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The
cohomology of a group G can be defined in terms of resolutions. Let Z
be the group of integers considered as a trivial ZG-module. A free ZG-resolution of Z is a
sequence of ZG-module homomorphisms ... → Mn → Mn-1
→ ... → M1 → M0
satisfying:
... → TMn → TMn-1
→ ... → TM1 → TM0 .
This sequence will generally not
satisfy the above exactness condition, and one defines the integral homology of
G to be
for all n>0. By changing the
definition of TMn
one arrives at the definition of homology Hn(G,A) and
cohomology Hn(G,A) with coefficients in a ZG-module A.
Needless to say, homology and cohomology are invariants of G, and do
not depend on the particular choice of free ZG-resolution.
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There
are two steps to computing group
homology:
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gap>
F:=FreeGroup(2);;x:=F.1;;y:=F.2;; gap> G:=F/[x^2,y^512,(x*y)^2];; D_512:=Image(IsomorphismPermGroup(G));; gap> R:=ResolutionFiniteGroup(D_512,26);; gap> TR:=TensorWithIntegers(R);; gap> Homology(TR,25); [ 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2 ] |
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We
see that H25(D256,Z) = (Z2)14. Homology with other coefficients can also be calculated from R. For instance, the following additional command shows that the 25th homology of D256 with coefficients in the trivial module Z2 is the vector space H25(D256,Z2) = (Z2)26. |
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gap>
T2R:=TensorWithIntegersModP(R,2);; gap> Homology(T2R,25); 26 |
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(At
this point we should note that there exist more efficient methods for
computing the homology of a finite group G over the finite field Zp
with trivial action. One approach, which has been used very succesfully by Jon Carlson on small p-groups, is to regard the group ring ZpG as a vector space of rank |G|. Each term in a ZpG-resolution R is then also a vector space, and one can use linear algebra techniques to construct R. Details can be found here. The approach will not work so well on large groups G since one ends up having to compute null-spaces of large matrices. Consider for instance our computation for G=D256. Any resolution is going to have at least 26 Z2G-generators in dimension 25, and 25 generators in dimension 24. So one has to compute the nullspace of a matrix with (512)2×26×25 = 170393600 entries. An efficient approach to mod p homology computations of larger groups has been developed by David Green. His approach involves non-abelian Gröbner basis techniques and is described in the book [D.J. Green, Gröbner bases and the computation of group cohomology, Lecture Notes in Math., No. 1828 (Springer, 2003)] and on the corresponding web page.) For integral computations to succeed the ZG-rank of the resolution R should not be too large in any given dimension. The construction of small ZG-resolutions is an interesting problem and at first glance might seem to be a purely algebraic one. However, we can benefit from the advice of Sir Michael Atiyah. |
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