Alexander invariants of ribbon tangles and planar algebras

TL;DR

This paper introduces an Alexander invariant for ribbon tangles in 4-dimensional space, connecting topological, algebraic, and diagrammatic approaches, and generalizing classical link invariants.

Contribution

It constructs a new invariant for ribbon tangles that extends classical Alexander invariants and relates to planar algebras and diagrammatic representations.

Findings

The invariant induces a functor compatible with tangle compositions.

It generalizes the Burau-Gassner representation for ribbon braids.

Provides a combinatorial and diagrammatic description of the invariant.

Abstract

Ribbon tangles are proper embeddings of tori and cylinders in the $4$-ball~$B^4$, "bounding" $3$-manifolds with only ribbon disks as singularities. We construct an Alexander invariant $\mathsf{A}$ of ribbon tangles equipped with a representation of the fundamental group of their exterior in a free abelian group $G$. This invariant induces a functor in a certain category $\mathsf{R}ib_G$ of tangles, which restricts to the exterior powers of Burau-Gassner representation for ribbon braids, that are analogous to usual braids in this context. We define a circuit algebra $\mathsf{C}ob_G$ over the operad of smooth cobordisms, inspired by diagrammatic planar algebras introduced by Jones, and prove that the invariant $\mathsf{A}$ commutes with the compositions in this algebra. On the other hand, ribbon tangles admit diagrammatic representations, throught welded diagrams. We give a simple combinatorial description of $\mathsf{A}$ and of the algebra $\mathsf{C}ob_G$, and observe that our construction is a topological incarnation of the Alexander invariant of Archibald. When restricted to diagrams without virtual crossings, $\mathsf{A}$ provides a purely local description of the usual Alexander poynomial of links, and extends the construction by Bigelow, Cattabriga and the second author.