On the chain of commuting operators on Banach spaces and Lomonosov's invariant subspace theorem

Abstract

An operator $T$ on a Banach space is said to be of chain $N$ if there exist non-scalar operators $S_1,...,S_{N-1}$ and a non-zero compact $K$ such that $$T \leftrightarrow S_1 \leftrightarrow S_2 \leftrightarrow ...\leftrightarrow S_{N-1} \leftrightarrow K,$$ where $A\leftrightarrow B$ means $AB=BA$. A connection of this theory to the Lomonosov's Invariant Subspace Theorem is highlighted. It is shown that for every weighted shift $T$ it is of chain $3$. In particular, every non-Lomonosov operator from from the work of Hadwin et al. is of chain $3$. An example of an operator on a separable Hilbert space is given, such that it fails to be connected to a compact operator via a chain of any length.