Published online by Cambridge University Press: 20 November 2018
We study reducibility of representations parabolically induced from discrete series representations of $S{{U}_{n}}(F)$ for $F$ a $p$-adic field of characteristic zero. We use the approach of studying the relation between $R$-groups when a reductive subgroup of a quasi-split group and the full group have the same derived group. We use restriction to show the quotient of $R$-groups is in natural bijection with a group of characters. Applying this to $S{{U}_{n}}(F)\,\subset \,{{U}_{n}}(F)$ we show the $R$ group for $S{{U}_{n}}$ is the semidirect product of an $R$-group for ${{U}_{n}}(F)$ and this group of characters. We derive results on nonabelian $R$-groups and generic elliptic representations as well.