English

$S^2$-bundles over 2-orbifolds

Geometric Topology 2013-04-10 v3

Abstract

Let MM be a closed 4-manifold with π2(M)Z\pi_2(M)\cong{Z}. Then MM is homotopy equivalent to either CP2CP^2, or the total space of an orbifold bundle with general fibre S2S^2 over a 2-orbifold BB, or the total space of an RP2RP^2-bundle over an aspherical surface. If π=π1(M)1\pi=\pi_1(M)\not=1 there are at most two such bundle spaces with given action u:πAut(π2(M))u:\pi\to{Aut}(\pi_2(M)). The bundle space has the geometry S2×E2\mathbb{S}^2\times\mathbb{E}^2 (if χ(M)=0\chi(M)=0) or S2×H2\mathbb{S}^2\times\mathbb{H}^2 (if χ(M)<0\chi(M)<0), except when BB is orientable and π\pi is generated by involutions, in which case the action is unique and there is one non-geometric orbifold bundle.

Keywords

Cite

@article{arxiv.1008.4186,
  title  = {$S^2$-bundles over 2-orbifolds},
  author = {Jonathan A. Hillman},
  journal= {arXiv preprint arXiv:1008.4186},
  year   = {2013}
}

Comments

We have completed the determination of which $S^2$-orbifold bundles are geometric, and also computed the second Wu class for each such manifold. Further minor changes have been made (principally to the section on surgery) in the second revision

R2 v1 2026-06-21T16:04:48.947Z