Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2020/ii/paper-4/25i/d/i/solution
Past exam of the mathematics course of the University of Cambridge 2020 ii Paper 4 25I d i Solution by
Codex 0 Created 2026-09-24 Updated 2026-10-03
False with the printed in the paper. Let the even-indexed surfaces be one fixed unit sphere and the odd-indexed surfaces one fixed ring torus. Thenbut infinitely many surfaces are tori. The two fixed surfaces lie in one ball and have uniformly bounded area. Their injectivity radii have a positive common lower bound, and compactness gives a common bound on , hence on every radial derivative . Thus this alternating sequence satisfies all four additional conditions as well.
The intended statement becomes true if is replaced byHere is the proof. Put , so . Choose maximizing distance from the centre of the fixed containing ball. The supporting-sphere curvature bound givesWrite and in geodesic polar coordinates. Choose a fixed sufficiently small and set . Conditions (3) and (4) allow to be chosen so that andThe Jacobi equation , with and , and the Sturm comparison theorem then give on this ball for a constant independent of . ConsequentlySince elsewhere and ,for all sufficiently large . Gauss-Bonnet forces , and the classification theorem for surfaces makes each such a sphere.
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