Nilpotent maximal subgroup abelianity theorem for subnormal skew linear groups

Let $D$ be an infinite division ring, let $n$ be a natural number, let $N$ be a subnormal subgroup of $\mathrm{GL}_n(D)$, and let $M$ be a nilpotent maximal subgroup of $N$. Nilpotent maximal subgroup abelianity theorem. If $n=1$ or the center of $D$ contains at least five elements, then $M$ is abelian. This result generalizes earlier work showing abelianity under additional hypotheses and metabelianity without a dimension condition; the source presents it as one of the paper's main results.