Is a limit of profinite spaces profinite

category-theorygeneral-topologylimits-colimits

Here is the statement of Lemma 5.22.3 in the Stacks Project:

Lemma. A cofiltered limit of profinite spaces is profinite.

And here is the proof:

Proof. Let us use the characterization of profinite spaces in Lemma 5.22.2. By Lemma 5.14.1 the limit exists. By Theorem 5.14.4 the limit is quasi-compact. A cofiltered limit of totally disconnected spaces is totally disconnected (details omitted). A cofiltered limit of Hausdorff spaces is Hausdorff (details omitted). This finishes the proof.

If we remove the word "cofiltered" we get:

Lemma. A limit of profinite spaces is profinite.

And here is the proof:

Proof. Let us use the characterization of profinite spaces in Lemma 5.22.2. By Lemma 5.14.1 the limit exists. By Theorem 5.14.4 the limit is quasi-compact. A limit of totally disconnected spaces is totally disconnected (details omitted). A limit of Hausdorff spaces is Hausdorff (details omitted). This finishes the proof.

I don't see what's wrong with this second version.

What am I missing?

Best Answer

As Kevin Carlson explained in a comment, the answer is Yes.

More precisely, say that a space is compact if it is quasi-compact and Hausdorff. Then the Stacks Project text can be amended as follows:

Lemma. A limit of profinite spaces is profinite.

Proof. Let us use the characterization of profinite spaces in Lemma 5.22.2. By Lemma 5.14.1 the limit exists. By Theorem 5.14.4 the limit is compact. A limit of totally disconnected spaces is totally disconnected (details omitted). This finishes the proof.

The key point is that the equalizer of a double arrow $X\rightrightarrows Y$ is closed if $Y$ is Hausdorff (because it is the inverse image of the diagonal under the obvious map $X\to Y\times Y$, and the diagonal is closed since $Y$ is Hausdorff).

(I confess with shame that, when I wrote the question, I thought that equalizers were always closed. I'm sure I knew that this was not so when I leaned general topology, but I forgot it. I know it's silly...)