A presentation of relative unitary Steinberg groups

Abstract: We find an explicit presentation of relative odd unitary Steinberg groups constructed by odd form rings and of relative doubly laced Steinberg groups over commutative rings, i.e. the Steinberg groups associated with the Chevalley group schemes of the types $\mathsf B_\ell$, $\mathsf C_\ell$, $\mathsf F_4$ for $\ell \geq 3$. For simply laced root systems such result is already known.