Past exam of the mathematics course of the University of Cambridge / 2005 / iii / Paper 18 / 4 / i

Let $p:E\to S^r$ be the Serre fibration. For $r\ge 1$, cover the base by $U=S^r\setminus \{s\}$ and a small open ball $V$ about $s$. Write $A=p^{-1}U$ and $B=p^{-1}V$. The contractible-base assumption gives $A\simeq F$ and identifies the pair $(B,A\cap B)$, up to fiberwise homotopy, with $(F\times V,F\times (V\setminus \{s\}))$.

Excision for the open cover $E=A\cup B$ gives $H_i(E,A)\cong H_i(B,A\cap B)$. The relative base pair hasby the deformation retraction of the punctured ball onto $S^{r-1}$ and its relative homology sequence. The relative Künneth theorem therefore givesThere is no Tor term because the only nonzero base group is free. The long exact sequence of a pair, replacing $A$ by its homotopy-equivalent fiber, now becomesThe last arrow is the relative boundary transported through these identifications; it can encode twisting, so it has not been set to zero. This proves the relative homology of a fibration over a sphere sequence. In the dimension-zero case, the total space is the disjoint union of the two fibers and the same formula is the split sequence for that union.