Sharp Pointwise Weyl Laws for Schr\"odinger Operators with Singular Potentials on Flat Tori

The Weyl law of the Laplacian on the flat torus $\mathbb{T}^n$ is concerning the number of eigenvalues $\le\lambda^2$, which is equivalent to counting the lattice points inside the ball of radius $\lambda$ in $\mathbb{R}^n$. The leading term in the Weyl law is $c_n\lambda^n$, while the sharp error term $O(\lambda^{n-2})$ is only known in dimension $n\ge5$. Determining the sharp error term in lower dimensions is a famous open problem (e.g. Gauss circle problem). In this paper, we show that under a type of singular perturbations one can obtain the pointwise Weyl law with a sharp error term in any dimensions. Moreover, this result verifies the sharpness of the general theorems for the Schr\"odinger operators $H_V=-\Delta_{g}+V$ in the previous work of the authors, and extends the 3-dimensional results of Frank-Sabin to any dimensions.Comment: 58 pages. To appear in Communications in Mathematical Physic