# Abelian category

Abelian category

In mathematics, an abelian category is a category in which morphisms and objects can be added and in which kernels and cokernels exist and have desirable properties. The motivating prototype example of an abelian category is the category of abelian groups, Ab. The theory originated in a tentative attempt to unify several cohomology theories by Alexander Grothendieck. Abelian categories are very "stable" categories, for example they are regular and they satisfy the Snake lemma. The class of Abelian categories is closed under several categorical constructions, for example, the category of chain complexes of an Abelian category, or the category of functors from a small category to an Abelian category are Abelian as well. These stability properties make them inevitable in homological algebra and beyond; the theory has major applications in algebraic geometry, cohomology and pure category theory.

Definitions

A category is abelian if
*it has a zero object,
*it has all "pullbacks" and "pushouts", and
*all monomorphisms and epimorphisms are normal.

By a theorem of Peter Freyd, this definition is equivalent to the following "piecemeal" definition:
* A category is "preadditive" if it is enriched over the monoidal category Ab of abelian groups. This means that all hom-sets are abelian groups and the composition of morphisms is bilinear.
* A preadditive category is "additive" if every finite set of objects has a biproduct. This means that we can form finite direct sums and direct products.
* An additive category is "preabelian" if every morphism has both a kernel and a cokernel.
* Finally, a preabelian category is abelian if every monomorphism and every epimorphism is normal. This means that every monomorphism is a kernel of some morphism, and every epimorphism is a cokernel of some morphism.

Note that the enriched structure on hom-sets is a "consequence" of the three axioms of the first definition. This highlights the foundational relevance of the category of Abelian groups in the theory and its canonical nature.

The concept of exact sequence arises naturally in this setting, and it turns out that exact functors, i.e. the functors preserving exact sequences in various senses, are the relevant functors between Abelian categories. This "exactness" concept has been axiomatized in the theory of exact categories, forming a very special case of regular categories.

Examples

* As mentioned above, the category of all abelian groups is an abelian category. The category of all finitely generated abelian groups is also an abelian category, as is the category of all finite abelian groups.
* If "R" is a ring, then the category of all left (or right) modules over "R" is an abelian category. In fact, it can be shown that any small abelian category is equivalent to a full subcategory of such a category of modules ("Mitchell's embedding theorem").
* If "R" is a left-noetherian ring, then the category of finitely generated left modules over "R" is abelian. In particular, the category of finitely generated modules over a noetherian commutative ring is abelian; in this way, abelian categories show up in commutative algebra.
* As special cases of the two previous examples: the category of vector spaces over a fixed field "k" is abelian, as is the category of finite-dimensional vector spaces over "k".
* If "X" is a topological space, then the category of all (real or complex) vector bundles on "X" is not usually an abelian category, as there can be monomorphisms that are not kernels.
* If "X" is a topological space, then the category of all sheaves of abelian groups on "X" is an abelian category. More generally, the category of sheaves of abelian groups on a Grothendieck site is an abelian category. In this way, abelian categories show up in algebraic topology and algebraic geometry.
* If C is a small category and A is an abelian category, then the category of all functors from C to A forms an abelian category (the morphisms of this category are the natural transformations between functors). If C is small and preadditive, then the category of all additive functors from C to A also forms an abelian category. The latter is a generalization of the "R"-module example, since a ring can be understood as a preadditive category with a single object.

Grothendieck's Axioms

In his Tôhoku article, Grothendieck listed four additional axioms (and their duals) that an abelian category A might satisfy. These axioms are still in common use to this day. They are the following:
* AB3) For every set {Ai} of objects of A, the coproduct ∐Ai exists in A (i.e. A is cocomplete).
* AB4) A satisfies AB3), and the coproduct of a family of monomorphisms is a monomorphism.
* AB5) A satisfies AB3), and filtered colimits of exact sequences are exact.and their duals
* AB3*) For every set {Ai} of objects of A, the product ΠAi exists in A (i.e. A is complete).
* AB4*) A satisfies AB3*), and the product of a family of epimorphisms is an epimorphism.
* AB5*) A satisfies AB3*), and filtered limits of exact sequences are exact.

Axioms AB1) and AB2) were also given. They are what make an additive category abelian. Specifically:
* AB1) Every morphism has a kernel and a cokernel.
* AB2) For every morphism "f", the canonical morphism from coim "f" to im "f" is an isomorphism.

Grothendieck also gave axioms AB6) and AB6*).

Elementary properties

Given any pair "A", "B" of objects in an abelian category, there is a special zero morphism from "A" to "B".This can be defined as the zero element of the hom-set Hom("A","B"), since this is an abelian group.Alternatively, it can be defined as the unique composition "A" → 0 → "B", where 0 is the zero object of the abelian category.

In an abelian category, every morphism "f" can be written as the composition of an epimorphism followed by a monomorphism.This epimorphism is called the "coimage" of "f", while the monomorphism is called the "image" of "f".

Subobjects and quotient objects are well-behaved in abelian categories.For example, the poset of subobjects of any given object "A" is a bounded lattice.

Every abelian category A is a module over the monoidal category of finitely generated abelian groups; that is, we can form a tensor product of a finitely generated abelian group "G" and any object "A" of A.The abelian category is also a comodule; Hom("G","A") can be interpreted as an object of A.If A is complete, then we can remove the requirement that "G" be finitely generated; most generally, we can form finitary enriched limits in A.

Related concepts

Abelian categories are the most general setting for homological algebra.All of the constructions used in that field are relevant, such as exact sequences, and especially short exact sequences, and derived functors.Important theorems that apply in all abelian categories include the five lemma (and the short five lemma as a special case), as well as the snake lemma (and the nine lemma as a special case).

History

Abelian categories were introduced by Alexander Grothendieck in his famous Tôhoku paper in the middle of the 1950s in order to unify various cohomology theories. At the time, there was a cohomology theory for sheaves, and a cohomology theory for groups. The two were defined completely differently, but they had formally almost identical properties. In fact, much of category theory was developed as a language to study these similarities. Grothendieck managed to unify the two theories: they both arise as derived functors on abelian categories; on the one hand the abelian category of sheaves of abelian groups on a topological space, on the other hand the abelian category of "G"-modules for a given group "G".

References

* P. Freyd. "Abelian Categories," Harper and Row, New York, 1964. [http://www.tac.mta.ca/tac/reprints/articles/3/tr3abs.html Available online.]
* Alexandre Grothendieck, "Sur quelques points d'algèbre homologique", Tôhoku Mathematics Journal, 1957
* Barry Mitchell: "Theory of Categories", New York, Academic Press, 1965.
* N. Popescu: "Abelian categories with applications to rings and modules", Academic Press, London, 1973.

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