The toral contractions and $Γ$-distinguished $Γ$-contractions (2204.08380v3)

Abstract: We consider distinguished varieties in the bidisc $\mathbb D^2$ and the symmetrized bidisc $\mathbb{G}_2$. A commuting pair of Hilbert space operators $(S,P)$ for which $\Gamma=\overline{\mathbb{G}}_2$ is a spectral set is called a \textit{$\Gamma$-contraction}. A commuting pair of contractions $(T_1,T_2)$ is said to be \textit{toral} if there is a toral polynomial $p\in \mathbb C[z_1,z_2]$ such that $p(T_1,T_2)=0$. Similarly, a $\Gamma$-contraction $(S,P)$ is called $\Gamma$-\textit{distinguished} if $(S,P)$ is annihilated by a $\Gamma$-distinguished polynomial in $\mathbb C[z_1,z_2]$. The main results af this article are the following: we find a necessary and sufficient condition such that a toral pair of contractions dilates to a toral pair of isometries. Also, we characterize all $\Gamma$-distinguished $\Gamma$-contractions that admit $\Gamma$-distinguished $\Gamma$-isometric dilations. We determine the distinguished boundary of a distinguished variety in $\mathbb D^2$ and $\mathbb G_2$. Then we prove that a pair of commuting contractions $(T_1,T_2)$ (or a $\Gamma$-contraction $(S,P)$) dilates to a toral pair of isometries (or a $\Gamma$-distinguished $\Gamma$-isometry) if and only if there is a toral polynomial (or $\Gamma$-distinguished polynomial) $p\in \mathbb C[z_1,z_2]$ such that $Z(p) \cap \overline{\mathbb D}^2$ (or $Z(p) \cap \overline{\mathbb G}_2$) is a complete spectral set for $(T_1,T_2)$ (or $(S,P)$). We provide several examples at places to show the contrasts between the theory of toral contractions and $\Gamma$-distinguished $\Gamma$-contractions.