Past exam of the mathematics course of the University of Cambridge 2020 ii Paper 4 21F b Solution Created 2026-09-24 Updated 2026-09-29
Let be the surface with boundary obtained by deleting the interiors of the disks. By excision,so and the other relative groups vanish. In the long exact sequence in relative homology, the mapsends an orientation class to , up to an overall choice of sign. It is therefore injective and has cokernel . Exactness gives and a short exact sequenceThis sequence splits because its right-hand group is a free abelian group. Since is connected, we obtain