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Corollary 5.15.19030D740FC9C6

Corollary 5.15.19.030D740FC9C6 Let $\varphi ,\psi \in \mathrm{QPSH}(X)$. Then the set

\[ \{ x\in X: \varphi (x)>\psi (x)\} \]

is $\mathcal{F}$-open.

Proof. It suffices to show that for any $x\in X$ such that $\varphi (x)>\psi (x)$, the same holds on an $\mathcal{F}$-neighborhood $U$ of $x$. Observe that $\varphi (x)\neq -\infty $. If $\psi (x)\neq -\infty $, then it suffices to apply proposition 5.15.18. Otherwise, take

\[ U\coloneqq \{ y\in X: \varphi (y)>\varphi (x)-1\} \cap \{ y\in X: \psi (y)<\varphi (x)-1\} . \]

This is an $\mathcal{F}$-open neighborhood of $x$ contained in $\{ \varphi >\psi \} $. $\square$

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