The Schur–Potapov Algorithm in the General Matrix Case and Its Application to the Matricial Schur Problem

Abstract

AbstractThis paper is a generalization of the topic handled in Bogner et al. (Oper Theory 1(1):55–95, 2007a, Oper Theory 1(2):235–278, 2007b) where the Schur–Potapov algorithm (SP-algorithm) was handled in the context of non-degenerate $${p\times q}$$ p × q  Schur sequences and non-degenerate $${p\times q}$$ p × q  Schur functions. In particular, the interplay between both types of algorithms was intensively studied there. This was itself a generalization of the classical Schur algorithm (Schur in J Reine Angew Math 148:122–145, 1918) to the non-degenerate matrix case. In treating the matrix case a result due to Potapov (Potapov in Trudy Moskov Mat Obšč 4:125–236, 1955) concerning particular linear fractional transformations of contractive $${p\times q}$$ p × q  matrices was used. For this reason, the notation SP-algorithm was already chosen in Dubovoj et al. (Matricial version of the classical Schur problem, volume 129 of Teubner-Texte zur Mathematik [Teubner Texts in Mathematics], B. G. Teubner Verlagsgesellschaft mbH, Stuttgart, 1992). We are going to introduce both types of SP-algorithms as well for arbitrary $${p\times q}$$ p × q  Schur sequences as for arbitrary $${p\times q}$$ p × q  Schur functions. Again we will intensively discuss the interplay between both types of algorithms. Applying the SP-algorithm, a complete treatment of the matricial Schur problem in the most general case is established. A one-step extension problem for finite $${p\times q}$$ p × q  Schur sequences is considered. Central $${p\times q}$$ p × q  Schur sequences are studied under the view of SP-parameters.