Sharp Pointwise Weyl Laws for Schrödinger Operators with Singular Potentials on Flat Tori

The Weyl law of the Laplacian on the flat torus 𝕋^n is concerning the number of eigenvalues ≤λ ^2 , which is equivalent to counting the lattice points inside the ball of radius λ in ℝ^n . The leading term in the Weyl law is c_nλ ^n , while the sharp error term O(λ ^n-2) is only known in dimension n≥ 5 . 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. This result establishes the sharpness of the general theorems for the Schrödinger operators H_V=-Δ _g+V in the previous work (Huang and Zhang (Adv Math, arXiv:2103.05531 )) of the authors, and extends the 3-dimensional results of Frank and Sabin (Sharp Weyl laws with singular potentials. arXiv:2007.04284 ) to any dimensions by using a different approach. Our approach is a combination of Fourier analysis techniques on the flat torus, Li–Yau’s heat kernel estimates, Blair–Sire–Sogge’s eigenfunction estimates, and Duhamel’s principle for the wave equation.