Definition. A Schur function
is defined as a complex function analytic and of modulus not exceeding
unity in
.
Property. The function
Proof.
Suppose is positive real. Then for , rere is PR. Consequently, is minimum phase which implies all roots of lie in the unit circle. Thus is analytic in . Also,
By the maximum modulus theorem, takes on its maximum value in on the boundary. Thus is Schur.
Conversely, suppose is Schur. Solving Eq. (C.84) for and taking the real part on the unit circle yields
If is constant, then is PR. If is not constant, then by the maximum principle, for . By Rouche's theorem applied on a circle of radius , , on which , the function has the same number of zeros as the function in . Hence, is minimum phase which implies is analytic for . Thus is PR.