Cusps of hyperbolic 4-manifolds and rational homology spheres

TL;DR

This paper constructs a hyperbolic 4-manifold with specific cusp sections that are rational homology spheres, demonstrating spectral properties and addressing classification issues in flat 3-manifolds.

Contribution

It constructs a hyperbolic 4-manifold with Hantzsche-Wendt cusp sections and refines the classification of certain flat 3-manifolds, addressing open questions.

Findings

The Laplacian on 2-forms has purely discrete spectrum.

The constructed manifold's cusp sections are rational homology spheres.

Refinement of the classification of flat 3-manifolds from cube colorings.

Abstract

In the present paper, we construct a cusped hyperbolic $4$-manifold with all cusp sections homeomorphic to the Hantzsche-Wendt manifold, which is a rational homology sphere. By a result of Gol\'enia and Moroianu, the Laplacian on $2$-forms on such a manifold has purely discrete spectrum. This shows that one of the main results of Mazzeo and Phillips from 1990 cannot hold without additional assumptions on the homology of the cusps. This also answers a question by Gol\'enia and Moroianu from 2012. We also correct and refine the incomplete classification of compact orientable flat $3$-manifolds arising from cube colourings provided earlier by the last two authors.