Past exam of the mathematics course of the University of Cambridge / 2002 / ib / Paper 3 / 13G / c / Solution

For nonnegative integers $m,n$, put $B(z)=z^m[(z-a)/(1-\bar{a}z){]}^n$. Since $|a|<1$, the denominator has no zero on or inside the unit circle. On $|z|=1$,so $|B(z)|=1$. Consequently $|-a|<|B(z)|$, and Rouché's theorem gives the same interior zero count for $B-a$ as for $B$. The latter has $m$ zeros at zero and $n$ at $a$, with the multiplicities combining when $a=0$. ThereforeThis is the interior zero count for a finite Blaschke product minus a constant. If $m+n=0$, the nonzero constant $1-a$ has zero zeros, so the same conclusion includes that endpoint case.