Past exam of the mathematics course of the University of Cambridge 2019 ii Paper 4 21F Solution 2026-10-03
For a continuous self-map of a finite CW complex, its Lefschetz number over isThe Lefschetz fixed-point theorem states that implies that has a fixed point. Consequently every fixed-point-free self-map has Lefschetz number zero.
The space in the question is a cyclic point-gluing of two-spheres. Before gluing, the spheres have total Euler characteristic . Each of the point identifications lowers the Euler characteristic by one, soEquivalently, , with rational homology dimensions in degrees zero, one, and two.