Idempotents in Intersection of the Kernel and the Image of Locally Finite Derivations and $\mathcal E$-derivations

Abstract

Let $K$ be a field of characteristic zero, $\mathcal A$ a $K$-algebra and $\delta$ a $K$-derivation of $\mathcal A$ or $K$-$\mathcal E$-derivation of $\mathcal A$ (i.e., $\delta=\operatorname{Id}_A-\phi$ for some $K$-algebra endomorphism $\phi$ of $\mathcal A$). Motivated by the Idempotent conjecture proposed in [Z4], we first show that for every idempotent $e$ lying in both the kernel ${\mathcal A}^\delta$ and the image $\operatorname{Im}\delta \!:=\delta ({\mathcal A})$ of $\delta$, the principal ideal $(e)\subseteq \operatorname{Im} \delta$ if $\delta$ is a locally finite $K$-derivation or a locally nilpotent $K$-$\mathcal E$-derivation of $\mathcal A$; and $e{\mathcal A}, {\mathcal A}e \subseteq \operatorname{Im} \delta$ if $\delta$ is a locally finite $K$-$\mathcal E$-derivation of $\mathcal A$. Consequently, the Idempotent conjecture holds for all locally finite $K$-derivations and all locally nilpotent $K$-$\mathcal E$-derivations of $\mathcal A$. We then show that $1_{\mathcal A} \in \operatorname{Im} \delta$, (if and) only if $\delta$ is surjective, which generalizes the same result [GN, W] for locally nilpotent $K$-derivations of commutative $K$-algebras to locally finite $K$-derivations and $K$-$\mathcal E$-derivations $\delta$ of all $K$-algebras $\mathcal A$.