Where i can find the proof of Ostrowski theorem

Where i can find any proof of next theorem?

Theorem of Ostrowski: Let $\sum_{n=0}^{\infty}a_nz^n$ be a power series with radius of convergence 1, which is analytically continuable beyond unit disk. Then the power series is overconvergent if it possesses Hadamard-Ostrowski gaps.

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