The Riesz transform for homogeneous Schrödinger operators on metric cones

Abstract: We consider Schroedinger operators on metric cones whose cross section is a closed Riemannian manifold $(Y, h)$ of dimension $d-1 \geq 2$. Thus the metric on the cone $M = (0, \infty)_r \times Y$ is $dr^2 + r^2 h$. Let $\Delta$ be the Friedrichs Laplacian on $M$ and $V_0$ be a smooth function on $Y$, such that $\Delta_Y + V_0 + (d-2)^2/4$ is a strictly positive operator on $L^2(Y)$, with lowest eigenvalue $\mu^2_0 $ and second lowest eigenvalue $\mu^2_1$, with $\mu_0, \mu_1 > 0$. The operator we consider is $H = \Delta + V_0/r^2$, a Schr\"odinger operator with inverse square potential on $M$; notice that $H$ is homogeneous of degree -2. We study the Riesz transform $T = \nabla H^{-1/2}$ and determine the precise range of $p$ for which $T$ is bounded on $L^p(M)$. This is achieved by making a precise analysis of the operator $(H + 1)^{-1}$ and determining the complete asymptotics of its integral kernel. We prove that if $V$ is not identically zero, then the range of $p$ for $L^p$ boundedness is $$ d/ \Big(min(1+d/2+\mu_0, d \Big) < p < d / \Big(max(d/2-\mu_0, 0) \Big),$$ while if $V$ is identically zero, then the range is $$ 1 < p < d / \Big(max(d/2-\mu_1, 0 \Big).$$ The result in the case $V$ identically zero was first obtained in a paper by H.-Q. Li.