A note on the Jordan decomposition

Abstract

In this article we prove that the elliptic, hyperbolic and nilpotent (or unipotent) additive (or multiplicative) Jordan components of an endomorphism $X$ (or an isomorphism $g$) of a finite dimensional vector space are given by polynomials in $X$ (or in $g$). By using this, we provide a simple proof that, for an element $X$ of a linear semisimple Lie algebra $\g$ (or $g$ of a linear semisimple connected Lie group $G$), its three Jordan components lie again in the algebra (in the group). This was previously unknown for linear Lie groups other then $\Int(\g)$. This implies that, for this class of algebras and groups, the usual linear Jordan decomposition coincides with the abstract Jordan decomposition.