Quasinilpotency of commutators under a weakened Kleinecke--Shirokov condition

Abstract

Kramar and Kramar Fijavž proved that a commutator $Y=AB-BA$ of two complex square matrices is nilpotent whenever $Y$ commutes with an iterated commutator $δ_Y^{k}(A)$, where $δ_Y(T)=YT-TY$, and asked whether this result can be generalized to bounded operators on infinite-dimensional spaces. We show that for $k\geq 2$ such a commutator need not be quasinilpotent. On the other hand, for every $k$ it is quasinilpotent whenever it is compact, or more generally whenever it is a Riesz operator or $AY-YA$ is compact. Moreover, its Riesz projection at any non-zero isolated point of its spectrum always has infinite rank.