Multiplicities in the length spectrum and growth rate of Salem numbers

Abstract

We prove that mean multiplicities in the length spectrum of a non-compact arithmetic hyperbolic orbifold of dimension $n \geqslant 4$ have exponential growth rate $$ \langle g(L) \rangle \geqslant c \frac{e^{([n/2] - 1)L}}{L^{1 + \delta_{5, 7}(n) }}, $$ extending the analogous result for even dimensions of Belolipetsky, Lal\'in, Murillo and Thompson. Our proof is based on the study of (square-rootable) Salem numbers. As a counterpart, we also prove an asymptotic formula for the distribution of square-rootable Salem numbers by adapting the argument of G\"otze and Gusakova. It shows that one can not obtain a better estimate on mean multiplicities using our approach.