Two-Parameter Novikov-Shubin Invariants for Fibre Bundles

In this paper we construct a two-parameter version of spectral density functions and Novikov-Shubin invariants on fibre bundles. The aim of this approach is to gain a better understanding of how the near-zero spectrum of the Hodge Laplace operators on the fibre and the base of a fibre bundle contribute separately to the near-zero spectrum of the Laplace operators of the total space. We show that this two-parameter generalisation of the classical spectral density function still satisfies several invariance properties. As an example, we compute it explicitly for the three-dimensional Heisenberg group.