Smooth Billiards with a Large Weyl Remainder

Abstract

We prove an estimate from below for the remainder in Weyl's law for smooth star-shaped planar domains admitting an appropriate one-parameter family of periodic billiard trajectories. Examples include ellipses and smooth domains of constant width. Our results confirm a prediction of P. Sarnak who proved a similar statement for surfaces without boundary. The proof is based on the analysis of the singularity in the wave trace, occurring at time T>0T>0 equal to the length of the closed trajectories belonging to the family. Such families of periodic orbits produce large singularities in the wave trace, which in turn imply estimates from below for the absolute value of the Weyl remainder averaged over a spectral window. We also obtain lower bounds for the error term in higher dimensions. In this case, the main contribution to the Weyl remainder typically comes from the singularity at zero of the wave trace. However, for certain domains, such as the Euclidean ball, the dimension of the family of periodic trajectories is large enough to dominate the contribution of the singularity at zero.