On Similarity to Contraction Semigroups in Hilbert Space

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-semigroup in Hilbert space, then T(t) is similar to a contraction semigroup. Analogously, if T is a power-bounded quasi-compact operator, then T is similar to a contraction. This latter fact is a simultaneous generalization of well known results of Sz.-Nagy and Rota. Some characterizations of bounded quasi-compact semigroups and power-bounded quasi-compact operators are also given.