Abelian Category

An Abelian category is a category for which the constructions and techniques of homological algebra are available. The basic examples of such categories are the category of Abelian groups and, more generally, the category of modules over a ring. Abelian categories are widely used in algebra, algebraic geometry, and topology.

Many of the same constructions that are found in categories of modules, such as kernels, exact sequences, and commutative diagrams are available in Abelian categories. A disadvantage that must be overcome is the fact that the objects in a category do not necessarily have elements that can be manipulated directly, so the traditional definitions do not work. As a result, methods must be developed that allow definition and manipulation of objects without the use of elements.

As an example, consider the definition of the kernel of a morphism, which states that given f:E->F, the kernel of f is defined to be a morphism K->E such that all morphisms g:X->E such that f degreesg=0, factor through K. Notice that this definition does not guarantee that the kernel exists, but only gives properties which uniquely identify it if it does exist. Similar work must be done to define the cokernel of a morphism.

With these definitions, a category can be defined as Abelian if it satisfies the following five properties:

1. For two objects E and F, the set Mor(E,F) of morphisms from E to F has the structure of an Abelian group. This group structure must be arranged so that composition of morphisms is bilinear.

2. There is an object, denoted 0, that is both an initial object and a terminal object.

3. Products and coproducts of finite collections of objects always exist.

4. Kernels and cokernels always exist.

5. If f:E->F is a morphism whose kernel is 0, then f is the kernel of its cokernel. If f:E->F is a morphism whose cokernel is 0, then f is the cokernel of its kernel.

A category that satisfies only the first three properties is called an additive category.

Examples of Abelian categories include:

1. For a commutative ring R, the category of modules over R is an Abelian category. This is the basic example.

2. The category of vector bundles over a fixed topological space is an Abelian category.

3. The category of sheaves over a topological space is an Abelian category.

Freyd's theorem states that every Abelian category is a subcategory of some category of modules over a ring. Mitchell (1964) has strengthened this, saying every Abelian category is a full subcategory of a category of modules over a ring. Despite this result, the terminology and methods of Abelian categories remain useful and powerful.

See also

Abelian Group, Category, Cokernel, Category Product, Freyd's Theorem, Module, Module Kernel, Morphism, Product

This entry contributed by John Renze

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Freyd, P. Abelian Categories: An Introduction to the Theory of Functors. New York: Harper & Row, 1964.Grothendieck, A. "Sur quelques points d'algèbre homologique." Tôhoku Math. J. 9, 119-221, 1957.Hilton, P. and Stammbach, U. A Course in Homological Algebra, 2nd ed. New York: Springer-Verlag, 1997.Lang, S. Algebra, rev. 3rd ed. New York: Springer Verlag, 158-159, 2002.Mac Lane, S. and Gehring, F. W. Categories for the Working Mathematician, 2nd ed. New York: Springer-Verlag, 1998.Mitchell, B. "The Full Imbedding Theorem." Amer. J. Math. 86, 619-637, 1964.

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Abelian Category

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Renze, John. "Abelian Category." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein.

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