# Signatures of topological branched covers

## TL;DR

This paper extends classical signature formulas for branched covers to topological four-manifolds with singularities, introducing intersection homology techniques and knot invariants to detect non-ribbon slice knots.

## Contribution

It generalizes signature computations to topological settings with singularities and introduces a new knot invariant-based obstruction for slice knots.

## Key findings

- Extended signature formulas to topological 4-manifolds with singularities.
- Developed a homotopy ribbon obstruction using knot invariants.
- Provided new tools for detecting non-ribbon slice knots.

## Abstract

Let $X^4$ and $Y^4$ be smooth manifolds and $f: X\to Y$ a branched cover with branching set $B$. Classically, if $B$ is smoothly embedded in $Y$, the signature $\sigma(X)$ can be computed from data about $Y$, $B$ and the local degrees of $f$. When $f$ is an irregular dihedral cover and $B\subset Y$ smoothly embedded away from a cone singularity whose link is $K$, the second author gave a formula for the contribution $\Xi(K)$ to $\sigma(X)$ resulting from the non-smooth point. We extend the above results to the case where $Y$ is a {\it topological} four-manifold and $B$ is locally flat, away from the possible singularity. Owing to the presence of non-locally-flat points on $B$, $X$ in this setting is a stratified pseudomanifold, and we use the Intersection Homology signature of $X$, $\sigma_{IH}(X)$. For any knot $K$ whose determinant is not $\pm 1$, a homotopy ribbon obstruction is derived from $\Xi(K)$, providing a new technique to potentially detect slice knots that are not ribbon.