Characteristic Functions of Unitary Colligations

Let 5 and 8 be Krein spaces. By S(3 ,8 ) we denote the (generalized Schur) class of all functions O, defined and holomorphic at z = 0 and with values in L(5,8), the space of bounded Linear operators from 5 to 8 (we write L ( 8 ) for L(3,S)). It is known, see Section 3 below, that any OeS(3,8) is in some neighborhood of z = 0 the characteristic function On of a unitary colligation A = (J?,S,@;U) with outer spaces 3,O and a Krein space R as inner space, that is, there exists a unitary mapping U with matrix representation