Let \(f\) be nonconstant and holomorphic on an open set containing the closed unit disk, and suppose \[ |f(z)|=1\qquad(|z|=1). \]
Show that \(f\) has a zero in the open unit disk.
Show that for every \(|w_0|<1\), the equation \(f(z)=w_0\) has a root in the open unit disk.
Thus the image of \(f\) contains the unit disk.