Signature for flat unitary bundles over surfaces with boundary

TL;DR

This paper unifies the concepts of signature, Toledo, and rho invariants for flat unitary bundles over surfaces with boundary, extending classical invariants to broader Lie groups and relating them through a signature formula.

Contribution

It introduces a unified framework connecting signature, Toledo, and rho invariants for representations into classical Lie groups, extending the rho invariant beyond U(p).

Findings

Extended rho invariant to classical groups via embeddings into U(p,q)

Unified the invariants through a signature formula for manifolds with boundary

Connected local system signatures with Toledo and rho invariants

Abstract

This paper deals with the representations of the fundamental groups of compact surfaces with boundary into classical simple Lie groups of Hermitian type. We relate work on the signature of the associated local systems of Atiyah-Patodi-Singer, to Burger-Iozzi-Wienhard's Toledo invariant. To measure the difference, we extend Atiyah-Patodi-Singer's rho invariant, initially defined on $\mathrm{U}(p)$, to discontinuous class functions, first on $\mathrm{U}(p,q)$, and then on other classical groups via embeddings into $\mathrm{U}(p,q)$. In this way, we present three different invariants -- signature, Toledo and rho invariant -- in a unifying way, which is a version of the classical signature formula of Atiyah-Patodi-Singer for manifolds with boundary.