Spectral characterization of sums of commutators I

Abstract

Suppose $\Cal J$ is a two-sided quasi-Banach ideal of compact operators on a separable infinite-dimensional Hilbert space $\Cal H$. We show that an operator $T\in\Cal J$ can be expressed as finite linear combination of commutators $[A,B]$ where $A\in\Cal J$ and $B\in\Cal B(\Cal H)$ if and only its eigenvalues $(\lambda_n)$ (arranged in decreasing order of absolute value, repeated according to algebraic multiplicity and augmented by zeros if necessary) satisfy the condition that the diagonal operator $\diag\{\frac1n(\lambda_1+\cdots +\lambda_n)\}$ is a member of $\Cal J.$ This answers (for quasi-Banach ideals) a question raised by Dykema, Figiel, Weiss and Wodzicki.