Rational dilation of tetrablock contractions revisited

Abstract: A classical result of Sz.-Nagy asserts that a Hilbert-space contraction operator $T$ can be lifted to an isometry $V$. A more general multivariable setting of recent interest for these ideas is the case where (i) the unit disk is replaced by a certain domain contained in ${\mathbb C}^3$ (called the {\em tetrablock}), (ii) the contraction operator $T$ is replaced by a commutative triple $(T_1, T_2, T)$ of Hilbert-space operators having ${\mathbb E}$ as a spectral set (a tetrablock contraction) . The rational dilation question for this setting is whether a tetrablock contraction $(T_1, T_2, T)$ can be lifted to a tetrablock isometry $(V_1, V_2, V)$ (a commutative operator tuple which extends to a tetrablock-unitary tuple $(U_1, U_2, U)$---a commutative tuple of normal operators with joint spectrum contained in the distinguished boundary of the tetrablock). We discuss necessary conditions for a tetrablock contraction to have a tetrablock-isometric lift. We present an example of a tetrablock contraction which does have a tetrablock-isometric lift but violates a condition previously thought to be necessary for the existence of such a lift. Thus the question of whether a tetrablock contraction always has a tetrablock-isometric lift appears to be unresolved at this time.