Explicit spectral gap for Schottky subgroups of $\mathrm{SL}(2,\mathbb{Z})$

Irving Calderón, Michael Magee

Let $\Gamma$ be a Schottky subgroup of $\mathrm{SL}(2,\mathbb{Z})$. We establish a uniform and explicit lower bound of the second eigenvalue of the Laplace-Beltrami operator of congrue…