“…This was first discovered by Godefroy and Shapiro, who produced in [19] their well-known criterion that if the eigenvectors of an operator T associated to eigenvalues of modulus greater than 1 and smaller than 1, respectively, span a dense subspace of the space, then T is hypercyclic. This was developed by Bourdon and Shapiro in [13], and then in the works [4], [5], [6]. The notion of frequent hypercyclicity was introduced and investigated there, and the study of operators from the ergodic-theoretic point of view was also developed there, building on early work of Flytzanis [17].…”