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Let $pi$ be a group equipped with an action of a second group $G$ by automorphisms. We define the equivariant cohomological dimension ${sf cd}_G(pi)$, the equivariant geometric dimension ${sf gd}_G(pi)$, and the equivariant Lusternik-Schnirelmann category ${sf cat}_G(pi)$ in terms of the Bredon dimensions and classifying space of the family of subgroups of the semi-direct product $pirtimes G$ consisting of sub-conjugates of $G$. When $G$ is finite, we extend theorems of Eilenberg-Ganea and Stallings-Swan to the equivariant setting, thereby showing that all three invariants coincide (except for the possibility of a $G$-group $pi$ with ${sf cat}_G(pi)={sf cd}_G(pi)=2$ and ${sf gd}_G(pi)=3$). A main ingredient is the purely algebraic result that the cohomological dimension of any finite group with respect to any family of proper subgroups is greater than one. This implies a Stallings-Swan type result for families of subgroups which do not contain all finite subgroups.
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We develop the theory of equivariant sheaves over profinite spaces, where the group is also taken to be profinite. We construct a good notion of equivariant presheaves, with a suitable sheafification functor. Using these results on equivariant presheaves, we give explicit constructions of products of equivariant sheaves of R-modules. We introduce an equivariant analogue of skyscraper sheaves, which allows us to show that the category of equivariant sheaves of R-modules over a profinite space has enough injectives. This paper also provides the basic theory for results by the authors on giving an algebraic model for rational G-spectra in terms of equivariant sheaves over profinite spaces. For those results, we need a notion of Weyl-G-sheaves over the space of closed subgroups of G. We show that Weyl-G-sheaves of R-modules form an abelian category, with enough injectives, that is a full subcategory of equivariant sheaves of R-modules. Moreover, we show that the inclusion functor has a right adjoint.
If $G$ has $4$-periodic cohomology, then D2 complexes over $G$ are determined up to polarised homotopy by their Euler characteristic if and only if $G$ has at most two one-dimensional quaternionic representations. We use this to solve Walls D2 problem for several infinite families of non-abelian groups and, in these cases, also show that any finite Poincar{e} $3$-complex $X$ with $pi_1(X)=G$ admits a cell structure with a single $3$-cell. The proof involves cancellation theorems for $mathbb{Z} G$ modules where $G$ has periodic cohomology.
Let P be an extraspecial p-group which is neither dihedral of order 8, nor of odd order p^3 and exponent p. Let G be a finite group having P as a Sylow p-subgroup. Then the mod-p cohomology ring of G coincides with that of the normalizer N_G(P).
We prove that certain families of Coxeter groups and inclusions $W_1hookrightarrow W_2hookrightarrow...$ satisfy homological stability, meaning that in each degree the homology $H_ast(BW_n)$ is eventually independent of $n$. This gives a uniform treatment of homological stability for the families of Coxeter groups of type $A_n$, $B_n$ and $D_n$, recovering existing results in the first two cases, and giving a new result in the third. The key step in our proof is to show that a certain simplicial complex with $W_n$-action is highly connected. To do this we show that the barycentric subdivision is an instance of the basic construction, and then use Daviss description of the basic construction as an increasing union of chambers to deduce the required connectivity.
We show that the mod $ell$ cohomology of any finite group of Lie type in characteristic $p$ different from $ell$ admits the structure of a module over the mod $ell$ cohomology of the free loop space of the classifying space $BG$ of the corresponding compact Lie group $G$, via ring and module structures constructed from string topology, a la Chas-Sullivan. If a certain fundamental class in the homology of the finite group of Lie type is non-trivial, then this module structure becomes free of rank one, and provides a structured isomorphism between the two cohomology rings equipped with the cup product, up to a filtration. We verify the nontriviality of the fundamental class in a range of cases, including all simply connected untwisted classical groups over the field of $q$ elements, with $q$ congruent to 1 mod $ell$. We also show how to deal with twistings and get rid of the congruence condition by replacing $BG$ by a certain $ell$-compact fixed point group depending on the order of $q$ mod $ell$, without changing the finite group. With this modification, we know of no examples where the fundamental class is trivial, raising the possibility of a general structural answer to an open question of Tezuka, who speculated about the existence of an isomorphism between the two cohomology rings.
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