Paper detail

The inverse commutant lifting problem: characterization of associated Redheffer linear-fractional maps

It is known that the set of all solutions of a commutant lifting and other interpolation problems admits a Redheffer linear-fractional parametrization. The method of unitary coupling identifies solutions of the lifting problem with minimal unitary extensions of a partially defined isometry constructed explicitly from the problem data. A special role is played by a particular unitary extension, called the central or universal unitary extension. The coefficient matrix for the Redheffer linear-fractional map has a simple expression in terms of the universal unitary extension. The universal unitary extension can be seen as a unitary coupling of four unitary operators (two bilateral shift operators together with two unitary operators coming from the problem data) which has special geometric structure. We use this special geometric structure to obtain an inverse theorem (Theorem 8.4 as well as Theorem 9.3) which characterizes the coefficient matrices for a Redheffer linear-fractional map arising in this way from a lifting problem. When expressed in terms of Hellinger-space functional models (Theorem 10.3), these results lead to generalizations of classical results of Arov and to characterizations of the coefficient matrix-measures of the lifting problem in terms of the density properties of the corresponding model spaces. The main tool is the formalism of unitary scattering systems developed in [18], [45].