## 5.27 Miscellany

The following lemma applies to the underlying topological space associated to a quasi-separated scheme.

Lemma 5.27.1. Let $X$ be a topological space which

has a basis of the topology consisting of quasi-compact opens, and

has the property that the intersection of any two quasi-compact opens is quasi-compact.

Then

$X$ is locally quasi-compact,

a quasi-compact open $U \subset X$ is retrocompact,

any quasi-compact open $U \subset X$ has a cofinal system of open coverings $\mathcal{U} : U = \bigcup _{j\in J} U_ j$ with $J$ finite and all $U_ j$ and $U_ j \cap U_{j'}$ quasi-compact,

add more here.

**Proof.**
Omitted.
$\square$

Definition 5.27.2. Let $X$ be a topological space. We say $x \in X$ is an *isolated point* of $X$ if $\{ x\} $ is open in $X$.

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