A note on interpolation in the generalized Schur class
  Realization theory for operator colligations on Pontryagin spaces is used to study interpolation and factorization in generalized Schur classes. Several criteria are derived which imply that a given function is almost the restriction of a generalized Schur function. The role of realization theory in coefficient problems is also discussed. 
D. Alpay, T. Constantinescu, A. Dijksma, and J. Rovnyak