A functorial extension of the Magnus representation to the category of three-dimensional cobordisms

TL;DR

This paper introduces a functorial extension of the Magnus representation to 3-dimensional cobordisms, creating a new monoidal functor that generalizes existing representations and connects to Alexander invariants.

Contribution

It constructs a monoidal functor from the cobordism category to pointed Lagrangian relations, extending the Magnus representation and linking it to Alexander invariants.

Findings

Defined the Magnus functor for 3D cobordisms.

Connected the Magnus functor to Alexander invariants.

Provided a factorization formula for the Alexander functor.

Abstract

Let $R$ be an integral domain and $G$ be a subgroup of its group of units. We consider the category $\mathbf{\mathsf{Cob}}_G$ of 3-dimensional cobordisms between oriented surfaces with connected boundary, equipped with a representation of their fundamental group in $G$. Under some mild conditions on $R$, we construct a monoidal functor from $\mathbf{\mathsf{Cob}}_G$ to the category $\mathbf{\mathsf{pLagr}}_R$ consisting of "pointed Lagrangian relations" between skew-Hermitian $R$-modules. We call it the "Magnus functor" since it contains the Magnus representation of mapping class groups as a special case. Our construction is inspired from the work of Cimasoni and Turaev on the extension of the Burau representation of braid groups to the category of tangles. It can also be regarded as a $G$-equivariant version of a TQFT-like functor that has been described by Donaldson. The study and computation of the Magnus functor is carried out using classical techniques of low-dimensional topology. When $G$ is a free abelian group and $R = Z[G]$ is the group ring of $G$, we relate the Magnus functor to the "Alexander functor" (which has been introduced in a prior work using Alexander-type invariants), and we deduce a factorization formula for the latter.