Since I cannot concentrate anyway, here comes the third installment of my reading Merkulov. We now get to the funky stuff – introducing sheaves and getting to what should be at the very start of basically any modern geometry course. At least if I am to believe the geometers I know.
So, let’s launch straight to it. A presheaf on the topological space is just a contravariant functor from to , where is the category of open subsets of with morphisms being inclusion maps.
So that’s the one-line definition. But what does it mean?
Well, a functor is a map between categories that takes objects to objects and morphisms to morphisms. So we have that is an abelian group for any open set . For such a map to really be a functor, it has to be sane in a rather precisely defined sense: namely morphism composition should still be associative and the identity endomorphism on a group shouldn’t actually, ya’know, change the morphisms before or after it.
For the functor to be contravariant means precisely that for we get – all arrows reverse by application of the functor.
And for our definition of presheaves? We can read out that for every inclusion of open subsets of our space we get a group homomorphism . Functoriality requires these homomorphisms to be sane – i.e. and whenever .
We will most often, as soon as it is clear what sheaf we work with, stick to denoting the with and call it the restriction of to .
Our first example will be the constant presheaf: for a topological space and an abelian group, we define if is non-empty, and . The restriction homomorphisms are the identity whenever the subset is nonempty and the zero homomorphism to the empty set.
Completely analogously, presheaves of sets, graded vector spaces, algebras, modules over an algebra et.c. can be defined. Throughout, the presheaves are just contravariant functors from to the relevant category.
Presheaves over presheaves
Suppose is a presheaf of rings with restrictions and is a presheaf of abelian groups with restrictions on some topological space such that whenever we know that is a -module and for any we also know that for and so that the restrictions agree with the module structure. Then we call a presheaf of modules over a presheaf of rings . By adding structure to the modules in we can define presheaves of algebras or Lie algebras et.c. over a fixed presheaf of (graded) commutative rings .
Let be a graded -algebra for a field . A derivation of of degree is an element satisfying the condition
Let be the vector space of all derivations of degree and let
Now, has a natural -module structure by where for . Furthermore, there is a natural structure of graded Lie algebra to with brackets given as .
Now, if is a presheaf of algebras, then the associated collection is naturally a presheaf of -modules. It is also a presheaf of Lie algebras on the space.
For an open subset of a fixed topological space let be the vector space of all complex valued continuous functions on . For every pair of open subsets let be the usual restriction of a continuous function to , so for . Then is a presheaf of continuous functions on and is often denoted by .
If is a smooth manifold we can take smooth functions everywhere instead, we get a presheaf of smooth functions on .
If is a complex manifold we can take holomorphic functions everywhere to get a presheaf of holomorphic functions on .
A presheaf on a topological manifold is called a sheaf if, for every open set and every family of open subsets with the following conditions are satisfied:
If for all , and we have
for every family of elements with the property that there is some such that for all .
So a sheaf is a presheaf such that if for a covering of an open set equality on all covering sets implies equality in the covered set and where if a family seems to have come from an object higher up, then that object really does exist.
Note that the constant presheaf above is only a sheaf if either the abelian group is trivial or the space contains no non-intersecting open sets. If both of these conditions fail, then for a pair of non-intersecting and distinct we know that and thus that but there is no such that and since all restrictions are identity homomorphisms and . Thus it’s no sheaf.
This defect, however, is easily fixed. We define for an open subset to be the set of all locally constant maps . Clearly, if is connected, then since each map then is uniquely determined by its image. The restriction maps are then the usual restrictions of maps. The result is a sheaf of locally constant functions with values in , and is often denoted by the same symbol .
All the presheaves , and are sheaves though. Their presheaves of derivations are also sheaves.
The sheaf is called the structure sheaf of a smooth manifold , whereas the associated sheaf is called the tangent sheaf or the sheaf of smooth vector fields on .
The sheaf is called the structure sheaf of a complex manifold and the tangent sheaf or sheaf or holomorphic vector fields on is defined as above as the sheaf of derivations. Note that any complex manifold also has a structure of smooth real manifold and thus also has the associated sheaves and .
Let’s define a presheaf on the complex plane as a complex manifold. For open define to be the vector space of all bounded holomorphic functions on , with the restriction being the usual restriction of a holomorphic function. This is obviously a presheaf. However, it’s not a sheaf, since with a covering for and a family of bounded holomorphic functions . These functions are compatible, however, there is no bounded holomorphic function on such that .
From this we learn that non-local properties on presheaves often cause the presheaf to fail being a sheaf.
Morphisms and categorical structure
A morphism of (pre)sheaves is defined in the obvious way – as a family of homomorphisms of abelian groups such that the obvious diagrams commute. That is it doesn’t matter if you first restrict and then follow the morphism or first follow the morphism and then restrict – the result should be the same. These morphisms make the definition needed to have a category of sheaves, and in this category, we receive the usual definition of an isomorphism, of inclusions, et.c.
Note that the following inclusion maps are all morphisms of sheaves of rings:
For the next installment, we’re going to stalks and exact sequences. And germs! Wouldya look at that? We’ve gone from maritime terminology to agricultural terminology…