Sheaves

We define sheaves on a topological space, with values in an arbitrary category with products.

The sheaf condition for a F : presheaf C X requires that the morphism F.obj U ⟶ ∏ F.obj (U i) (where U is some open set which is the union of the U i) is the equalizer of the two morphisms ∏ F.obj (U i) ⟶ ∏ F.obj (U i ⊓ U j).

We provide the instance category (sheaf C X) as the full subcategory of presheaves, and the fully faithful functor sheaf.forget : sheaf C X ⥤ presheaf C X.

Equivalent conditions

While the "official" definition is in terms of an equalizer diagram, in src/topology/sheaves/sheaf_condition/pairwise_intersections.lean and in src/topology/sheaves/sheaf_condition/open_le_cover.lean we provide two equivalent conditions (and prove they are equivalent).

The first is that F.obj U is the limit point of the diagram consisting of all the F.obj (U i) and F.obj (U i ⊓ U j). (That is, we explode the equalizer of two products out into its component pieces.)

The second is that F.obj U is the limit point of the diagram constisting of all the F.obj V, for those V : opens X such that V ≤ U i for some i. (This condition is particularly easy to state, and perhaps should become the "official" definition.)