Essential spherical isometries

TL;DR

This paper extends the characterization of essential spherical isometries on Hilbert spaces, showing that a recent dimension condition can be removed for cases where the dimension n>1, broadening the understanding of these operators.

Contribution

It proves that the dimension condition in Chavan's characterization of essential spherical isometries is unnecessary when the dimension n exceeds 1.

Findings

The dimension condition in Chavan's theorem is redundant for n>1.

Essential spherical isometries can be characterized without kernel dimension restrictions.

The result generalizes previous characterizations to a broader class of operators.

Abstract

A result due to Williams, Stampfli and Fillmore shows that an essential isometry $T$ on a Hilbert space $\mathcal{H}$ is a compact perturbation of an isometry if and only if ind$(T)\le 0$. A recent result of S. Chavan yields an analogous characterization of essential spherical isometries $T=(T_1,\dots,T_n)\in\mathcal{B}(\mathcal{H})^n$ with dim($\bigcap_{i=1}^n\ker(T_i))\le$ dim$(\bigcap_{i=1}^n\ker(T_i^*))$. In the present note we show that in dimension $n>1$ the result of Chavan holds without any condition on the dimensions of the joint kernels of $T$ and $T^*$.