Lemma 5.15.10. Let $X$ be a topological space. Every constructible subset of $X$ is retrocompact.

**Proof.**
Let $E = \bigcup _{i = 1, \ldots , n} U_ i \cap V_ i^ c$ with $U_ i, V_ i$ retrocompact open in $X$. Let $W \subset X$ be quasi-compact open. Then $E \cap W = \bigcup _{i = 1, \ldots , n} U_ i \cap V_ i^ c \cap W$. Thus it suffices to show that $U \cap V^ c \cap W$ is quasi-compact if $U, V$ are retrocompact open and $W$ is quasi-compact open. This is true because $U \cap V^ c \cap W$ is a closed subset of the quasi-compact $U \cap W$ so Lemma 5.12.3 applies.
$\square$

## Post a comment

Your email address will not be published. Required fields are marked.

In your comment you can use Markdown and LaTeX style mathematics (enclose it like `$\pi$`

). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).

Unfortunately JavaScript is disabled in your browser, so the comment preview function will not work.

All contributions are licensed under the GNU Free Documentation License.

## Comments (0)