Rational tetra-inner functions and the special variety of the tetrablock

Abstract

The set \[ \overline{\mathbb{E}}= \{ x \in {\mathbb{C}}^3: \quad 1-x_1 z - x_2 w + x_3 zw \neq 0 \mbox{ whenever } |z| < 1, |w| < 1 \} \] is called the tetrablock and has intriguing complex-geometric properties. It is polynomially convex, nonconvex and starlike about $0$. It has a group of automorphisms parametrised by ${\mathrm{Aut}~} {\mathbb{D}} \times {\mathrm{Aut}~} {\mathbb{D}} \times {\mathbb{Z}}_2$ and its distinguished boundary $b\overline{\mathbb{E}}$ is homeomorphic to the solid torus $\overline{\mathbb{D}} \times {\mathbb{T}}$. It has a special subvariety \[\mathcal{R}_{\mathbb{\overline{E}}} = \big\{ (x_{1}, x_{2}, x_{3}) \in \overline{\mathbb{E}} : x_{1}x_{2}=x_{3} \big\}, \] called the royal variety of $\overline{\mathbb{E}}$, which is a complex geodesic of ${\mathbb{E}}$ that is invariant under all automorphisms of ${\mathbb{E}}$. We exploit this geometry to develop an explicit and detailed structure theory for the rational maps from the unit disc ${\mathbb{D}}$ to $\overline{\mathbb{E}}$ that map the unit circle ${\mathbb{T}}$ to the distinguished boundary $b\overline{\mathbb{E}}$ of $\overline{\mathbb{E}}$. Such maps are called rational $\mathbb{ \overline{ E}}$-inner functions. We show that, for each nonconstant rational $\mathbb{ \overline{ E}}$-inner function $x$, either $x(\overline{\mathbb{D}}) \subseteq \mathcal{R}_{\mathbb{\overline{E}}} \cap \overline{\mathbb{E}}$ or $x(\overline{\mathbb{D}})$ meets $\mathcal{R}_{\mathbb{\overline{E}}}$ exactly $deg(x)$ times. We study convex subsets of the set $\mathcal{J}$ of all rational $\mathbb{ \overline{ E}}$-inner functions and extreme points of $\mathcal{J}$.