Instanton 2-torsion and Dehn surgeries

Abstract

In our earlier work on $2$-torsion in instanton Floer homology, we considered only integral surgeries on a knot $K\subset S^3$ and showed that the absence of $2$-torsion forces $K$ to be fibered. The present paper extends the result to all rational surgeries. We prove that if the framed instanton homology $I^{♯}(S_r^3(K);\mathbb{Z})$ is $2$-torsion-free for some $r\in {\mathbb{Q}}_{+}$, then $K$ is an instanton L-space knot and $r>2g(K)-1$. Leveraging this $2$-torsion perspective, we also obtain new small-surgery obstructions: If either $S_5^3(K)$ or $S_{11/2}^3(K)$ is $SU(2)$-abelian, then $K$ must be the unknot or the right-handed trefoil. This result sharpens the small-$SU(2)$-abelian surgery theorems of Kronheimer--Mrowka, Baldwin--Sivek, and Baldwin--Li--Sivek--Ye.

30 pages, 5 figures. Comments are welcome