Limit multiplicities for ${\rm SL}_2(\mathcal{O}_F)$ in ${\rm SL}_2(\mathbb{R}^{r_1}\oplus\mathbb{C}^{r_2})$

Abstract

We prove that the family of lattices ${\rm SL}_2(\mathcal{O}_F)$, $F$ running over number fields with fixed archimedean signature $(r_1, r_2)$, in ${\rm SL}_2(\mathbb{R}^{r_1}\oplus\mathbb{C}^{r_2})$ has the limit multiplicity property.