English

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

Functional Analysis 2019-06-03 v2

Abstract

The question if 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 [G16], cyclic polynomially bounded operators which are not similar to contractions was constructed. The construction was 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. In this paper, a cyclic polynomially bounded operator T0T_0 such that T0T_0 is not similar to a contraction and ωa(T0)=O\omega_a(T_0)=\mathbb O, is constructed. Here ωa(z)=exp(az+1z1)\omega_a(z)=\exp(a\frac{z+1}{z-1}), zDz\in\mathbb D, a>0a>0, and D\mathbb D is the open unit disk. To obtain such T0T_0, a slight modification of the construction from [G16] is needed.

Keywords

Cite

@article{arxiv.1803.03626,
  title  = {Examples of cyclic polynomially bounded operators that are not similar to contractions, II},
  author = {Maria Gamal'},
  journal= {arXiv preprint arXiv:1803.03626},
  year   = {2019}
}

Comments

Submitted to Acta Sci. Math. (Szeged). Lemma 2.3 is improved, Lemma 2.5 is added, misprint in the proof of Lemma 2.12 (old 2.11) is corrected. Improved Sec. 4 will appear separately