Splitting in a complete local ring and decomposition its group of units

Abstract

Let $(R,M,k)$ be a complete local ring (not necessarily Noetherian). As the first main result of this article, we prove that in the unequal characteristic case $\Char(R)\neq\Char(k)$, the natural surjective map between the groups of units $R^{\ast }\to k^{\ast }$ admits a splitting. Next, we reprove by a new method that $R$ is equi-characteristic, i.e., $\Char(R)=\Char(k)$ if and only if the natural surjective ring map $R\to k$ admits a splitting, or equivalently, $R$ has a coefficient field. In our proof there is no need for the existence of the coefficient fields for equi-characteristic complete local rings, whose existence is the most difficult part of the known proof. This is one of the main contributions of the article. \\ As an application, we show that for any complete local ring $(R,M,k)$ the following short exact sequence of Abelian groups: $$\xymatrix{1\ar[r]&1+M\ar[r]& R^{\ast}\ar[r]&k^{\ast} \ar[r]&1}$$ is always split. In particular, we have an isomorphism of Abelian groups $R^{\ast }\simeq (1+M)\times k^{\ast }$. We also show with an example that the above exact sequence does not split for many incomplete local rings.

16 pages