Characteristic functions of operators similar to unitary operators
Pratibha G. Ghatage · Proceedings of the American Mathematical Society · 1976
Suppose T T denotes an operator which is similar to a unitary operator and θ T {\theta _T} denotes its characteristic function—considered as an operator on H 2 ( D T ) {H^2}({D_T}) . It is proved that θ T {\theta _T} is a bounded operator if and only if it is the inverse of a bounded operator.