String topology of finite groups of Lie type

Abstract

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 class in the homology of the finite group of Lie type, arising from the fundamental class of $G$, is nontrivial, then this module structure is free of rank one, providing a highly structured isomorphism between the two cohomologies. We verify the nontriviality of the 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 avoid 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 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.

80 pages. v2: Major revision. Improvements include comparison of string products, asymptotic existence of fundamental classes, and improved ring structure preservation. v3: Fix reference

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• A polarized view of string topology
• The classification of 2-compact groups
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• A finite loop space not rationally equivalent to a compact Lie group
• The bv algebra in string topology of classifying spaces
• Finite Chevalley groups and loop groups
• Cohomology of classifying spaces of loop groups and finite Chevalley groups associated with spin groups

Cited by in corpus (1)

• Torsion Free Endotrivial Modules for Finite Groups of Lie Type