Let $n$ be an integer greater than or equal to $3$ and $(R,\Delta)$ a Hermitian form ring where $R$ is commutative. We prove that if $H$ is a subgroup of the odd-dimensional unitary group $U_{ 2n+1}(R,\Delta)$ normalised by a relative elementary subgroup $EU_{ 2n+1}((R,\Delta),(I,\Omega))$, then there is an odd form ideal $(J,\Sigma)$ such that $EU_{ 2n+1}((R,\Delta),(JI^{ k},\Omega_{ \min}^{ JI^k}\overset{ \cdot}{ +}\Sigma\circ I^{ k}))\leq H \leq CU_{ 2n+1}((R,\Delta),(J,\Sigma))$ where $k=12$ if $n=3$ respectively $k=10$ if $n\geq 4$. As a conseqence of this result we obtain a sandwich theorem for subnormal subgroups of odd-dimensional unitary groups.
Categories: math.KT