Overgroups of elementary block-diagonal subgroups in the classical symplectic group over an arbitrary commutative ring

In this paper we prove a sandwich classification theorem for subgroups of the classical symplectic group over an arbitrary commutative ring $R$ that contain the elementary block-diagonal (or subsystem) subgroup $\operatorname{Ep}(\nu, R)$correspondingtoaunitaryequivalencerealation$\nu$ such that all self-conjugate equivalence classes of $\nu$ are of size at least 4 and all not-self-conjugate classes of $\nu$ are of size at least 5. Namely, given a subgroup $H$ of $\mathrm{Sp}(2n,R)$ such that $\operatorname{Ep}(\nu, R) \le H$weshowthatthereexistsauniqueexactmajorformnetofideals$(\sigma, \Gamma)$over$R$suchthat$\operatorname{Ep}(\sigma, \Gamma) \le H \le N_{\operatorname{Sp}(2n,R)}(\operatorname{Sp}(\sigma, \Gamma))$. Further, we describe the normalizer $N_{\mathrm{Sp}(2n,R)}(\mathrm{Sp}(\sigma ,\mathrm{\Gamma }))$ in terms of congruences.