On extreme points of measures which implement an isometric embedding of   model spaces
  In 1996 A. Alexandrov solved an isometric embedding problem for model spaces $K_\Theta$ with an arbitrary inner function $\Theta$. We find all extreme points of this convex set of measures in the case when $\Theta$ is a finite Blaschke product, and obtain some partial results for generic inner functions. 
L. Golinskii
math.CV math.FA