I was reading the solution of a problem, and I did not understand one step. Let's consider the power series expansion of a holomorphic function $ f: D\to D$ (where D is the unit open disk) and $f(0)=0$. $
$ f(z)= \sum\limits_{k = 1}^\infty {a_k z^k } $ Well I don't understand why: $ \sum\limits_{k = 1}^\infty {\left| {a_k } \right|^2 } = \mathop {\lim }\limits_{r \to 1} \int\limits_0^{2\pi } {\left| {f\left( {re^{it} } \right)} \right|^2 dt} $
I don't know if this result holds only in this kind of functions, or it's more general (other holomorphic functions)