T. Bermúdez | Searcharxiv

arXiv2019

Some examples of $m$-isometries

We obtain the admissible sets on the unit circle to be the spectrum of a strict $m$-isometry on an $n$-finite dimensional Hilbert space. This property gives a better picture of the correct spectrum of an $m$-isometry. We determine that the only $m$-isometries on $\mathbb{R}^2$ are $3$-isometries and isometries giving by $\pm I+Q$, where $Q$ is a nilpotent operator. Moreover, on real Hilbert space, we obtain that $m$-isometries preserve volumes. Also we present a way to construct a strict $(m+1)$-isometry with an $m$-isometry given, using ideas of Aleman and Suciu \cite[Proposition 5.2]{AS} on infinite dimensional Hilbert space.

math.FA↗

arXiv2014

On convex-cyclic operators

We give a Hahn-Banach Characterization for convex-cyclicity. We also obtain an example of a bounded linear operator $S$ on a Banach space with $σ_{p}(S^*)=\emptyset$ such that $S$ is convex-cyclic, but $S$ is not weakly hypercyclic and $S^2 $ is not convex-cyclic. This solved two questions of Rezaei in \cite{Rezaei} when $σ_p(S^*)=\varnothing$. %Recently, León-Saavedra and Romero de la Rosa \cite{LeRo} provide an example of a convex-cyclic operator $S$ such that the power $S^n$ fails to be convex-cyclic with $σ_p(S^*)\neq \varnothing$. In fact they solved tree questions posed by Rezaei in \cite{Rezaei}. Moreover, we prove that $m$-isometries are not convex-cyclic and that $\varepsilon$-hypercyclic operators are convex-cyclic. We also characterize the diagonalizable normal operators that are convex-cyclic and give a condition on the eigenvalues of an arbitrary operator for it to be convex-cyclic. We show that certain adjoint multiplication operators are convex-cyclic and show that some are convex-cyclic but no convex polynomial of the operator is hypercyclic. Also some adjoint multiplication operators are convex-cyclic but not 1-weakly hypercyclic.

math.FA↗