Almost invertible operators

Abstract

We prove that a bounded linear operator $T$ is a direct sum of an invertible operator and an operator with at most countable spectrum iff $0\notin\mbox{acc}^{\omega_{1}}\,\sigma(T),$ where $\omega_{1}$ is the smallest uncountable ordinal and $\mbox{acc}^{\omega_{1}}\,\sigma(T)$ is the $\omega_{1}$-th Cantor-Bendixson derivative of $\sigma(T).$