Images of Ideals under Derivations and $\mathcal E$-Derivations of Univariate Polynomial Algebras over a Field of Characteristic Zero

Abstract

Let $K$ be a field of characteristic zero and $x$ a free variable. A $K$-$\mathcal E$-derivation of $K[x]$ is a $K$-linear map of the form $\operatorname{I}-\phi$ for some $K$-algebra endomorphism $\phi$ of $K[x]$, where $\operatorname{I}$ denotes the identity map of $K[x]$. In this paper we study the image of an ideal of $K[x]$ under some $K$-derivations and $K$-$\mathcal E$-derivations of $K[x]$. We show that the LFED conjecture proposed in [Z4] holds for all $K$-$\mathcal E$-derivations and all locally finite $K$-derivations of $K[x]$. We also show that the LNED conjecture proposed in [Z4] holds for all locally nilpotent $K$-derivations of $K[x]$, and also for all locally nilpotent $K$-$\mathcal E$-derivations of $K[x]$ and the ideals $uK[x]$ such that either $u=0$, or $\operatorname{deg}\, u\le 1$, or $u$ has at least one repeated root in the algebraic closure of $K$. As a bi-product, the homogeneous Mathieu subspaces (Mathieu-Zhao spaces) of the univariate polynomial algebra over an arbitrary field have also been classified.