The Stacks project

5.29 Colimits of spaces

The category of topological spaces has coproducts. Namely, if $I$ is a set and for $i \in I$ we are given a topological space $X_ i$ then we endow the set $\coprod _{i \in I} X_ i$ with the coproduct topology. As a basis for this topology we use sets of the form $U_ i$ where $U_ i \subset X_ i$ is open.

The category of topological spaces has coequalizers. Namely, if $a, b : X \to Y$ are morphisms of topological spaces, then the coequalizer of $a$ and $b$ is the coequalizer $Y/\sim $ in the category of sets endowed with the quotient topology (Section 5.6).

Lemma 5.29.1. The category of topological spaces has colimits and the forgetful functor to sets commutes with them.

Proof. This follows from the discussion above and Categories, Lemma 4.14.12. Another proof of existence of colimits is sketched in Categories, Remark 4.25.2. It follows from the above that the forgetful functor commutes with colimits. Another way to see this is to use Categories, Lemma 4.24.5 and use that the forgetful functor has a right adjoint, namely the functor which assigns to a set the corresponding chaotic (or indiscrete) topological space. $\square$