Bloch's Theorem

Theorem
Let $f: \C \to \C$ be a holomorphic function in the unit disk $\cmod z \le 1$.

Let $\cmod {\map {f'} 0} = 1$.

Then there exists:
 * a disk $D$ of radius $B$
 * an analytic function $\phi$ in $D$ such that $map f {\map \phi z} = z$ for all $z$ in $D$.

where $B > \dfrac 1 {72}$ is an absolute constant.

The lower bound $\dfrac 1 {72}$ is not the best possible.

Also see

 * Landau's Theorem
 * Value of Bloch's Constant
 * Value of Landau's Constant