Rational dilation on the symmetrized tridisc: failure, success and unknown

Abstract

The closed symmetrized tridisc $\Gamma_3$ and its distinguished boundary $b\Gamma_3$ are the sets $\Gamma_3=\{ (z_1+z_2+z_3,z_1z_2+z_2z_3+z_3z_1,z_1z_2z_3): \,|z_i|\leq 1, i=1,2,3 \}\subseteq \mathbb C^3$ $b\Gamma_3=\{ (z_1+z_2+z_3,z_1z_2+z_2z_3+z_3z_1,z_1z_2z_3): \,|z_i|= 1, i=1,2,3 \}\subseteq \Gamma_3.$ A triple of commuting operators $(S_1,S_2,P)$ defined on a Hilbert space $\mathcal H$ for which $\Gamma_3$ is a spectral set is called a $\Gamma_3$-contraction. In this article we show by a counter example that there are $\Gamma_3$-contractions which do not dilate. It is also shown that under certain conditions a $\Gamma_3$-contraction can have normal $b\Gamma_3$ dilation. We determine several classes of $\Gamma_3$-contractions which dilate and show explicit construction of their dilations. A concrete functional model is provided for the $\Gamma_3$-contractions which dilate. Various characterizations for $\Gamma_3$-unitaries and $\Gamma_3$-isometries are obtained; the classes of $\Gamma_3$-unitaries and $\Gamma_3$-isometries are analogous to the unitaries and isometries in one variable operator theory. Also we find out a model for the class of pure $\Gamma_3$-isometries. En route we study the geometry of the sets $\Gamma_3$ and $b\Gamma_3$ and provide variety of characterizations for them.