Integral representations of isotropic semi-classical functions and applications

TL;DR

This paper develops integral representations for isotropic semi-classical functions, demonstrating their invariance under Fourier integral operators and expressing them as superpositions of coherent states, with applications to elliptic operators and eigenvalue problems.

Contribution

It provides oscillatory integral formulas for isotropic functions, shows their equivariance under FIOs, and links them to coherent states and spectral analysis.

Findings

Oscillatory integral expressions for isotropic functions

Isotropic functions are superpositions of coherent states

Application to eigenvalue counting functions

Abstract

In \cite{GUW} we introduced a class of "semi-classical functions of isotropic type", starting with a model case and applying Fourier integral operators associated with canonical transformations. These functions are a substantial generalization of the "oscillatory functions of Lagrangian type" that have played major role in semi-classical and micro-local analysis. In this paper we exhibit more clearly the nature of these isotropic functions by obtaining oscillatory integral expressions for them. Then we use these to prove that the classes of isotropic functions are equivariant with respect to the action of general FIOs (under the usual clean-intersection hypothesis). The simplest examples of isotropic states are the "coherent states", a class of oscillatory functions that has played a pivotal role in mathematics and theoretical physics beginning with their introduction by of Schr\"odinger in the 1920's. We prove that every oscillatory function of isotropic type can be expressed as a superposition of coherent states, and examine some implications of that fact. We also show that certain functions of elliptic operators have isotropic functions for Schwartz kernels. This lead us to a result on an eigenvalue counting function that appears to be new (Corollary \ref{cor:altWeyl}).