As most cohomological invariants, the cohomological dimension involves a choice of a "ring of coefficients" R, with a prominent special case given by R = Z, the ring of integers. Let G be a discrete group, R a non-zero ring with a unit, and RG the group ring. The group G has cohomological dimension less than or equal to n, denoted cdR(G) ≤ n, if the trivial RG-module R has a projective resolution of length n, i.e. there are projectiveRG-modules P0, ..., Pn and RG-module homomorphisms dk: PkPk − 1 (k = 1, ..., n) and d0: P0R, such that the image of dk coincides with the kernel of dk − 1 for k = 1, ..., n and the kernel of dn is trivial.
Equivalently, the cohomological dimension is less than or equal to n if for an arbitrary RG-module M, the cohomology of G with coefficients in M vanishes in degrees k > n, that is, Hk(G,M) = 0 whenever k > n. The p-cohomological dimension for prime p is similarly defined in terms of the p-torsion groups Hk(G,M){p}.[1]
The smallest n such that the cohomological dimension of G is less than or equal to n is the cohomological dimension of G (with coefficients R), which is denoted .
A free resolution of can be obtained from a free action of the group G on a contractible topological spaceX. In particular, if X is a contractible CW complex of dimension n with a free action of a discrete group G that permutes the cells, then .
Examplesedit
In the first group of examples, let the ring R of coefficients be .
A free group has cohomological dimension one. As shown by John Stallings (for finitely generated group) and Richard Swan (in full generality), this property characterizes free groups. This result is known as the Stallings–Swan theorem.[2] The Stallings-Swan theorem for a group G says that G is free if and only if every extension by G with abelian kernel is split.[3]
More generally, the fundamental group of a closed, connected, orientable asphericalmanifold of dimensionn has cohomological dimension n. In particular, the fundamental group of a closed orientable hyperbolic n-manifold has cohomological dimension n.
Nontrivial finite groups have infinite cohomological dimension over . More generally, the same is true for groups with nontrivial torsion.
Now consider the case of a general ring R.
A group G has cohomological dimension 0 if and only if its group ring RG is semisimple. Thus a finite group has cohomological dimension 0 if and only if its order (or, equivalently, the orders of its elements) is invertible in R.
Generalizing the Stallings–Swan theorem for , Martin Dunwoody proved that a group has cohomological dimension at most one over an arbitrary ring R if and only if it is the fundamental group of a connected graph of finite groups whose orders are invertible in R.
Cohomological dimension of a fieldedit
The p-cohomological dimension of a field K is the p-cohomological dimension of the Galois group of a separable closure of K.[4] The cohomological dimension of K is the supremum of the p-cohomological dimension over all primes p.[5]
Examplesedit
Every field of non-zero characteristicp has p-cohomological dimension at most 1.[6]
Every finite field has absolute Galois group isomorphic to and so has cohomological dimension 1.[7]
The field of formal Laurent series over an algebraically closed fieldk of non-zero characteristic also has absolute Galois group isomorphic to and so cohomological dimension 1.[7]
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Dydak, Jerzy (2002). "Cohomological dimension theory". In Daverman, R. J. (ed.). Handbook of geometric topology. Amsterdam: North-Holland. pp. 423–470. ISBN 0-444-82432-4. MR 1886675. Zbl 0992.55001.
Gille, Philippe; Szamuely, Tamás (2006). Central simple algebras and Galois cohomology. Cambridge Studies in Advanced Mathematics. Vol. 101. Cambridge: Cambridge University Press. ISBN 0-521-86103-9. Zbl 1137.12001.
Shatz, Stephen S. (1972). Profinite groups, arithmetic, and geometry. Annals of Mathematics Studies. Vol. 67. Princeton, NJ: Princeton University Press. ISBN 0-691-08017-8. MR 0347778. Zbl 0236.12002.