Instanton dimensions of knot surgeries over arbitrary fields

TL;DR

This paper derives a general dimension formula for instanton homology of knot surgeries over arbitrary fields, extending previous results and providing new insights into the properties of 3-manifolds obtained from knot surgeries.

Contribution

It generalizes the instanton homology dimension formula from complex numbers to all fields and extends non-abelianity results for certain knot surgeries.

Findings

Dimension formula for instanton homology over any field

Identification of non-$SU(2)$-abelian surgeries for specific parameters

Generalization of the distance-two surgery exact triangle to arbitrary coefficients

Abstract

Suppose $K \subset S^3$ is a knot and suppose $p$ and $q$ are co-prime integers with $q\ge 1$. For any field $\mathbb{K}$, we establish a dimension formula for the framed instanton homology of knot surgeries: $$ \dim I^\sharp(S^3_{p/q}(K); \mathbb{K}) = q \cdot r_{\mathbb{K}}(K) + |p - q \cdot \nu^\sharp_{\mathbb{K}}(K)| $$ for certain integers $r_{\mathbb{K}}(K)$ and $\nu^\sharp_{\mathbb{K}}(K)$, except possibly when $p/q = \nu^\sharp_{\mathbb{K}}(K)$ and $\nu^\sharp_{\mathbb{K}}(K)$ is even. This formula generalizes the result of Baldwin--Sivek from the case $\mathbb{K} = \mathbb{C}$ to arbitrary fields. Based on the result for $\mathbb{K} = \mathbb{Z}/2$, we obtain that $S^3_{p/q}(K)$ is not $SU(2)$-abelian for any knot $K$ other than the unknot and the right-handed trefoil whenever $p/q \in [0,6)$ and $p \in \{ a^e, 2a^e \}$ for some prime number $a$ and natural number $e$, thereby extending existing results for $p/q \in [0,5]$ and $p = a^e$. A byproduct of the techniques developed in this paper is that we generalize the distance-two surgery exact triangle by Culler--Daemi--Xie and Daemi--Miller-Eismeier--Lidman from $\mathbb{Z}/2$ coefficients to any coefficient ring.