Relative Milnor $K$-groups and differential forms of split nilpotent extensions

Abstract

Let $R$ be a commutative ring and $I\subset R$ be a nilpotent ideal such that the quotient $R/I$ splits out of $R$ as a ring. Let $N$ be a natural number such that ${I^N=0}$. We establish a canonical isomorphism between the relative Milnor $K$-group $K^{M}_{n+1}(R,I)$ and the quotient of the relative module of differential forms $\Omega^n_{R,I}/d\,\Omega^{n-1}_{R,I}$ assuming that $N!$ is invertible in $R$ and that the ring $R$ is weakly $5$-fold stable. The latter means that any $4$-tuple of elements in $R$ can be shifted by an invertible element to become a $4$-tuple of invertible elements.