Monday, 7 January 2008

at.algebraic topology - map of manifolds inducing iso on top cohomology, but not surjective on one other cohomology group

I think I have one: consider a torus in $mathbb{R}^{3}$ embedded as a surface of rotation, e.g. rotate a circle of radius $1$ in the zy plane with center $(0,2,0)$ around the $z$ axis. Now put a small sphere with center $(0,2,0)$ and radius $epsilon$. Then the map $T rightarrow S^{2}$ given by projection towards the center of the sphere should give an isomorphism in $H^{2}$ (since the degree is 1) but it is obviously not surjective in $H^{1}$

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