# Stacks of group representations

### Paul Balmer

UCLA, Los Angeles, USA

## Abstract

We start with a small paradigm shift about group representations, namely the observation that restriction to a subgroup can be understood as an extension-of-scalars. We deduce that, given a group $G$, the derived and the stable categories of representations of a subgroup $H$ can be constructed out of the corresponding category for $G$ by a purely triangulated-categorical construction, analogous to étale extension in algebraic geometry.

In the case of finite groups, we then use descent methods to investigate when modular representations of the subgroup $H$ can be extended to $G$. We show that the presheaves of plain, derived and stable representations all form stacks on the category of finite $G$-sets (or the orbit category of $G$), with respect to a suitable Grothendieck topology that we call the *sipp topology*.

When $H$ contains a Sylow subgroup of~$G$, we use sipp Čech cohomology to describe the kernel and the image of the homomorphism $T(G)\to T(H)$, where $T(-)$ denotes the group of endotrivial representations.

## Cite this article

Paul Balmer, Stacks of group representations. J. Eur. Math. Soc. 17 (2015), no. 1, pp. 189–228

DOI 10.4171/JEMS/501