Perturbed Traceless SU(2) Character Varieties of Tangle Sums

TL;DR

This paper develops a method to compute perturbed traceless SU(2) character varieties of tangles and their sums, providing insights into their role in instanton homology and supporting a conjecture about bounding cochains.

Contribution

It introduces a cut-and-paste technique to compute perturbed character varieties of tangle sums, advancing the understanding of their structure in relation to instanton homology.

Findings

Computed perturbed character varieties for a class of tangles.

Constructed perturbed character variety of tangle sums from individual tangles.

Proved a nontriviality result for bounding cochains in the conjecture.

Abstract

If a link $L$ can be decomposed into the union of two tangles $T\cup_{S^2} S$ along a 2-sphere intersecting $L$ in 4 points, then the intersections of perturbed traceless SU(2) character varieties of tangles in a space called the pillowcase form a set of generators for Kronheimer and Mrowka's reduced singular instanton homology, $I^\natural$. It is conjectured by Cazassus, Herald, Kirk, and Kotelskiy that with the addition of bounding cochains, the differential of $I^\natural$ can be recovered from these Lagrangians as well. This article gives a method to compute the perturbed character variety for a large class of tangles using cut-and-paste methods. In particular, given two tangles, $T$ and $S$, Conway defines the tangle sum $T+S$. Given the character varieties of $T$ and $S$, we show how to construct the perturbed character variety of $T+S$. This is done by first studying the perturbed character variety of a certain tangle $C_3$ properly embedded in $S^3$ with 3 balls removed. Using these results, we prove a nontriviality result for the bounding cochains in the conjecture of Cazassus, Herald, Kirk, and Kotelskiy.