# A $2$-compact group as a spets

## TL;DR

This paper explores the connection between exotic 2-compact groups and spetses, proposing a new set of characters for these spaces and proving an analogue of Robinson's weight conjecture in this context.

## Contribution

It introduces a novel set of characters for the exotic 2-compact group DI(4) and establishes a local counting formula analogous to Robinson's weight conjecture.

## Key findings

- Proposes Irr$( extbf{X}(q))$ as a set of characters for the space of homotopy fixed points.
- Includes unipotent characters from complex reflection groups within Irr$( extbf{X}(q))$.
- Proves an analogue of Robinson's weight conjecture for the fusion system Sol$(q)$.

## Abstract

In 1993, Brou\'{e}, Malle and Michel initiated the study of spetses on the Greek island bearing the same name. These are mysterious objects attached to non-real Weyl groups. In algebraic topology, a $p$-compact group $\mathbf{X}$ is a space which is a homotopy-theoretic $p$-local analogue of a compact Lie group. A connected $p$-compact group $\mathbf{X}$ is determined by its root datum which in turn determines its Weyl group $W_\mathbf{X}$. In this article we give strong numerical evidence for a connection between these two objects by considering the case when $\mathbf{X}$ is the exotic $2$-compact group DI$(4)$ constructed by Dwyer--Wilkerson and $W_\mathbf{X}$ is the complex reflection group $G_{24} \cong$ GL$_3(2) \times C_2$. Inspired by results in Deligne--Lusztig theory for classical groups, if $q$ is an odd prime power we propose a set Irr$(\mathbf{X}(q))$ of `ordinary irreducible characters' associated to the space $\mathbf{X}(q)$ of homotopy fixed points under the unstable Adams operation $\psi^q$. Notably Irr$(\mathbf{X}(q))$ includes the set of unipotent characters associated to $G_{24}$ constructed by Brou\'{e}, Malle and Michel from the Hecke algebra of $G_{24}$ using the theory of spetses. By regarding $\mathbf{X}(q)$ as the classifying space of a Benson--Solomon fusion system Sol$(q)$ we formulate and prove an analogue of Robinson's ordinary weight conjecture that the number of characters of defect $d$ in Irr$(\mathbf{X}(q))$ can be counted locally.

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Source: https://tomesphere.com/paper/1906.00898