The bitangential matrix Nevanlinna-Pick interpolation problem revisited
  We revisit four approaches to the BiTangential Operator Argument Nevanlinna-Pick (BTOA-NP) interpolation theorem on the right half plane: (1) the state-space approach of Ball-Gohberg-Rodman, (2) the Fundamental Matrix Inequality approach of the Potapov school, (3) a reproducing kernel space interpretation for the solution criterion, and (4) the Grassmannian/Kre\u{\i}n-space geometry approach of Ball-Helton. These four approaches lead to three distinct solution criteria which therefore must be equivalent to each other. We give alternative concrete direct proofs of each of these latter equivalences. In the final section we show how all the results extend to the case where one seeks to characterize interpolants in the Kre\u{\i}n-Langer generalized Schur class $\cS_{\kappa}$ of meromorphic matrix functions on the right half plane, with the integer $\kappa$ as small as possible. 
Joseph A. Ball and Vladimir Bolotnikov
math.CA