Complete systems of unitary invariants for some classes of $2$-isometries

Abstract

The unitary equivalence of $2$-isometric operators satisfying the so-called kernel condition is characterized. It relies on a model for such operators built on operator valued unilateral weighted shifts and on a characterization of the unitary equivalence of operator valued unilateral weighted shifts in a fairly general context. A complete system of unitary invariants for $2$-isometric weighted shifts on rooted directed trees satisfying the kernel condition is provided. It is formulated purely in the langauge of graph-theory, namely in terms of certain generation branching degrees. The membership of the Cauchy dual operators of $2$-isometries in classes $C_{0\cdot }$ and $C_{\cdot 0}$ is also studied.

The paper has 25 pages and 2 figures. It is the second part of arXiv:1702.01264v3 [math.FA] which focuses on unitary equivalence of some classes of $2$-isometries