Is there a long exact sequence associated to a ramified covering? - MathOverflow most recent 30 from http://mathoverflow.net 2013-06-19T15:26:46Z http://mathoverflow.net/feeds/question/57540 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/57540/is-there-a-long-exact-sequence-associated-to-a-ramified-covering Is there a long exact sequence associated to a ramified covering? Gao 2Man 2011-03-06T05:32:08Z 2013-01-28T18:38:40Z <p>A covering map $p:X\to Y$ between topological spaces can be viewed as a fiber bundle $\Sigma\to X\to Y$ with a discrete group $\Sigma=Gal(X/Y)$ as fiber. Such a fiber bundle leads to a long exact sequence of homotopy groups. In this case, if $Y$ is contractible then, of course, so is $X$.</p> <p>I'm wondering what happens if the covering map $p$ is ramified. Is there any relation between the homotopy groups of $\pi_n(X),\pi_n(Y)$ and $\Sigma$? I'm guessing that perhaps the fixed set $X^\Sigma$ might be involved. </p> <p>I'm particularly interested in two cases:</p> <p>1) When $\Sigma=\Sigma_2$, the two-element group. This occurs often in toric topology.</p> <p>2) What conditions can force $Y$ to be contractible (or just weakly null-homotopic). </p> http://mathoverflow.net/questions/57540/is-there-a-long-exact-sequence-associated-to-a-ramified-covering/57584#57584 Answer by John Klein for Is there a long exact sequence associated to a ramified covering? John Klein 2011-03-06T16:47:41Z 2011-03-06T16:47:41Z <p>If by ramified cover, you mean branched cover, then I am skeptical. For example, consider the suspension $\Sigma S^{\infty} \to \Sigma \Bbb RP^{\infty}$ of the double cover $S^\infty \to \Bbb RP^{\infty}$. This is a branched cover with branch locus $S^0$. The total space is contractible, but the homotopy groups of the base space are not completely known.</p> <p>By the way, there's a localization sequence (in the sense of fixed point theory) in <strong>homology</strong> for branched coverings associated with group actions. See for example:</p> <p>Cohomological Methods in Transformation Groups (Cambridge Studies in Advanced Mathematics)<br> by Christopher Allday, Volker Puppe </p> http://mathoverflow.net/questions/57540/is-there-a-long-exact-sequence-associated-to-a-ramified-covering/120138#120138 Answer by Mohammad F.Tehrani for Is there a long exact sequence associated to a ramified covering? Mohammad F.Tehrani 2013-01-28T18:38:40Z 2013-01-28T18:38:40Z <p>There is a paper "on the homology of double branched covers" by Lee, which is kind of related to your question.</p>