Some sufficient conditions for existence of hyperinvariant subspaces for operators intertwined with unitaries

Maria F. Gamal'

Published 2017-12-19Version 1

For a power bounded or polynomially bounded operator $T$ sufficient conditions for the existence of a nontrivial hyperinvariant subspace are given. The obtained hyperinvariant subspaces of $T$ have the form of the closure of the range of $\varphi(T)$. Here $\varphi$ is a singular inner function, if $T$ is polynomially bounded, or $\varphi$ is an analytic in the unit disc function with absolutely summable Taylor coefficients and singular inner part, if $T$ is supposed to be power bounded only. Also, an example of a quasianalytic contraction $T$ is given. The quasianalytic spectral set of $T$ is not the whole unit circle $\mathbb T$, while $\sigma(T)=\mathbb T$. Proofs are based on results by Esterle, Kellay, Borichev and Volberg.