Existence of Laurent Series

Theorem
Let $z_0 \in \C$ be a complex number.

Let $R>0$ be a real number.

Let $B'(z_0, R)$ be the open punctured disk at $z_0$ of radius $R$.

Let $f : B'(z_0, R) \to \C$ be holomorphic.

Then there exists a sequence $(a_n)_{n\in\Z}$ such that:
 * $f(z) = \displaystyle \sum_{n = -\infty}^\infty a_n \left({z - z_0}\right)^n$

for all $z \in B'(z_0, R)$.