Cosheafification

Abstract

It is proved that for any Grothendieck site $X$, there exists a coreflection (called $\mathbf{cosheafification}$) from the category of precosheaves on $X$ with values in a category $\mathbf{K}$, to the full subcategory of cosheaves, provided either $\mathbf{K}$ or $\mathbf{K}^{op}$ is locally presentable. If $\mathbf{K}$ is cocomplete, such a coreflection is built explicitly for the (pre)cosheaves with values in the category $\mathbf{Pro}% \left( \mathbf{K}\right) $ of pro-objects in $\mathbf{K}$. In the case of precosheaves on topological spaces, it is proved that any precosheaf with values in $\mathbf{Pro}\left( \mathbf{K}\right) $ is $\mathbf{smooth}$, i.e. is strongly locally isomorphic to a cosheaf. Constant cosheaves are constructed, and there are established connections with shape theory.