6
$\begingroup$

There is hint: Prove $H^{n-1}(N) \to H^{n-1}(M)$ is trivial. Just don't know how to prove this.

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    Further hint: Think about homology instead of cohomology. What *are* homology classes, anyways?2011-06-24
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    Related: [Is there a retraction of a non-orientable manifold to its boundary?](http://math.stackexchange.com/q/880016/10014), [Retraction to the Boundary on Compact Manifold](http://math.stackexchange.com/q/1257814/10014), [A manifold such that its boundary is a deformation retract of the manifold itself.](http://math.stackexchange.com/q/1359563/10014)2016-05-25

2 Answers 2