On the Gauss-Epple homomorphism of the braid group $B_n$, and generalizations to Artin groups of crystallographic type

Abstract

In this paper, we introduce a broad family of group homomorphisms that we name the Gauss-Epple homomorphisms. In the setting of braid groups, the Gauss-Epple invariant was originally defined by Epple based on a note of Gauss as an action of the braid group $B_n$ on the set $\{1,\dots ,n\}\times \mathbb{Z}$; we prove that it is well-defined. We consider the associated group homomorphism from $B_n$ to the symmetric group $\text{Sym}(\{1,\dots ,n\}\times \mathbb{Z})$. We prove that this homomorphism factors through ${\mathbb{Z}}^n⋊S_n$ (in fact, its image is an order 2 subgroup of the previous group). We also describe the kernel of the homomorphism and calculate the asymptotic probability that it contains a random braid of a given length. Furthermore, we discuss the super-Gauss-Epple homomorphism, a homomorphism which extends the generalization of the Gauss-Epple homomorphism and describe a related 1-cocycle of the symmetric group $S_n$ on the set of antisymmetric $n\times n$ matrices over the integers. We then generalize the super-Gauss-Epple homomorphism and the associated 1-cocycle to Artin groups of finite type. For future work, we suggest studying possible generalizations to complex reflection groups and computing the vector spaces of Gauss-Epple analogues.

22 pages, 13 figures