Looking for a Refined Monster

Abstract

We discuss some categorical aspects of the objects that appear in the construction of the Monster and other sporadic simple groups. We define the basic representation of the categorical torus $\mathcal T$ classified by an even symmetric bilinear form $I$ and of the semi-direct product of $\mathcal T$ with its canonical involution. We compute the centraliser of the basic representation of $\mathcal T\rtimes\{\pm1\}$ and find it to be a categorical extension of the extraspecial $2$-group with commutator $I\mod 2$. We study the inertia groupoid of a categorical torus and find that it is given by the torsor of the topological Looijenga line bundle, so that $2$-class functions on $\mathcal T$ are canonically theta-functions. We discuss how discontinuity of the categorical character in our formalism means that the character of the basic representation fails to be a categorical class function. We compute the automorphisms of $\mathcal T$ and of $\mathcal T\rtimes\{\pm1\}$ and relate these to the Conway groups.