Gromov, M.; Piatetski-Shapiro, I. Nonarithmetic groups in Lobachevsky spaces. Inst. Hautes Études Sci. Publ. Math. No. 66 (1988), 93—103. Vinberg, … Margulis’ amazing arithmeticity theorem says that irreducible lattices in Lie groups of high () rank are arithmetic. But has rank 1, so a question is how to produce non-arithmetic lattices. […]