On the differentiability of T(r,f)

Abstract

It is well known that T(r,f) is differentiable at least for r>r0. We show that, in fact, T(r,f) is differentiable for all but at most one value of r, and if T(r,f) fails to have a derivative for some value of r, then f is a constant times a quotient of finite Blaschke products.

Keywords

• Nevanlinna theory
• Nevanlinna characteristic function.