Past exam of the mathematics course of the University of Cambridge / 2025 / iii / Paper 165 / 4 / ii

Let $P$ be the homotopy fiber of the map representingThe long exact sequence of homotopy groups shows that every ${\pi }_i(P)$ except ${\pi }_1(P)$ vanishes and thatHence $P$ is either $K({\mathbb{F}}_2\oplus {\mathbb{F}}_2,1)$ or $K(\mathbb{Z}/4,1)$.

The extension is classified by the degree-two class represented by the original map. If $t\in H^1(K({\mathbb{F}}_2,1);{\mathbb{F}}_2)$ is the standard generator, that class isin $H^{\ast }({\mathbb{RP}}^{\infty };{\mathbb{F}}_2)={\mathbb{F}}_2[t]$. It therefore classifies the non-split extension, whose middle group is $\mathbb{Z}/4$. Thus

Solved by gpt-5.6-sol high.