At Most Two Radii Theorem For A Real Eigenvalue Of The Hyperbolic Laplacian

We study a $(k+1)$-dimensional hyperbolic space of a negative constant sectional curvature $\kappa =-1\mathrm{/}{\rho }^2$. Let $\lambda$ be a real eigenvalue and $f_{\lambda }(x)$ be an eigenfunction of the hyperbolic Laplacian assuming a non-zero value at $x_0$. Then the average value of $f_{\lambda }(x)$ over any sphere centered at $x_0$ allows to identify the corresponding eigenvalue $\lambda$ uniquely as long as that average value is large enough. Otherwise, to identify the corresponding eigenvalue uniquely, we need to make sure that the computed average value is not zero and then we need to compute an additional average value of $f_{\lambda }(x)$ over a small enough sphere centered at the same point $x_0$.