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| | 285G, Lecture 5: Finite time extinction of the third homotopy group, I « What’s new |
 | | As a consequence, every j-cycle, being the combination of singular simplices in a singular complex involving singular simplices of dimension at most j, can be expressed as the boundary of a (j+1)-chain, and the claim follows. |
 | | Of course, being 3-dimensional, all higher homology groups vanish, and so M is a homology sphere. |
 | | To be precise, a map between simply-connected spaces which is an isomorphism on integral homology groups is an isomorphism on homotopy groups, by the relative Hurewicz theorem; and a map between connected CW complexes which is an isomorphism on homotopy groups is a homotopy equivalence, by Whitehead’s theorem. |
| terrytao.wordpress.com /2008/04/15/285g-lecture-5-finite-time-extinction-of-the-third-homotopy-group-i (4734 words) |
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