Induced Dirac-Schr\"odinger operators on $S^1$-semi-free quotients

TL;DR

This paper constructs a Dirac-Schr"odinger operator on the orbit space of a semi-free $S^1$-action on a manifold, showing it matches Lott's signature and addressing issues related to the Witt condition.

Contribution

It introduces a new induced Dirac-Schr"odinger operator on stratified orbit spaces, ensuring self-adjointness and index computation even without the Witt condition.

Findings

The operator's index equals Lott's signature under the Witt condition.

The operator is essentially self-adjoint due to a zero order term.

The operator can be split to compute the index without the Witt condition.

Abstract

John Lott has computed an integer-valued signature for the orbit space of a compact orientable $(4k+1)$ manifold with a semi-free $S^1$-action, which is a homotopy invariant of that space, but he did not construct a Dirac type operator which has this signature as its index. In this Thesis, we construct such operator on the orbit space, a Thom-Mather stratified space with one singular stratum of positive dimension, and we show that it is essentially unique and that its index coincides with Lott's signature, at least when the stratified space satisfies the so called Witt condition. We call this operator the induced Dirac-Schr\"odinger operator. The strategy of the construction is to "push down" an appropriate $S^1$-invariant first order transversally elliptic operator to the quotient space. The Witt condition, a topological condition which in this case depends on the codimension of the fixed point set, has various analytic consequences. In particular, when not satisfied, the Hodge-de Rham operator on the quotient space does not need to be essentially self-adjoint and therefore a choice of boundary conditions is required. This choice freedom is not natural in view of the fact that Lott's signature is well defined independently of the Witt condition. The Dirac-Schr\"odinger operator constructed in this Thesis differs from the Hodge-de Rham operator by a zero order term which ensures it to be essentially self-adjoint. Moreover, this zero order term anti-commutes with the chirality involution allowing the whole operator to split so that the index can be computed even if the Witt condition is not satisfied.