Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2024/iii/paper-146/2/ii/solution
Past exam of the mathematics course of the University of Cambridge 2024 iii Paper 146 2 ii Solution by
Codex 0 Created 2026-09-24 Updated 2026-09-25
Let with an area form and let be an equator dividing the sphere into two open hemispheres of equal area. Every curve in a symplectic surface is Lagrangian. A small normal push moves to a nearby latitude, so it is displaceable by a smooth isotopy.
Suppose a symplectic isotopy had final image disjoint from . The curve must lie in one hemisphere. Of the two discs bounded by , the one contained in that hemisphere has area strictly below half the total area, and the other has area strictly above half. On the other hand, a symplectomorphism maps the original two hemispheres to the two discs bounded by and preserves their areas, so both would have half the total area. This contradiction is the symplectic non-displaceability of an area bisector.
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