Resonances for obstacles in hyperbolic space

Abstract

We consider scattering by star-shaped obstacles in hyperbolic space and show that resonances satisfy a universal bound $\mathrm{Im}\lambda \le -\frac{1}{2}$ which is optimal in dimension $2$. In odd dimensions we also show that $\mathrm{Im}\lambda \le -\frac{\mu }{\rho }$ for a universal constant $\mu$, where $\rho$ is the radius of a ball containing the obstacle; this gives an improvement for small obstacles. In dimensions $3$ and higher the proofs follow the classical vector field approach of Morawetz, while in dimension $2$ we obtain our bound by working with spaces coming from general relativity. We also show that in odd dimensions resonances of small obstacles are close, in a suitable sense, to Euclidean resonances.

37 pages, 10 figures. v2: added dedication to C. S. Morawetz, fixed typos. v3: published version, added section 6.3

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