Operator Theory

The tangential trigonometric moment problem on an interval and related topics.- 1. Introduction.- 2. Some lemmas on matrix-valued rational functions.- 3. The main result.- 4. The Nevanlinna-Pick problem.- References.- Maximum entropy and joint norm bounds for operator extensions.- 1. Introduction.- 2. A sharp bound in the 2x2 case.- 3. The maximum entropy method.- 4. An application to integral operators.- References.- Bitangential interpolation for input-output operators of time varying systems: the discrete time case.- 0. Introduction.- 1. Residue calculus and generalized point evaluation.- 2. Pairs of diagonal operators and homogeneous one-sided interpolation.- 3. Bitangential interpolation data set.- 4. Bitangential interpolation in geometric terms.- 5. Intermezzo about admissible Sylvester data sets.- 6. Construction of a particular solution.- 7. Parametrization of all solutions (without norm constraints).- 8. Input-output operators of time-varying systems.- 9. Parametrization of all contractive input-output operators satisfying the bitangential interpolation conditions.- References.- Two-sided tangential interpolation of real rational matrix functions.- 1. Introduction.- 2. Minimal realizations.- 3. Local data.- 4. Two-sided tangential interpolation: existence of real interpolants.- 5. Two-sided tangential interpolation with real-valued data: Description of interpolants.- 6. Degrees of interpolants.- 7. Generalized Nevanlinna-Pick interpolation for real rational matrix functions.- References.- On the spectra of operator completion problems.- 1. Introduction.- 2. Case of finite dimensional spaces.- 3. Case of infinite dimensional spaces.- References.- The exact H2 estimate for the central H? interpolant.- 1. An improved Kaftal-Larson-Weiss estimate.- 2. Some formulas for DB?.- 3. The role of DA?2II0*.- 4. The four block problem.- 5. Optimal solutions.- References.- On mixed H2 - H? tangential interpolation.- 1. Introduction.- 2. Formulas for the central solution.- 3. A state space approach.- 4. Applications of the H2 - H? tangential interpolation problem.- References.- On a completion problem for matrices.- 1. Introduction.- 2. Main theorems in the finite dimensional case.- 3. The full range case.- 4. The proof of the main theorems in the finite dimensional case.- 5. Infinite dimensional case.- References.