Local presentability and representable presheaves over the category of topological spaces

2010-05-10T16:38:58Z

Is the category of topological spaces locally presentable? n-lab claims that it is not locally FINITELY presentable, but how about for some larger cardinal? Here I really mean the 1-category of topological spaces and am not willing to identify it with simplicial sets. Essentially, I want to know if (after I fix appropriate Grothendieck universes) representable presheaves on Top are characterized by those presheaves which send colimits in Top to limits in Set, which would follow from local presentablility.

Answer by Daniel Schäppi for Local presentability and representable presheaves over the category of topological spaces

2010-05-11T01:01:26Z

The category of topological spaces is not locally $\lambda$-presentable for any $\lambda$. The reason for this is the existence of spaces which aren't $\lambda$-presentable (a.k.a. $\lambda$-small) for any $\lambda$ (in a locally presentable category every object is $\lambda$-presentable for some $\lambda$). An example of such a space is the Sierpinski space; a proof of this can be found in Mark Hovey's book on model categories, on page 49.

There is a convenient category of topological spaces which is locally presentable, the category of $\Delta$-generated spaces. This category contains most of the spaces usually studied by algebraic topologists (for example, the geometric realization of any simplicial set is a $\Delta$-generated space). Daniel Dugger has some expository notes on this here. A proof that the category of $\Delta$-generated spaces is locally presentable can be found this paper of L. Fajstrup and J. Rosický.

The second question was already answered in the comments: if $G\colon \mathbf{Top}^{\mathrm{op}} \rightarrow \mathbf{Set}$ is continuous, then it has a left adjoint $F$ by the special adjoint functor theorem. Therefore we have natural isomorphisms

$G(X) \cong \mathbf{Set}(\ast,GX) \cong \mathbf{Top}^{\mathrm{op}}(F(\ast),X)=\mathbf{Top}(X,F(\ast))$,

which shows that $G$ is represented by $F(\ast)$.

Edit: added the missing op's mentioned in the comment.

Local presentability and representable presheaves over the category of topological spaces - MathOverflow

Local presentability and representable presheaves over the category of topological spaces

Answer by Daniel Schäppi for Local presentability and representable presheaves over the category of topological spaces