mathematics
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, the category
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Ab has the abelian group
In mathematics
Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no g ...
s as objects and group homomorphism
Image:Group homomorphism ver.2.svg, 250px, Image of a group homomorphism (h) from G (left) to H (right). The smaller oval inside H is the image of h. N is the Kernel_(algebra)#Group_homomorphisms, kernel of h and aN is a coset of N.
In mathematics ...
s as morphism
In mathematics
Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It h ...
s. This is the prototype of an abelian category
In mathematics
Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no gener ...
: indeed, every small
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abelian category
In mathematics
Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no gener ...
can be embedded in Ab.Mac Lane (1998) p.209
Properties
Thezero object
In category theory
Category theory formalizes mathematical structure and its concepts in terms of a Graph labeling, labeled directed graph called a ''Category (mathematics), category'', whose nodes are called ''objects'', and whose labelled d ...
of Ab is the trivial group which consists only of its neutral element.
The monomorphisms in Ab are the injective group homomorphisms, the epimorphisms are the surjective group homomorphisms, and the isomorphisms are the bijective group homomorphisms.
Ab is a full subcategory of Grp, the category of groups, category of ''all'' groups. The main difference between Ab and Grp is that the sum of two homomorphisms ''f'' and ''g'' between abelian groups is again a group homomorphism:
:(''f''+''g'')(''x''+''y'') = ''f''(''x''+''y'') + ''g''(''x''+''y'') = ''f''(''x'') + ''f''(''y'') + ''g''(''x'') + ''g''(''y'')
: = ''f''(''x'') + ''g''(''x'') + ''f''(''y'') + ''g''(''y'') = (''f''+''g'')(''x'') + (''f''+''g'')(''y'')
The third equality requires the group to be abelian. This addition of morphism turns Ab into a preadditive category, and because the direct sum of abelian groups, direct sum of finitely many abelian groups yields a biproduct, we indeed have an additive category.
In Ab, the notion of kernel (category theory), kernel in the category theory sense coincides with kernel (algebra), kernel in the algebraic sense, i.e. the categorical kernel of the morphism ''f'' : ''A'' → ''B'' is the subgroup ''K'' of ''A'' defined by ''K'' = , together with the inclusion homomorphism ''i'' : ''K'' → ''A''. The same is true for cokernels; the cokernel of ''f'' is the quotient group ''C'' = ''B'' / ''f''(''A'') together with the natural projection ''p'' : ''B'' → ''C''. (Note a further crucial difference between Ab and Grp: in Grp it can happen that ''f''(''A'') is not a normal subgroup of ''B'', and that therefore the quotient group ''B'' / ''f''(''A'') cannot be formed.) With these concrete descriptions of kernels and cokernels, it is quite easy to check that Ab is indeed an abelian category
In mathematics
Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no gener ...
.
The product (category theory), product in Ab is given by the direct product of groups, product of groups, formed by taking the cartesian product of the underlying sets and performing the group operation componentwise. Because Ab has kernels, one can then show that Ab is a complete category. The coproduct in Ab is given by the direct sum; since Ab has cokernels, it follows that Ab is also cocomplete.
We have a forgetful functor Ab → category of sets, Set which assigns to each abelian group the underlying Set (mathematics), set, and to each group homomorphism the underlying function (mathematics), function. This functor is faithful functor, faithful, and therefore Ab is a concrete category. The forgetful functor has a adjoint functors, left adjoint (which associates to a given set the free abelian group with that set as basis) but does not have a right adjoint.
Taking direct limits in Ab is an exact functor. Since the group of integers Z serves as a Generator (category theory), generator, the category Ab is therefore a Grothendieck category; indeed it is the prototypical example of a Grothendieck category.
An object in Ab is injective module, injective if and only if it is a divisible group; it is projective module, projective if and only if it is a free abelian group. The category has a projective generator (Z) and an injective cogenerator (Q/Z).
Given two abelian groups ''A'' and ''B'', their tensor product ''A''⊗''B'' is defined; it is again an abelian group. With this notion of product, Ab is a closed monoidal category, closed monoidal category, symmetric monoidal category.
Ab is not a topos since e.g. it has a zero object.
See also
* Category of modules * Abelian sheaf - many facts about the category of abelian groups continue to hold for the category of sheaves of abelian groupsReferences
* * * {{DEFAULTSORT:Category Of Abelian Groups Categories in category theory, Abelian groups Group theory