Abstract interpolation problem in generalized Schur classes
  An indefinite variant of the abstract interpolation problem is considered. Associated to this problem is a model Pontryagin space isometric operator V. All the solutions of the problem are shown to be in a one-to-one correspondence with a subset of the set of all unitary extenions U of V. These unitary extension U of V are realized as unitary colligations with the indefinite de Branges-Rovnyak space as a state space. 
D. Baidiuk
math.FA