# Factorization of Dirac Operators on Almost-Regular Fibrations of $\mathrm{Spin}^c$ Manifolds

### Jens Kaad

Department of Mathematics and Computer Science, University of Southern Denmark, Campusvej 55, 5230, Odense M, Denmark### Walter D. van Suijlekom

Institute for Mathematics, Astrophysics and Particle Physics, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands

## Abstract

We establish the factorization of the Dirac operator on an almost-regular fibration of $\mathrm{spin}^c$ manifolds in unbounded KK-theory. As a first intermediate result we establish that any vertically elliptic and symmetric first-order differential operator on a proper submersion defines an unbounded Kasparov module, and thus represents a class in KK-theory. Then, we generalize our previous results on factorizations of Dirac operators to proper Riemannian submersions of $\mathrm{spin}^c$ manifolds. This allows us to show that the Dirac operator on the total space of an almost-regular fibration can be written as the tensor sum of a vertically elliptic family of Dirac operators with the horizontal Dirac operator, up to an explicit 'obstructing' curvature term. We conclude by showing that the tensor sum factorization represents the interior Kasparov product in bivariant K-theory.

## Cite this article

Jens Kaad, Walter D. van Suijlekom, Factorization of Dirac Operators on Almost-Regular Fibrations of $\mathrm{Spin}^c$ Manifolds. Doc. Math. 25 (2020), pp. 2049–2084

DOI 10.4171/DM/793