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May 13, 2015 at 18:03 comment added Fernando Muro Warning! Contains self promotion. You can find a full proof in arxiv.org/abs/1111.2723 I phrase it in dual terms (epi instead of mono) but it's equivalent by taking opposite model category.
May 13, 2015 at 17:36 comment added JMP i think, but i haven't got the details, you can use the next Lemma in the pdf (Lemma 2.4) as a stepping stone. i.e. prove Map->Map implies and is implied by Lemma2.4, then prove x->x*x implies and is implied by Lemma2.4, then you have your equivalence.
May 13, 2015 at 17:22 answer added Qiaochu Yuan timeline score: 5
May 13, 2015 at 17:17 answer added AAK timeline score: 9
May 13, 2015 at 17:06 comment added Tyler Lawson The key tool that you may be missing is the Mayer-Vietoris exact sequence for the homotopy groups of a homotopy pullback (mathoverflow.net/questions/3398/…)
May 13, 2015 at 16:53 comment added Qiaochu Yuan As a warmup, show that a morphism $f : x \to y$ in an ordinary category is a monomorphism iff $x \times_y x$ exists and $x \to x \times_y x$ is an isomorphism.
May 13, 2015 at 16:15 history asked Zhaoting Wei CC BY-SA 3.0