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.