Complex-analytic implies holomorphic

This article gives the statement, and possibly proof, of a basic fact in complex analysis.
View a complete list of basic facts in complex analysis

Statement

Suppose $U\subset \mathbb{C}$ is a domain and $f:U\to \mathbb{C}$ is a complex-analytic function: for every point $z_0\in U$, there exists a real number $r>0$ and a power series $\sum _{n=0}^{\mathrm{\infty }}{a}_n(z-z_0{)}^n$ such that the power series converges and agrees with $f$ in the ball of radius $r$.

Then, $f$ is a holomorphic function: it is complex-differentiable, and the complex differential is a continuous function. In fact, $f$ is differentiable infinitely often.

Facts used

1. Power series is infinitely differentiable in disk of convergence

Proof

The proof follows directly from fact (1).