Sunday, 23 July 2006

rt.representation theory - Interesting representations/cohomology of surface groups?

For purposes of my own, I'm interested in constructing connected spaces, without recourse to geometric realisation or the like, that have non-trivial homotopy groups in dimension 1 and 2 and are not products of Eilenberg-MacLane spaces. This rules out obvious constructions using crossed modules (at least, obvious to me).



One idea is this: take a representation rho:GammagtoSO(3)rho:GammagtoSO(3) of Gammag=pi1(Sigmag)Gammag=pi1(Sigmag), the fundamental group of a compact, connected, orientable surface or genus gg. Then form the associated sphere bundle X=widetildeSigmagtimesrhoS2toSigmagX=widetildeSigmagtimesrhoS2toSigmag.



Then, unless my calculation is wrong, XX is a 2-type space with pi2(X)=mathbbZpi2(X)=mathbbZ, pi1(X)=Gammagpi1(X)=Gammag and kk-invariant ainH3(Gammag,mathbbZ)ainH3(Gammag,mathbbZ).



So my question is,




are there any non-trivial representations rhorho or cohomology classes aa?




and secondarily,




would a non-trivial representation rhorho give rise to a non-trivial kk-invariant in the above situation?


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