Connected essential spectrum: the case of differential forms

Abstract

In this article we prove that, over complete manifolds of dimension $n$ with vanishing curvature at infinity, the essential spectrum of the Hodge Laplacian on differential $k$-forms is a connected interval for $0\le k\le n$. The main idea is to show that large balls of these manifolds, which capture their spectrum, are close in the Gromov-Hausdorff sense to product manifolds. We achieve this by carefully describing the collapsed limits of these balls. Then, via a new generalized version of the classical Weyl criterion, we demonstrate that very rough test forms that we get from the $\epsilon$-approximation maps can be used to show that the essential spectrum is a connected interval. We also prove that, under a weaker condition where the Ricci curvature is asymptotically nonnegative, the essential spectrum on $k$-forms is $[0,\infty )$, but only for $0\le k\le q$ and $n-q\le k\le n$ for some integer $q\ge 1$ which depends the structure of the manifolds at infinity.

The present article is a full generalization of some of the results of the preprint arXiv:1801.02952