Cohomological dimension explained

In abstract algebra, cohomological dimension is an invariant of a group which measures the homological complexity of its representations. It has important applications in geometric group theory, topology, and algebraic number theory.

Cohomological dimension of a group

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 \operatorname{cd} _{R(G)\le} n , if the trivial RG -module R has a projective resolution of length n, i.e. there are projective RG -modules P _{0,...} , P _{n} and RG -module homomorphisms d _{k\colon} P _{k\to} P _{k-1} (k = 1 , ... , n) and d _{0\colon} P _{0\to} R , such that the image of d _{k} coincides with the kernel of d _{k-1} for k = 1 , ... , n and the kernel of d _{n} 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, H ^{k(G,M)} = 0 whenever k>n . The p-cohomological dimension for prime p is similarly defined in terms of the p-torsion groups H ^{k(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

n = \operatorname{cd} _{R} (G) .

A free resolution of

\Z can be obtained from a free action of the group G on a contractible topological space X. 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 \operatorname{cd} _{\Z} (G)\le n .

Examples

In the first group of examples, let the ring R of coefficients be

\Z .

• 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]
• The fundamental group of a compact, connected, orientable Riemann surface other than the sphere has cohomological dimension two.
• More generally, the fundamental group of a closed, connected, orientable aspherical manifold of dimension n 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

\Z . 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

R = \Z , 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 field

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]

Examples

• Every field of non-zero characteristic p has p-cohomological dimension at most 1.^[6]
• Every finite field has absolute Galois group isomorphic to

\hat{\Z} and so has cohomological dimension 1.^[7] k((t)) over an algebraically closed field k of non-zero characteristic also has absolute Galois group isomorphic to \hat{\Z} and so cohomological dimension 1.^[7]

See also

• Eilenberg−Ganea conjecture
• Group cohomology
• Global dimension

References

• Book: Brown, Kenneth S. . Kenneth Brown (mathematician)

. Kenneth Brown (mathematician). Cohomology of groups . Corrected reprint of the 1982 original . . 87 . . New York . 1994 . 1324339 . 0-387-90688-6 . 0584.20036 .

• Book: Dicks, Warren . 10.1007/BFb0088140 . Groups, Trees, and Projective Modules . Lecture Notes in Mathematics . 790 . . Berlin . 1980 . 0584790 . 3-540-09974-3 . 0427.20016 .
• Book: Dydak, Jerzy . Cohomological dimension theory . Handbook of geometric topology . 423–470 . . Amsterdam . 2002. Robert Daverman . Daverman . R. J. . 0-444-82432-4 . 1886675 . 0992.55001 .
• Book: Gille . Philippe . Szamuely . Tamás . Central simple algebras and Galois cohomology . Cambridge Studies in Advanced Mathematics . 101 . Cambridge . . 2006 . 0-521-86103-9 . 1137.12001 .
• Book: Serre, Jean-Pierre . Jean-Pierre Serre

. Jean-Pierre Serre . Galois cohomology . . 1997. 3-540-61990-9 . 0902.12004 .

• Book: Shatz, Stephen S. . Profinite groups, arithmetic, and geometry . Annals of Mathematics Studies . 67 . Princeton, NJ . . 1972 . 0-691-08017-8 . 0236.12002 . 0347778 .
• John R. . Stallings . John R. Stallings . On torsion-free groups with infinitely many ends . . Second Series . 88 . 1968 . 312–334 . 0228573 . 0238.20036 . 0003-486X . 10.2307/1970577.
• Swan . Richard G. . Richard G. Swan . Groups of cohomological dimension one . . 12 . 1969 . 585–610 . 0240177 . 0188.07001. 0021-8693 . 10.1016/0021-8693(69)90030-1. free .

Notes and References

1. Gille & Szamuely (2006) p.136
2. Book: Baumslag, Gilbert. Gilbert Baumslag

   . Gilbert Baumslag. Topics in Combinatorial Group Theory. 2012. 16. Springer Basel AG.

3. Gruenberg. Karl W.. Karl W. Gruenberg. Review of Homology in group theory by Urs Stammbach. . 81. 1975. 851–854. 10.1090/S0002-9904-1975-13858-4. free.
4. Shatz (1972) p.94
5. Gille & Szamuely (2006) p.138
6. Gille & Szamuely (2006) p.139
7. Gille & Szamuely (2006) p.140