Triangularizability of trace-class operators with increasing spectrum

TL;DR

This paper investigates whether trace-class kernel operators with increasing spectrum relative to standard compressions have non-trivial invariant standard subspaces, extending previous results from finite-rank and discrete cases.

Contribution

It extends the analysis of invariant subspaces to trace-class kernel operators with increasing spectrum, providing new insights beyond finite-rank and discrete measure space cases.

Findings

Confirmed existence of non-trivial invariant standard subspaces for trace-class operators with increasing spectrum.

Strengthened previous results for finite-rank operators in the context of increasing spectrum.

Extended the scope of invariant subspace results to trace-class kernel operators.

Abstract

For any measurable set $E$ of a measure space $(X, \mu)$, let $P_E$ be the (orthogonal) projection on the Hilbert space $L^2(X, \mu)$ with the range $ran \, P_E = \{f \in L^2(X, \mu) : f = 0 \ \ a.e. \ on \ E^c\}$ that is called a standard subspace of $L^2(X, \mu)$. Let $T$ be an operator on $L^2(X, \mu)$ having increasing spectrum relative to standard compressions, that is, for any measurable sets $E$ and $F$ with $E \subseteq F$, the spectrum of the operator $P_E T|_{ran \, P_E}$ is contained in the spectrum of the operator $P_F T|_{ran \, P_F}$. In 2009, Marcoux, Mastnak and Radjavi asked whether the operator $T$ has a non-trivial invariant standard subspace. They answered this question affirmatively when either the measure space $(X, \mu)$ is discrete or the operator $T$ has finite rank. We study this problem in the case of trace-class kernel operators. We also slightly strengthen the above-mentioned result for finite-rank operators.