under construction
(also nonabelian homological algebra)
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In the context of homological algebra the right derived functor of the hom-functor is called the -functor . It derives its name from the fact that the derived hom-functor between abelian groups classifies abelian group extensions of by . (This is a special case of the general classification of principal ∞-bundles/∞-group extensions by general cohomology/group cohomology.)
Together with the Tor-functor it is one of the central objects of interest in homological algebra.
Given an abelian category we may consider the hom-functor Ab either as a functor in first or in second argument, and compute the corresponding right derived functors.
If they exist, the classical right derived functors of either functor agree and also agree with the homology of the mixed double complex obtained by taking simultaneously a projective resolution of the first contravariant argument and an injective resolution of the second covariant argument. The last construction is called the balanced .
Alternatively, one can consider the derived category and define
or define -groups as groups of abelian extensions of length , discussed below at Relation to extensions.
We give the definition following the discussion at derived functors in homological algebra.
Let be an abelian category with enough projectives. And let be any object. Consider the contravariant hom-functor
This is a left exact functor.
Therefore to derive it by resolutions we need to consider injective resolutions in the opposite category . But these are projective resolutions in itself.
For any object and a projective resolution, and for , the th -group of with coefficients in is the degree- cochain cohomology
of the cochain complex .
The following proposition expands a bit on the meaning of this definition. Write
for the enriched hom of chain complexes.
The th Ext-group is canonically identified with the 0-th homology of this enriched hom from the resolution of to the -fold delooping/suspension chain complex of (concentrated on in degree ):
or equivalently, if we think of degree chain homology as the 0th homotopy group (under Dold-Kan correspondence) and write the -fold suspension/delooping of as :
This is a special case of the general discussion at cochain cohomology.
By the discussion at internal hom of chain complexes, the 0-cycles of are chain maps of the form
By the definition of chain maps this are precisely those morphisms such that
which exhibits as a degree- cochain in the cochain complex .
Similarly, the -boundaries in come from chain homotopies :
in that
This are precisely the degree- coboundaries in .
This perspective on the -group as being the homotopy classes of maps out of (a resolution of) to is made more manifest in the discussion in terms of derived categories below. It connects -groups and their relation to extensions to the general context of cohomology and ∞-group extensions. See at abelian sheaf cohomology for more on this.
(…)
(…)
For all and , the -functor sends direct sums in the first variable and direct products in the second variable to direct products:
and
(e.g. Weibel, Prop. 3.3.4)
In particular, preserves the finite coproduct (a biproduct) in both variables:
We discuss how the group is identified with the group of extensions of by . In particular for and Ab this means that classified ordinary group extensions of by .
This is the relation that the name “” derives from. At infinity-group extension is discussed how this relation is a special case of the more general relation that identifies derived hom-spaces with -principal ∞-bundles over .
For two objects, an extension of by is a short exact sequence
An homomorphism of two such extensions and is a morphism in fitting into a commuting diagram of the form
All these homomophisms are necessarily isomorphisms, by the short five lemma.
Write for the set of isomorphism classes of such extensions.
Under Baer sum becomes an abelian group.
Define a morphism
by the following construction:
Choose a projective presentation of . Then for an extension consider the commuting diagram
where
is any choice of lift of through , which exists by definition since is a projective object;
is the induced morphism on the fibers, which exists by the exactness of the two sequences.
By prop. , represents an element . Let this be the image of the map to be defined:
This construction is independent of the choice of and involved and hence the map is well-defined.
The map from def. is a natural isomorphism of abelian groups
(…)
Given .
Let be a projective resolution. Let be a representative of .
Consider the pullback
(…)
For a discrete group with its group ring, over the integers, and for a linear -representation, hence a -module, the group cohomology of with coefficients in is
(…)
(…)
We discuss some facts helpful for the construction of -groups in certain situations.
The covariant hom-functor is generally a left exact functor. By the construction of via projective resolutions, def. , it is sufficient to show that it is also a right exact functor if is projective. In fact, this is one of the equivalent characterizations of projective objects (ee the section projective object – in abelian categories – equivalent characterizations for details).
For two objects, and
a short exact sequence with a projective object, hence exhibiting a projective presentation of , there is an exact sequence
exhibiting as the cokernel of .
The Yoneda product? is a pairing
(…)
A derived hom-functor such as the on chain complexes compute general notions of cohomology (see the discussion there). Here we list some specific incarnations of the -construction in the context of cohomology.
The universal coefficient theorem identifies, under suitable conditions, cohomology to the dual of homology up to -groups.
Various notions of cohomology groups in the context of algebra can be expressed as -groups, for instance:
For a discrete group, with its group ring over a commutative ring K, and for a -linear -representation (hence a -module), the group cohomology of with coefficients in is
For an associative algebra over some field and an -bimodule, hence an -module,
is the Hochschild cohomology of with coefficients in .
For a Lie algebra over a commutative ring , with universal enveloping algebra , and a Lie algebra module (hence an -module), the Lie algebra cohomology of with coefficients in is
(group extensions of the integers are trivial)
The -group of the integers with coefficients in any AbelianGroups is trivial:
(since the integers are already projective, e.g. Boardman, Prop. 19)
(group extensions of finite cyclic groups)
The -group of the cyclic group of order with coefficients in any AbelianGroups is the quotient group :
(e.g. Boardman, Prop. 20)
But:
(e.g. here)
(e.g. Boardman, Cor. 21)
In fact the last case of Example generalizes beyond cyclic groups:
(group extension by the rational numbers are trivial)
The -group of any AbelianGroups with coefficients in the rational numbers is trivial:
(e.g. Boardman, Prop. 22)
Less immediate is this example:
(group extension of rational numbers by the integers)
The -group of the rational numbers by the integers is the quotient group
of the group of adeles (without the real numbers-factor), i.e. the restricted product for the p-adic numbers restricted along the inclusion of the p-adic integers;
by the rational numbers (…):
Original discussion:
Samuel Eilenberg, Saunders MacLane, Group Extensions and Homology, Annals of Mathematics 43 4 (1942) 757-831 [doi:10.2307/1968966, jstor:1968966]
Nobuo Yoneda, On ext and exact sequences, PhD thesis, Journal of the Faculty of Science, Imperial University of Tokyo, 1960 (pdf, CiNii:naid/500000325773)
Saunders MacLane, Group Extensions by primary abelian groups, Transactions of the American Mathematical Society, 95 1 (1960) 1-16 [jstor:1993327]
Texbook accounts (see also most references at homological algebra):
Charles Weibel, An Introduction to Homological Algebra, Cambridge Studies in Adv. Math. 38, CUP 1994
Henri Cartan, Samuel Eilenberg, Homological algebra, Princeton Univ. Press 1956.
S. I . Gelfand, Yuri Manin, Methods of homological algebra
A systematic discussion from the point of view of derived categories is in
Lecture notes:
Kiyoshi Igusa, 25 The Ext Functor (pdf)
Michael Boardman, Some Common and Groups (pdf, pdf)
Patrick Morandi, Ext Groups and Ext Functors, (pdf)
(warning: the last section on resolutions for cocycles for group (abelian) extensions is not correct)
See also:
Last revised on August 23, 2023 at 14:15:44. See the history of this page for a list of all contributions to it.