Fermionic Dyson expansions and stochastic Duistermaat-Heckman localization on loop spaces

TL;DR

This paper develops a Fermionic calculus approach to analyze iterated operator integrals related to noncommutative geometry and loop spaces, deriving regularity results and stochastic representations, culminating in a localization formula on loop spaces of spin manifolds.

Contribution

It introduces a Fermionic calculus framework to construct an enlarged Hilbert space and semigroup, providing new regularity and stochastic representations for iterated operator integrals in geometric analysis.

Findings

Established regularity results for iterated operator integrals.

Derived a stochastic representation when H is a covariant Laplacian.

Proved a stochastic refinement of the Duistermaat-Heckman localization formula on loop spaces.

Abstract

Given a self-adjoint operator $H\geq 0$ and (appropriate) densely defined and closed operators $P_{1},\dots, P_{n}$ in a Hilbert space $\mathscr{H}$, we provide a systematic study of bounded operators given by iterated integrals \begin{align}\label{oh} \int_{\{ 0\leq s_1\leq \dots\leq s_n\leq t\}}\mathrm{e}^{-s_1H}P_{1}\mathrm{e}^{-(s_2-s_1)H}P_{2}\cdots \mathrm{e}^{-(s_n-s_{n-1})H}P_{n} \mathrm{e}^{-(t-s_n)H}\, \mathrm{d} s_{1} \ldots \mathrm{d} s_{n},\quad t>0. \end{align} These operators arise naturally in noncommutative geometry and the geometry of loop spaces. Using Fermionic calculus, we give a natural construction of an enlarged Hilbert space $\mathscr{H}^{(n)}$ and an analytic semigroup $\mathrm{e}^{-t (H^{(n)}+P^{(n)} )}$ thereon, such that $\mathrm{e}^{-t (H^{(n)}+P^{(n)} )}$ composed from the left with (essentially) a Fermionic integration gives precisely the above iterated operator integral. This formula allows to establish important regularity results for the latter, and to derive a stochastic representation for it, in case $H$ is a covariant Laplacian and the $P_{j}$'s are first-order differential operators. Finally, with $H$ given as the square of the Dirac operator on a spin manifold, this representation is used to derive a stochastic refinement of the Duistermaat-Heckman localization formula on the loop space of a spin manifold.