Problem
1 2 π ∫ 0 2 π | f ( r e i θ ) | 2 d θ = ∑ n ≥ 0 | a n | 2 r 2 n {\displaystyle {\frac {1}{2\pi }}\int \limits _{0}^{2\pi }\left|f(re^{i\theta })\right|^{2}d\theta =\sum \limits _{n\geq 0}\left|a_{n}\right|^{2}r^{2n}} wobei f ( z ) = ∑ n ≥ 0 a n z n {\displaystyle f(z)=\sum \limits _{n\geq 0}a_{n}z^{n}}