7
$\begingroup$

Does there exist a complete classification of all fiber bundles $\Bbb S^k\longrightarrow\Bbb S^d\longrightarrow B$, that is, fibrations of $\smash{\Bbb S^d}$ with each fiber homeomorphic to $\smash{\Bbb S^k}$ for some fixed $k\le d$.

The Wikipedia page on Hopf fibrations contains a list of some real/complex/quaternionic/octonionic fibrations. In other words: is this list complete?

I am then interested, which of the base spaces $B$ that appear in above classification admit a topological/Lie group structure (compatible with its present topology).


Update

The last part of my question about topological/Lie group structure (which should have been a separate question from the start) was partially answered here. The projective spaces listed there are exactly the base spaces of sphere fibrations by great spheres (according to "On fibrations with flat fibres" by Ovsienko and Tabachnikov). It says nothing about the general case, though.

$\endgroup$
11
  • 1
    $\begingroup$ A related reference is Ovsienko-Tabachnikov article on Hopf Fibrations and Hurwitz-Radon Numbers (ovsienko.perso.math.cnrs.fr/Publis/Hopf1.pdf). $\endgroup$
    – F. C.
    Commented Nov 3, 2019 at 12:58
  • $\begingroup$ This story is relevant to the study of the Blaschke conjecture in Riemannian geometry, as it provides some of the topological restrictions on Blaschke manifolds. $\endgroup$
    – Ben McKay
    Commented Nov 3, 2019 at 20:36
  • $\begingroup$ @F.C. Thank you for hinting me to this very well written article! It seems to answer my question for the most part. Footnote 5 of this article reads "The next step would be a classification of affine Hopf fibrations. See [10, 19] for partial results. [...]". This suggests that the full question is still open. It turned out that I am actually already satisfied with a classification of fibrations by great circles, and this was achieved according to the paper "On fibrations with flat fibres.", also by Ovsienko and Tabachnikov. These are exactly the Hopf fibrations also found on Wikipedia. $\endgroup$
    – M. Winter
    Commented Nov 3, 2019 at 21:05
  • $\begingroup$ On whether $B$ can admit a topological group structure, the answer is no. In fact, $B$ is not even an H-space. Indeed, theorem 6.10 in [Browder, W., Torsion in H-spaces. Ann. of Math. (2) 74 (1961), 24–51] says that if an H-space has finitely generated cohomology that vanish is all large degrees, then the first nonzero homotopy group of the space occurs in odd dimension. But $S^8$, $CP^m$, and homology $HP^m$ do not have this property. By Hurewicz theorem, their first nonzero homotopy groups occurs in degrees 8, 2, 4 respectively. $\endgroup$ Commented Nov 5, 2019 at 2:52
  • $\begingroup$ What you linked to at stackexchange answers neither your question there, nor the one here, because in your case $B$ is simply-connected so that it equals to its universal cover. $\endgroup$ Commented Nov 5, 2019 at 15:05

1 Answer 1

13
$\begingroup$

I'll assume that $1\leq k<d$, the other cases being easy. Then the long exact sequence of homotopy groups shows that $B$ is simply connected, so we have a Serre spectral sequence with untwisted coefficients: $$ E_2^{ij} = H^i(B)\otimes H^j(S^k) \Longrightarrow H^{i+j}(S^d), $$ with $d_r\colon E_r^{ij}\to E_r^{i+r,j-r+1}$. Let $u$ and $v$ be the generators of $H^k(S^k)$ and $H^d(S^d)$. The only possible differential is $d_{k+1}\colon H^i(B)u\to H^{i+k+1}(B)$, and this is has the form $au\mapsto ax$ for some $x\in H^{k+1}(B)$. The only way the spectral sequence can converge to $H^*(S^d)$ is if $k$ is odd and $H^*(B)=\mathbb{Z}[x]/x^{r+1}$ with $(r+1)(k+1)=v+1$. We can now apply Adams's theorem on elements of Hopf invariant one to the first attaching map in $B$ to see that $k\in\{1,3,7\}$. I think that there is an argument along similar lines that if $k=7$ we can only have $r\leq 2$, but I don't remember details. Thus, your fibration looks cohomologically like one of the standard fibrations $S^1\to S^{2r+1}\to \mathbb{C}P^r$ or $S^3\to S^{4r+3}\to \mathbb{H}P^r$ or $S^7\to S^{8r+1}\to\mathbb{O}P^r$. For the case $k=1$ we can use $[X,\mathbb{C}P^\infty]=H^2(X)$ to get a map $B\to\mathbb{C}P^\infty$ and check that it restricts to give a homotopy equivalence $B\to\mathbb{C}P^r$. In the other cases I think it is also true that $B$ is homotopy equivalent to $\mathbb{H}P^r$ or $\mathbb{O}P^r$, but the argument is more complicated and again I do not remember details. [UPDATE: See comment below from Igor Belegradek: this last sentence is apparently wrong.]

$\endgroup$
6
  • 8
    $\begingroup$ The original reference is [Browder, William, Higher torsion in H-spaces, Trans. Amer. Math. Soc. 108 (1963), 353–375] where it is shown that the fiber must be $S^1, S^7, S^3$ in which cases the respective bases are homotopy $CP^m$, $S^8$, and (I think) the rational homology $HP^m$. There are some examples in the last case which aren't homotopy $HP^m$. $\endgroup$ Commented Nov 3, 2019 at 13:47
  • $\begingroup$ If $d < k$ it seems like you get $H^*(B) = \mathbb{Z}[x,y]/y^2$ where $|x| = k+1$ and $|y| = d$. It seems like there could be nontrivial examples of this? Or am I missing something? $\endgroup$ Commented Nov 5, 2019 at 17:59
  • $\begingroup$ @KevinCasto we are assuming that we have an actual fibre bundle rather than just a fibration, so $B$ will have dimension $d-k$ and so cannot have cohomology in degrees above that. $\endgroup$ Commented Nov 5, 2019 at 18:11
  • $\begingroup$ @NeilStrickland Fair enough! I do think the fibration question is interesting though. e.g., is there a fibration $S^3 \to S^d \to B$, where $H^*(B) = H^*(S^d \times \mathbb{H}P^\infty)$? $\endgroup$ Commented Nov 5, 2019 at 19:11
  • $\begingroup$ Oh duh, of course there is, because $\mathbb{H}P^\infty = BS^3$. Same with $S^1 \to S^d \to S^d \times \mathbb{C}P^\infty$ $\endgroup$ Commented Nov 5, 2019 at 19:20

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .