Triple linking and rational homology cobordism

Abstract

If a rational homology 3-sphere $M$ bounds a rational homology 4-ball $W$, then the kernel of the inclusion-induced homomorphism $H_1(M;\mathbb{Z})\to H_1(W;\mathbb{Z})$ is a Lagrangian for the $\mathbb{Q}/\mathbb{Z}$-valued torsion linking form $\lambda_2$ on $H_1(M;\mathbb{Z})$. In this short paper, we prove that the Freedman-Krushkal triple torsion linking form $\lambda_3$ (arXiv:2506.11941v3) vanishes on this Lagrangian under the assumption that $H_2(W;\mathbb{Z})=0$. We then pose several questions about topological rational homology cobordism.