(x):x\in A\}. Then the following hold.

1. (Restriction) The map f∣Af|_{A} is continuous from (A,TA)(A,\mathcal{T}_{A}) to (Y,TY)(Y,\mathcal{T}_{Y}).

2. (Image of a compact subset) If AA is compact in XX, then f(A)f(A) is compact in YY.

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