# Schur Algorithm in The Class {\mathcal {SI}}  of J-contractive Functions Intertwining Solutions of Linear Differential Equations Academic Article

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### abstract

• In the PhD thesis of the second author under the supervision of the third author was defined the class $${\mathcal{SI}}$$ of J-contractive functions, depending on a parameter and arising as transfer functions of overdetermined conservative 2D systems invariant in one direction. In this paper we extend and solve in the class $${\mathcal{SI}}$$, a number of problems originally set for the class $${\mathcal{S}}$$ of functions contractive in the open right-half plane, and unitary on the imaginary line with respect to some preassigned signature matrix J. The problems we consider include the Schur algorithm and the Nevanlinna–Pick interpolation problem. The arguments rely on a correspondence between elements in a given subclass of $${\mathcal{SI}}$$ and elements in $${\mathcal{S}}$$. Another important tool in the arguments is a new result pertaining to the classical tangential Schur algorithm.

### publication date

• October 21, 2012