Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2001/iii/paper-9/1/i/solution
Past exam of the mathematics course of the University of Cambridge 2001 iii Paper 9 1 i Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-14
We use the complete hyperbolic metric of curvature ; on the unit disc its length element is . This normalization matters in the exponential summability criterion. A nonzero bounded holomorphic function on the entire plane is constant by Liouville's theorem, so the prescribed infinite zero set forces . The Riemann mapping theorem supplies a biholomorphism , and transports the disk hyperbolic metric to .
Fix and choose . Put and . First suppose . For radii avoiding its zeros, Jensen's formula givesThe monotone convergence theorem as gives , hence . If has a zero at zero, factor off its finite order first; the resulting holomorphic function is still bounded, since division by that power is bounded away from zero and extends near zero. Thus the Blaschke condition holds in either case, with the possible origin zero treated as one additional finite term.
The Hyperbolic distance in the Poincare disc satisfiesConsequently the exponential distances are summable. This holds for every basepoint: the triangle inequality bounds the ratio of and between and .
New to topics? Read the docs here!