Homomorphisms of Topological Groups which are Automatically Fiber Bundles? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-18T12:22:57Z http://mathoverflow.net/feeds/question/20183 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/20183/homomorphisms-of-topological-groups-which-are-automatically-fiber-bundles Homomorphisms of Topological Groups which are Automatically Fiber Bundles? Chris Schommer-Pries 2010-04-02T19:24:16Z 2012-05-06T03:35:51Z <p>Suppose I have a surjective homomorphism of topological groups $f:E \to G$. Let K be the kernel of f. The topological group K acts on E in an obvious way. When is this a fiber bundle over G? (It will then be a K-principal bundle over G). </p> <p>I'm writing a paper where I make a claim about when this holds. I thought I had a reference but when I went looking for it, my claim was not in the reference. </p> <p>I don't want to consider examples that are too pathological so lets assume everything is Hausdorff and paracompact. (However if people are familiar with the more general setting, I'd be curious about that too!).</p> <p>Clearly a necessary condition is that $G \cong E/K$, so let's assume this is the case. By homogeneity it is enough to show that f admits a local section in a neighborhood of the identity element of G. So my question is equivalent to asking if there are conditions I can impose on E and G which will ensure that f admit local sections near the identity of G. </p> <p>I know by work of G. Segal ("Cohomology of Topological Groups" Symposia Math. Vol IV 1970 pg 377, in the appendix) that if G is abelian and locally contractible then the sequence $$G \to EG \to BG$$ is of this kind. </p> <p>I want to know: </p> <ol> <li>Does this hold when K is locally contractible?</li> <li>What if K is globally contractible?</li> <li>Are there any simple (but not tautological) conditions I can put on K, E, or G to make this hold?</li> </ol> <p>I'd also like to know some reasonable examples where this fails to be a principal bundle (if there are any). </p> http://mathoverflow.net/questions/20183/homomorphisms-of-topological-groups-which-are-automatically-fiber-bundles/96107#96107 Answer by Ricardo Andrade for Homomorphisms of Topological Groups which are Automatically Fiber Bundles? Ricardo Andrade 2012-05-06T03:35:51Z 2012-05-06T03:35:51Z <p>The following statement follows from results of Palais (see theorem 2.3.3 in "On the existence of slices for actions of non-compact Lie groups"). When $G$ is a Lie group, any principal $G$-bundle (in the sense of Husemoller's book, which does not assume local triviality) whose total space is completely regular is actually locally trivial.</p> <p>It follows that when $H$ is a Lie group, and a closed subgroup of a Hausdorff topological group $G$, then $G\to G/H$ is a locally trivial principal $H$-bundle.</p> <p>In a slightly different direction, it is a theorem of Skljarenko (theorem 15 of "On the topological structure of locally bicompact groups and their quotient spaces") that when $G$ is a locally compact Hausdorff group, and $H$ is a closed subgroup of $G$, then $G\to G/H$ is a Hurewicz fibration. In particular, if $G/H$ is (relatively) locally contractible then $G\to G/H$ is a locally trivial principal $H$-bundle. Again, by a result of Skljarenko, it is also enough to assume that $G/H$ is finite dimensional to conclude local triviality. See also Mostert's "Sections in principal fibre spaces".</p>