Monthly Archives: June 2013

Book: Group Cohomology and Algebraic Cycles

I’m about to deliver the final manuscript of my book Group Cohomology and Algebraic Cycles to Cambridge University Press. As a sort of early advertisement, here’s the draft preface. I thank them at the end of the preface, but I’d like to say here too: I thank Ben Antieau and Peter Symonds for many useful suggestions.

Group cohomology reveals a deep relation between algebra and topology. A group determines a topological space in a natural way, its classifying space. The cohomology ring of a group is defined to be the cohomology ring of its classifying space. The challenges are to understand how the algebraic properties of a group are related to its cohomology ring, and to compute the cohomology rings of particular groups.

A fundamental fact is that the cohomology ring of any finite group is finitely generated. So there is some finite description of the whole cohomology ring of a finite group, but it is not clear how to find it. A central problem in group cohomology is to find an upper bound for the degrees of generators and relations for the cohomology ring. If we can do that, then there are algorithms to compute the cohomology in low degrees and therefore compute the whole cohomology ring.

Peter Symonds made a spectacular advance in 2010: for any finite group G with a faithful complex representation of dimension n at least 2 and any prime number p, the mod p cohomology ring of G is generated by elements of degree at most n2 (Symonds 2010). Not only is this the first known bound for generators of the cohomology ring; it is nearly an optimal bound among arbitrary finite groups, as we will see.

This book proves Symonds’s theorem and several new variants and improvements of it. Some involve algebro-geometric analogs of the cohomology ring. Namely, Morel–Voevodsky and I independently showed how to view the classifying space of an algebraic group G (for example, a finite group) as a limit of algebraic varieties in a natural way. That allows the definition of the Chow ring of algebraic cycles on the classifying space BG (Morel and Voevodsky 1999, prop. 2.6; Totaro 1999).

A major goal of algebraic geometry is to compute the Chow ring for varieties of interest, since that says something meaningful about all subvarieties of the variety.

The fact that not all the cohomology of BG is represented by algebraic cycles (even for abelian groups G) is the source of Atiyah-Hirzebruch’s counterexamples to the integral Hodge conjecture (Atiyah and Hirzebruch 1962; Totaro 1997, 1999). It is a natural problem of “motivic homotopy theory” to understand the Chow ring and more generally the motivic cohomology of classifying spaces BG. Concretely, computing the Chow ring of BG essentially amounts to computing the Chow groups of the quotients by G of all representations of G. Such quotients are extremely special among all varieties, but they have been fundamental examples in algebraic geometry for more than 150 years. Computing their Chow groups is a fascinating problem. (Rationally, the calculations are easy; the interest is in integral or mod p calculations.)

Bloch generalized Chow groups to a bigraded family of groups, now called motivic cohomology. A great achievement of motivic homotopy theory is the proof by Voevodsky and Rost of the Bloch–Kato conjecture (Voevodsky 2011, theorem 6.16). A corollary, the Beilinson–Lichtenbaum conjecture, says that for any smooth variety over a field, a large range of motivic cohomology groups with finite coefficients map isomorphically to etale cohomology. Etale cohomology is a more computable theory, which coincides with ordinary cohomology in the case of complex varieties. Thus the Beilinson–Lichtenbaum conjecture is a powerful link between algebraic geometry and topology.

Chow groups are the motivic cohomology groups of most geometric interest, but they are also farthest from the motivic cohomology groups that are computed by the Beilinson–Lichtenbaum conjecture. A fundamental difficulty in computing Chow groups is “etale descent”: for a finite Galois etale morphism X → Y of schemes, how are the Chow groups of X and Y related? This is easy after tensoring with the rationals; the hard case of etale descent is to compute Chow groups integrally, or with finite coefficients. Etale descent is well understood for etale cohomology, and hence for many motivic cohomology groups with finite coefficients.

The problem of etale descent provides some motivation for trying to compute the Chow ring of classifying spaces of finite groups G. Computing the Chow ring of BG means computing the Chow ring of certain varieties Y which have a covering map X → Y with Galois group G (an approximation to EG → BG) such that X has trivial Chow groups. Thus the Chow ring of BG is a model case in seeking to understand etale descent for Chow groups.

Chow rings can be generalized in various ways, for example to algebraic cobordism and motivic cohomology. Another direction of generalization leads to unramified cohomology, cohomological invariants of algebraic groups (Garibaldi, Merkurjev, and Serre 2003), and obstructions to rationality for quotient varieties (Bogomolov 1987; Kahn and Ngan 2012). All of these invariants are worth computing for classifying spaces, but we largely focus on the most classical case of Chow rings. Some of our methods will certainly be useful for these more general invariants. For example, finding generators for the Chow ring (of any algebraic variety) automatically gives generators of its algebraic cobordism, by Levine and Morel (2007, theorem 1.2.19).

We now summarize the contents. Continue reading

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