[Math] Local compactness is preserved under continuous open onto mappings

compactnessgeneral-topologyopen-map

If $f$ is a continuous open mapping of a locally compact space $(X,\tau)$ onto a
topological space $(Y,\tau_1)$, show that $(Y,\tau_1)$ is locally compact.

The definition of locally compact is that all points have compact neighborhood. I know that without the "open mapping" condition, that statement is false, but I can't show how this condition make it different.

Best Answer

so take $y \in Y$ since $f$ is surjective there is $x \in X$ such that $f(x)=y$, so there is a compact neighborhood $U$ of $x$ , $f(U)$ is a compact subset of $Y$ and since $U$ is a neighborhood of $x$ so there is a open subset of $U$, $V$ such that $x \in V$, $f$ is a open mapping so $f(V)$ is open in $Y$ and $f(V) \subset f(U)$ such that $y \in f(V)$ so $f(U)$ is a compact neighborhood of $y$ as requested!

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