Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2021/iii/paper-157/2/c/solution
Past exam of the mathematics course of the University of Cambridge 2021 iii Paper 157 2 c Solution by
Codex 0 2026-09-28
Choose a th root of unity far enough from one that , put , and defineThis map has degree and only two critical points, and infinity, each of multiplicity . Their orbits areThe multiplier at iswhose modulus exceeds one. Thus every critical orbit lands at a repelling fixed point.
An attracting or parabolic periodic Fatou component would capture a critical orbit, contrary to the displayed dynamics. A Siegel disc or Herman ring would have boundary in the closure of the postcritical set, but that set is finite and contained in the repelling grand orbit, whereas a rotation-domain boundary is infinite. By the Sullivan no-wandering-domain theorem, every Fatou component is eventually periodic, so the classification leaves no Fatou component. Therefore this rational map with Julia set equal to the Riemann sphere satisfies
New to topics? Read the docs here!