  In this paper we would like to show the interrelation between the different
mathematical theories concerning the Schur interpolation problem, contractions
in Hilbert spaces, pseudocontinuation and Darlington synthesis. The main
objects of this article are contractive functions holomorphic in the unit disc
(Schur functions). Here they are considered, on the one hand, as characteristic
functions of contractions in Hilbert spaces and, on the other hand, as transfer
functions of open systems.
