Published online by Cambridge University Press: 20 November 2018
Let $\Gamma $ be a rank-one arithmetic subgroup of a semisimple Lie group
$G$. For fixed
$K$-Type, the spectral side of the Selberg trace formula defines a distribution on the space of infinitesimal characters of
$G$, whose discrete part encodes the dimensions of the spaces of square-integrable
$\Gamma $-automorphic forms. It is shown that this distribution converges to the Plancherel measure of
$G$ when
$\Gamma $ shrinks to the trivial group in a certain restricted way. The analogous assertion for cocompact lattices
$\Gamma $ follows from results of DeGeorge-Wallach and Delorme.