On the Picard Group of the Stable Module Category for Infinite Groups

We introduce the stable module ∞-category for groups of type Φ as an enhancement of the stable category defined by N. Mazza and P. Symonds. For groups of type Φ that act on a tree, we show that the stable module ∞-category decomposes in terms of the associated graph of groups. For groups that admit a finite-dimensional cocompact model for the classifying space for proper actions, we exhibit a decomposition in terms of the stable module ∞-categories of their finite subgroups. We use these decompositions to provide methods to compute the Picard group of the stable module category. In particular, we provide a description of the Picard group for countable locally finite p-groups.