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for questions about fiber bundles, including structure groups, principal bundles, and spaces of sections.

3 votes
Accepted

Reduction of the structure group of $\mathbb{R}^n$-fiber bundles to a special subgroup of $\...

Your group $G$ consists of homeomorphisms of $\Bbb R^n$ which induce homeomorphisms on $T^n$ with induced map $f_*: \pi_1(T^n) \to \pi_1(T^n)$ equal to the identity. (If $f$ satisfied $f(x+m) = f(x)+g …
mme's user avatar
  • 9,580
9 votes

Classifying space of semidirect product of groups

I am adding my comment as an answer. Every extension of groups $1 \to H \to G \to K \to 1$ corresponds to a fibration $$BH \to BG \to BK,$$ or a little more precisely at the space level $$EG/H \to (E …
mme's user avatar
  • 9,580
2 votes
Accepted

Free $S^1$-action on compact homogeneous spaces

Here is a counterexample where $K$ is connected. Let $W = SU_3/SO_3$ be the Wu manifold. This 5-dimensional manifold is simply-connected and has $H_2(W;\Bbb Z) = \Bbb Z/2$, and in particular, $\pi_2(W …
mme's user avatar
  • 9,580