Refined Weyl law for homogeneous perturbations of the harmonic oscillator

Abstract

Let $H$ denote the harmonic oscillator Hamiltonian on $\mathbb{R}^d,$ perturbed by an isotropic pseudodifferential operator of order $1.$ We consider the Schrödinger propagator $U(t)=e^{-itH},$ and find that while $\operatorname{singsupp} \operatorname{Tr} U(t) \subset 2 π\mathbb{Z}$ as in the unperturbed case, there exists a large class of perturbations in dimension $d \geq 2$ for which the singularities of $\operatorname{Tr} U(t)$ at nonzero multiples of $2 π$ are weaker than the singularity at $t=0$. The remainder term in the Weyl law is of order $o(λ^{d-1})$, improving in these cases the $O(λ^{d-1})$ remainder previously established by Helffer--Robert.