Quotient of an abelian category
In mathematics, the quotient of an abelian category by a Serre subcategory is the abelian category which, intuitively, is obtained from by ignoring all objects from. There is a canonical exact functor whose kernel is, and is in a certain sense the most general abelian category with this property.
Forming Serre quotients of abelian categories is thus formally akin to forming quotients of groups. Serre quotients are somewhat similar to quotient categories, the difference being that with Serre quotients all involved categories are abelian and all functors are exact. Serre quotients also often have the character of localizations of categories, especially if the Serre subcategory is localizing.
Definition
Formally, is the category whose objects are those of and whose morphisms from X to Y are given by the direct limitwhere the limit is taken over subobjects and such that and. These pairs of subobjects are ordered by.
Composition of morphisms in is induced by the universal property of the direct limit.
The canonical functor sends an object X to itself and a morphism to the corresponding element of the direct limit with X′ = X and Y′ = 0.
An alternative, equivalent construction of the quotient category uses what is called a "calculus of fractions" to define the morphisms of. Here, one starts with the class of those morphisms in whose kernel and cokernel both belong to. This is a multiplicative system in the sense of Gabriel-Zisman, and one can localize the category at the system to obtain.
Examples
Let be a field and consider the abelian category of all vector spaces over. Then the full subcategory of finite-dimensional vector spaces is a Serre-subcategory of. The Serre quotient has as objects the -vector spaces, and the set of morphisms from to in is . This has the effect of identifying all finite-dimensional vector spaces with 0, and of identifying two linear maps whenever their difference has finite-dimensional image. This example shows that the Serre quotient can behave like a quotient category.For another example, take the abelian category Ab of all abelian groups and the Serre subcategory of all torsion abelian groups. The Serre quotient here is equivalent to the category of all vector spaces over the rationals, with the canonical functor given by tensoring with. Similarly, the Serre quotient of the category of finitely generated abelian groups by the subcategory of finitely generated torsion groups is equivalent to the category of finite-dimensional vectorspaces over. Here, the Serre quotient behaves like a localization.
Properties
The Serre quotient is an abelian category, and the canonical functor is exact and surjective on objects. The kernel of is, i.e., is zero in if and only if belongs to.The Serre quotient and canonical functor are characterized by the following universal property: if is any abelian category and is an exact functor such that is a zero in for each object, then there is a unique exact functor such that.
Given three abelian categories,,, we have
if and only if
Theorems involving Serre quotients
Serre's description of coherent sheaves on a projective scheme
According to a theorem by Jean-Pierre Serre, the category of coherent sheaves on a projective scheme can be described as the Serre quotientwhere denotes the category of finitely-generated graded modules over and is the Serre subcategory consisting of all those graded modules which are 0 in all degrees that are high enough, i.e. for which there exists such that for all.
A similar description exists for the category of quasi-coherent sheaves on, even if is not noetherian.