ACTA issues

Examples of cyclic polynomially bounded operators that are not similar to contractions

Maria F. Gamal'

Acta Sci. Math. (Szeged) 82:3-4(2016), 597-628

Abstract. The question if a polynomially bounded operator is similar to a contraction was posed by Halmos and was answered in the negative by Pisier. His counterexample is an operator of infinite multiplicity, while all its restrictions on invariant subspaces of finite multiplicity are similar to contractions. In the paper, cyclic polynomially bounded operators which are not similar to contractions and are quasisimilar to $C_0$-contractions or to isometries are constructed. The construction is based on a perturbation of the sequence of finite dimensional operators which is uniformly polynomially bounded, but is not uniformly completely polynomially bounded, constructed by Pisier.

DOI: 10.14232/actasm-016-016-4

AMS Subject Classification (1991): 47A65, 47A60, 47A16, 47A20, 47A55

Keyword(s): polynomially bounded operator, similarity, contraction, unilateral shift, isometry, $C_0$-contraction, $C_0$-operator

Received March 10, 2016, and in revised form August 15, 2016. (Registered under 16/2016.)