Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2006/iii/paper-10/1/solution
Past exam of the mathematics course of the University of Cambridge 2006 iii Paper 10 1 Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-14
A normal family is a family such that every sequence has a subsequence converging locally uniformly in the spherical metric to a meromorphic limit, possibly the constant infinity. For a family uniformly bounded in modulus, the possible limits are finite holomorphic functions, and ordinary locally uniform convergence suffices. The limit need not take values in the open unit disc; a sequence of constants can approach its boundary.
Here is a direct compactness proof for disc-valued maps. For each compact subset , choose a slightly larger compact neighborhood contained in . The Cauchy estimate on a fixed-radius disk around each point bounds independently of because . A finite covering of yields uniform boundedness and equicontinuity there. The Arzelà-Ascoli theorem, applied successively to a compact exhaustion and followed by a diagonal subsequence, gives a limit uniformly on every compact. The Cauchy integral formula passes to that limit on small circles, proving it is a holomorphic function. This proves the required normal family assertion rather than just invoking Montel theorem.
Every is a holomorphic function on the complex upper half-plane with values in the unit disc, so the same argument makes a normal family. If a subsequence converges locally uniformly to , then for every ,The identity theorem for holomorphic functions gives . Since every convergent subsequence has this same limit, the whole sequence converges locally uniformly to : otherwise a compact set and a subsequence with a fixed positive discrepancy would have a further locally uniformly convergent subsequence, a contradiction.
To obtain the nontangential limit, take any with and . Put . Then , , and the points lie, for large , in a fixed compact rectangle inside the half-plane. ThereforeThis is the normal-family proof of angular boundary convergence; using integer rescalings requires this step because the points need not lie exactly on one fixed scaled ray.
For a tangential counterexample, useIf with , then , so is a holomorphic function and . Also . ButTheir ratio tends to infinity. Thus the nontangential limit is zero while the unrestricted boundary limit does not exist.
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