Explicit presentation of relative Steinberg groups

Abstract: We find an explicit presentation of relative linear Steinberg groups $\mathrm{St}(n, R, I)$ for any ring $R$ and $n \geq 4$ by generators and relations as abstract groups. We also prove a similar result for relative simply laced Steinberg groups $\mathrm{St}(\Phi; R, I)$ for commutative $R$ and $\Phi \in {\mathsf A_\ell, \mathsf D_\ell, \mathsf E_\ell \mid \ell \geq 3}$.