A small presheaf on a category $C$ is a presheaf which is determined by a small amount of data. If $C$ is itself small, then every presheaf on $C$ is small, but this is no longer true when $C$ is large. In many cases, when $C$ is large, it is the small presheaves which seem to be more important and useful.
Let $C$ be a category which is locally small, but possibly large. A presheaf $F\colon C^{op}\to Set$ is small if it is the left Kan extension of some functor whose domain is a small category, or equivalently if it is a small colimit of representable functors.
Of course, if $C$ is itself small, then every presheaf is small.
We write $P C$ for the category of small presheaves on $C$. Observe that although the category of all presheaves on $C$ cannot be defined without the assumption of a universe, the category $P C$ can be so defined, using small diagrams in $C$ as proxies for small colimits of representable presheaves. Moreover $P C$ is locally small, and there is a Yoneda embedding $C\hookrightarrow P C$.
Of course, if $C$ is small, then $P C$ is the usual category of all presheaves on $C$.
Since small colimits of small colimits are small colimits, $P C$ is cocomplete. In fact, it is easily seen to be the free cocompletion of $C$, even when $C$ is not small. It is not, in general, complete, but we can characterize when it is (cf. Day–Lack).
$P C$ is complete if and only if for every small diagram in $C$, the category of cones over that diagram has a small weakly terminal set, i.e. there is a small set of cones such that every cone factors through one in that set.
If $C$ is either complete or small, then $P C$ is complete.
We also have:
If $C$ and $D$ are complete, then a functor $F\colon C\to D$ preserves small limits if and only if the functor $P F\colon P C \to P D$ (induced by left Kan extension) also preserves small limits.
These results can all be generalized to enriched categories, and also relativized to limits in some class $\Phi$ (which, for some purposes, we might want to assume to be “saturated”). See the paper by Day and Lack.
B. J. Day, Steve Lack, Limits of small functors, JPAA, <http://www.maths.usyd.edu.au/u/stevel/papers/small.pdf>
Boris Chorny, William Dwyer, The homotopy theory of small diagrams over large categories, Forum Math. 21 (2009), no. 2, 167–179; preprint
The results of Chorny–Dwyer are cited by Rosicky in Accessible categories and homotopy theory, <http://www.math.yorku.ca/~tholen/HB07Rosicky.pdf>
Last revised on April 11, 2016 at 15:08:59. See the history of this page for a list of all contributions to it.