Weyl's law for the cuspidal spectrum of SL(n)

Let $\mathrm{\Gamma }$ be a principal congruence subgroup of $S{L}_n(Z)$ and let $\sigma$ be an irreducible representation of SO(n). Let $N(T,\sigma )$ be the counting function of the eigenvalues of the Casimir operator acting in the space of cusp forms for $\mathrm{\Gamma }$ which transform under SO(n) according to $\sigma$. We prove that the counting function $N(T,\sigma )$ satisfies Weyl’s law as $T\to \mathrm{\infty }$. Especially this implies that there exist infinitely many cusp forms for the full modular group $S{L}_n(Z)$.