Disoriented homology and double branched covers

TL;DR

This paper introduces a practical method to compute the homology and intersection pairing of branched double covers of the 4-ball, extending classical results to more general surfaces and providing new computational tools.

Contribution

It defines disoriented homology for projections of links and surfaces, proving its isomorphism to the cover's homology and generalizing Gordon-Litherland's work to 4-dimensional settings.

Findings

Disoriented homology is isomorphic to the homology of the branched cover.

A pairing on the first disoriented homology equals the intersection pairing.

Generalization of Gordon-Litherland's signature formula to 4-ball surfaces.

Abstract

This paper provides a convenient and practical method to compute the homology and intersection pairing of a branched double cover of the 4-ball. To projections of links in the 3-ball, and to projections of surfaces in the 4-ball into the boundary sphere, we associate a sequence of homology groups, called the disoriented homology. We show that the disoriented homology is isomorphic to the homology of the double branched cover of the link or surface. We define a pairing on the first disoriented homology group of a surface and show that this is equal to the intersection pairing of the branched cover. These results generalize work of Gordon and Litherland, for embedded surfaces in the 3-sphere, to arbitrary surfaces in the 4-ball. We also give a generalization of the signature formula of Gordon-Litherland to the general setting. Our results are underpinned by a theorem describing a handle decomposition of the branched double cover of a codimension-2 submanifold in the $n$-ball, which generalizes previous results of Akbulut-Kirby and others.