Ilya Grigoriev
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 Mar 8 awarded Good Answer Jan 19 awarded Nice Answer Jan 15 awarded Notable Question Dec 31 awarded Good Answer Oct 15 awarded Yearling Oct 1 comment Most interesting mathematics mistake? @JimConant: Yes, of course. Aug 4 awarded Nice Answer Jun 10 awarded Popular Question Oct 15 awarded Yearling Jul 2 awarded Curious May 25 comment What is (co)homology, and how does a beginner gain intuition about it? @DanielMcLaury Hatcher's "Algebraic Topology" has a section on the "Dold-Thom theorem" as section 4.K. Look there. May 19 awarded Pundit May 16 comment Action of the mapping class group on separating spheres in a connected sum of aspherical 3-manifolds @RyanBudney I might be misconstruing your comment, but I think you are describing the generators of $Aut(\pi_1(M))$, like on page 9 of the Hatcher-Wahl paper arxiv.org/abs/0709.2173. The map $\Gamma \to Aut(\pi_1(M))$ has a kernel (also described there). I am not sure how all of those things act on $\pi_2(M)$. Even with the elements of $\Gamma$ whose action I understand, I'm not sure whether there is some clever way to show that $2 \cdot [S_1] = 0 \in \pi_2(M)//\Gamma$ (the fact that $g \cdot [S_1] = 0$ was already a surprise to me). May 16 revised Action of the mapping class group on separating spheres in a connected sum of aspherical 3-manifolds added 143 characters in body May 16 comment Action of the mapping class group on separating spheres in a connected sum of aspherical 3-manifolds @user39082 Yes, I do. That is a very good point, thank you. May 16 revised Action of the mapping class group on separating spheres in a connected sum of aspherical 3-manifolds added 193 characters in body May 16 revised Action of the mapping class group on separating spheres in a connected sum of aspherical 3-manifolds added 24 characters in body May 15 comment Action of the mapping class group on separating spheres in a connected sum of aspherical 3-manifolds @user39082: The mapping class group includes an element that rotates the picture I drew by 90 degrees clockwise. This takes $S_1$ to $S_2$, and thus identifies them in $\pi_2(M)//\Gamma$. May 15 answered How to prove the connected sum of two closed aspherical n-manfolds (n >2) is not asperical? May 15 revised Action of the mapping class group on separating spheres in a connected sum of aspherical 3-manifolds added 65 characters in body