The Lie module and its complexity

TL;DR

This paper determines the complexity of the Lie module for symmetric groups in characteristic p, confirming a conjecture by Erdmann, Lim, and Tan, and linking it to the largest p-power dividing n.

Contribution

It proves the conjecture that the complexity of the Lie module equals the largest p-power dividing n, using homological and representation-theoretic methods.

Findings

Complexity of Lie(n) equals the largest p-power dividing n.

Confirmed a conjecture by Erdmann, Lim, and Tan.

Connected homology computations with module complexity over symmetric groups.

Abstract

The complexity of a module is an important homological invariant that measures the polynomial rate of growth of its minimal projective resolution. For the symmetric group $\Sigma_n$, the Lie module $\mathsf{Lie}(n)$ has attracted a great deal of interest in recent years. We prove here that the complexity of $\mathsf{Lie}(n)$ in characteristic $p$ is $t$ where $p^t$ is the largest power of $p$ dividing $n$, thus proving a conjecture of Erdmann, Lim and Tan. The proof uses work of Arone and Kankaanrinta which describes the homology $\operatorname{H}_\bullet(\Sigma_n, \mathsf{Lie}(n))$ and earlier work of Hemmer and Nakano on complexity for modules over $\Sigma_n$ that involves restriction to Young subgroups.