Extremal functions of the Nevanlinna-Pick problem and Douglas algebras
Tom 105 / 1993
Studia Mathematica 105 (1993), 151-158
DOI: 10.4064/sm-105-2-151-158
Streszczenie
The Nevanlinna-Pick problem at the zeros of a Blaschke product B having a solution of norm smaller than one is studied. All its extremal solutions are invertible in the Douglas algebra D generated by B. If B is a finite product of sparse Blaschke products (Newman Blaschke products, Frostman Blaschke products) then so are all the extremal solutions. For a Blaschke product B a formula is given for the number C(B) such that if the NP-problem has a solution of norm smaller than C(B) then all its extremal solutions are Carleson Blaschke products, i.e. can be represented as finite products of interpolating Blaschke products.
