EN
An increasing sequence $(n_{k})_{k≥0}$ of positive integers is said to be a Jamison sequence if for every separable complex Banach space X and every T ∈ ℬ(X) which is partially power-bounded with respect to $(n_{k})_{k≥0}$, the set $σ_{p}(T) ∩ 𝕋$ is at most countable. We prove that for every separable infinite-dimensional complex Banach space X which admits an unconditional Schauder decomposition, and for any sequence $(n_{k})_{k≥0}$ which is not a Jamison sequence, there exists T ∈ ℬ(X) which is partially power-bounded with respect to $(n_{k})_{k≥0}$ and has the set $σ_{p}(T) ∩ 𝕋$ uncountable. We also investigate the notion of Jamison sequences for C₀-semigroups and we give an arithmetic characterization of such sequences.