Sharp energy decay rates for the damped wave equation on the torus via non-polynomial derivative bound conditions

Abstract: For the damped wave equation on the torus, when some geodesics never meet the positive set of the damping, energy decay rates are known to depend on derivative bounds and growth properties of the damping near the boundary of its support, as well as the geometry of the support of the damping. In this paper we obtain, sometimes sharp, energy decay rates for damping which satisfy more general non-polynomial derivative bounds and growth properties. We also show how these rates can depend on the geometry of the support of the damping. We prove general results and apply them to examples of damping growing exponentially slowly, polynomial-logarithmically, or logarithmically. The decay rates found for these examples interpolate between known sharp rates for purely polynomial damping. The proof relies on estimating the solution at very fine, non-poynomial, semiclassical scales to obtain resolvent estimates, which are then converted to energy decay rates via semigroup theory.