Boundedly Spaced Subsequences and Weak Dynamics

Abstract: The purpose of this paper is to characterize weak supercyclicity for Hilbert-space contractions, which is shown to be equivalent to characterizing weak supercyclicity for unitary operators$.$ This is naturally motivated by an open question that asks whether every weakly supercyclic power bounded operator is weakly stable (which in turn is naturally motivated by a result that asserts that every supercyclic power bounded operator is strongly stable)$.$ Precisely, weakly supercyclicity is investigated in light of boundedly spaced subsequences as discussed in Lemma 3.1$.$ The main result in Theorem 4.1 characterizes weakly l-sequentially supercyclic unitary operators $U!$ that are weakly unstable in terms of boundedly spaced subsequences of the power sequence ${U^n}.$ Remark 4.1 shows that characterizing any form of weak super-cyclicity for weakly unstable unitary operators is equivalent to characterizing any form of weak supercyclicity for weakly unstable contractions after the Nagy--Foia\c s--Langer decomposition.