Universal Jamison spaces and Jamison sequences for $C_0$-semigroups

TL;DR

This paper characterizes Jamison sequences for operators and $C_0$-semigroups, showing that non-Jamison sequences allow for operators with uncountably many unimodular eigenvalues on certain Banach spaces.

Contribution

It provides an arithmetic characterization of Jamison sequences for $C_0$-semigroups and demonstrates the existence of operators with uncountably many unimodular eigenvalues on spaces with an unconditional Schauder decomposition.

Findings

Characterization of Jamison sequences for $C_0$-semigroups.

Existence of operators with uncountably many unimodular eigenvalues.

Arithmetic criteria for identifying Jamison sequences.

Abstract

An increasing sequence of positive integers $(n_k)_{k\ge 0}$ is said to be a Jamison sequence if the following property holds true: for every separable complex Banach space $X$ and every $T\in \mathcal{B}(X)$ which is partially power-bounded with respect to $(n_k)_{k\ge 0}$, the set $\sigma_p(T)\cap \T$ is at most countable. We prove that a separable infinite-dimensional complex Banach space $X$ which admits an unconditional Schauder decomposition is such that for any sequence $(n_k)_{k\ge 0}$ which is not a Jamison sequence, there exists $T\in \mathcal{B}(X)$ which is partially power-bounded with respect to this sequence and such that the set $\sigma_p(T)\cap \T$ is uncountable. We also investigate the notion of Jamison sequences for $C_0$-semigroups and we give an arithmetic characterization of these sequences.