Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2007/iii/paper-11/3/a/solution

Use the curvature hyperbolic metric . For the disk Möbius transformation , set . The hyperbolic distance satisfies
The triangle inequality gives
Thus convergence of the exponential sum is independent of the base point. Taking the base point zero, it is equivalent to the Blaschke condition . This condition implies that only finitely many terms lie in any compact subdisk, including finitely many repetitions of any point.
To construct the required holomorphic function, for define the Blaschke factor
For , writing gives
Consequently, after the finitely many factors corresponding to small are removed, the sum of converges uniformly on each compact subdisk. Those factors are uniformly close to one, so their logarithms form an absolutely convergent series there. Their product tail is holomorphic and nonvanishing. Including the finitely many removed factors gives the Blaschke product
with exactly the requested zero multiplicities. Each finite product has modulus at most one in the disk, so the limit does too. Hence maps into the open disk and has exactly the prescribed zeros, proving the implication from the summability condition.

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