# Trisections, intersection forms and the Torelli group

## TL;DR

This paper explores the relationship between trisections of 4-manifolds, intersection forms, and the Torelli group, demonstrating how certain 4-manifold invariants can be used to understand their topology and classify them.

## Contribution

It establishes an analogy between 3-manifold regluing via the Johnson kernel and 4-manifold trisections, linking invariants to intersection form classification.

## Key findings

- Trisections of 4-manifolds can be related through the Johnson kernel.
- Invariants of homology 3-spheres can obstruct intersection forms of 4-manifolds.
- Casson invariant recovers Rohlin's theorem on spin 4-manifolds.

## Abstract

We apply mapping class group techniques and trisections to study intersection forms of smooth 4-manifolds. Johnson defined a well-known homomorphism from the Torelli group of a compact surface. Morita later showed that every homology 3-sphere can be obtained from the standard Heegaard decomposition of $S^3$ by regluing according to a map in the kernel of this homomorphism. We prove an analogous result for trisections of 4-manifolds. Specifically, if $X$ and $Y$ admit handle decompositions without 1- or 3-handles and have isomorphic intersection forms, then a trisection of $Y$ can be obtained from a trisection of $X$ by cutting and regluing by an element of the Johnson kernel. We also describe how invariants of homology 3-spheres can be applied, via this result, to obstruct intersection forms of smooth 4-manifolds. As an application, we use the Casson invariant to recover Rohlin's Theorem on the signature of spin 4-manifolds.