The Gysin braid for $S^3$-actions on manifolds

TL;DR

This paper extends the Gysin sequence for smooth $ ext{S}^3$ actions on manifolds to non-semi-free actions, establishing a Gysin braid that relates cohomology and intersection cohomology, with new exotic terms involved.

Contribution

It introduces a Gysin braid framework for non-semi-free $ ext{S}^3$ actions, generalizing previous semi-free cases and incorporating intersection cohomology and exotic terms.

Findings

Established a Gysin braid relating cohomology and intersection cohomology.

Identified exotic terms depending on fixed point sets and intersection cohomology.

Extended the understanding of cohomological relations for non-free group actions.

Abstract

Given a smooth action of the sphere $\mathbb S^3$ on a manifold $M$, we have previously constructed a Gysin sequence relating the cohomology of the manifold $M$ and that of the orbit space $M/\mathbb S^3$. This sequence involves an exotic term depending on the subset $M^{\mathbb S^1}$. Notice that the orbit space is a stratified pseudomanifold, a kind of singular spaces where intersection cohomology applies. In the case where the the action is semi-free, the first author has already constructed a Gysin sequence relating the cohomology of $M$ and the intersection cohomology of $M/\mathbb S^3$. What happens if the action is not semi-free? This is the goal of this work. The situation is more complicated and we do not find a Gysin sequence but a Gysin braid relating the cohomology of $M$ and the intersection cohomology of $M/\mathbb S^3$. This braid also contains an exotic term depending this time on the intersection cohomology of $M^{\mathbb S^1}$.