Let Γ be a principal congruence subgroup of SLn (Z) and let σ be an Γ irreducible unitary representation of SO(n). Let Ncus (λ, σ) be the counting function of the eigenvalues of the Casimir operator acting in the space of cusp forms for Γ which transform under SO(n) according to σ. In this paper we Γ prove that the counting function Ncus (λ, σ) satisfies Weyl’s law. Especially, this implies that there exist infinitely many cusp forms for the full modular group SLn (Z). Contents 1. Preliminaries 2. Heat kernel estimates 3. Estimations of the discrete spectrum 4
