Hey who can help me with this:

Letfbe holomorphic in a neighborhood of a closed disk $\displaystyle \bar{D}$. Then $\displaystyle g(z) = \int_{\delta D}\frac{f(\zeta)d\zeta}{\zeta -z}$ is a holomorphic function on $\displaystyle \mathbb{C}-\bar{D}$.

Why, and which one?