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Concept
Inverse image functor
Résumé
In mathematics, specifically in algebraic topology and algebraic geometry, an inverse image functor is a contravariant construction of sheaves; here “contravariant” in the sense given a map , the inverse image functor is a functor from the of sheaves on Y to the category of sheaves on X. The is the primary operation on sheaves, with the simplest definition. The inverse image exhibits some relatively subtle features.
Suppose we are given a sheaf on and that we want to transport to using a continuous map .
We will call the result the inverse image or sheaf . If we try to imitate the by setting
for each open set of , we immediately run into a problem: is not necessarily open. The best we could do is to approximate it by open sets, and even then we will get a presheaf and not a sheaf. Consequently, we define to be the sheaf associated to the presheaf:
(Here is an open subset of and the colimit runs over all open subsets of containing .)
For example, if is just the inclusion of a point of , then is just the stalk of at this point.
The restriction maps, as well as the functoriality of the inverse image follows from the universal property of direct limits.
When dealing with morphisms of locally ringed spaces, for example schemes in algebraic geometry, one often works with sheaves of -modules, where is the structure sheaf of . Then the functor is inappropriate, because in general it does not even give sheaves of -modules. In order to remedy this, one defines in this situation for a sheaf of -modules its inverse image by
While is more complicated to define than , the stalks are easier to compute: given a point , one has .
is an exact functor, as can be seen by the above calculation of the stalks.
is (in general) only right exact. If is exact, f is called flat.
is the left adjoint of the . This implies that there are natural unit and counit morphisms and . These morphisms yield a natural adjunction correspondence:
However, the morphisms and are almost never isomorphisms.
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Séances de cours associées (6)
Restriction Functors: Actions de groupe
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Concepts associés (9)
Gluing axiom
In mathematics, the gluing axiom is introduced to define what a sheaf on a topological space must satisfy, given that it is a presheaf, which is by definition a contravariant functor to a category which initially one takes to be the . Here is the partial order of open sets of ordered by inclusion maps; and considered as a category in the standard way, with a unique morphism if is a subset of , and none otherwise. As phrased in the sheaf article, there is a certain axiom that must satisfy, for any open cover of an open set of .
Stalk (sheaf)
The stalk of a sheaf is a mathematical construction capturing the behaviour of a sheaf around a given point. Sheaves are defined on open sets, but the underlying topological space consists of points. It is reasonable to attempt to isolate the behavior of a sheaf at a single fixed point of . Conceptually speaking, we do this by looking at small neighborhoods of the point. If we look at a sufficiently small neighborhood of , the behavior of the sheaf on that small neighborhood should be the same as the behavior of at that point.
Direct image functor
In mathematics, the direct image functor is a construction in sheaf theory that generalizes the global sections functor to the relative case. It is of fundamental importance in topology and algebraic geometry. Given a sheaf F defined on a topological space X and a continuous map f: X → Y, we can define a new sheaf f∗F on Y, called the direct image sheaf or the pushforward sheaf of F along f, such that the global sections of f∗F is given by the global sections of F.