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Mar 10, 2012 at 14:29 vote accept Francesco Genovese
Mar 10, 2012 at 14:29 comment added Francesco Genovese All right! I shall approve the answer; thanks!
Mar 10, 2012 at 12:55 comment added Thomas Nikolaus On can argue more or less as you say. Given the factorization $A \leftarrow A′ \to B′ \leftarrow B$ this gives a ZigZag in the homotopy category (applying the functor $\dgCat \to Ho(\dgCat)$ where the outer two morphisms are isomorphisms. Thus the morphism in the middle is an isomorphism iff the inner is one, i.e. a weak equuivalence.
Mar 10, 2012 at 12:20 comment added Francesco Genovese Yes, I found myself that the converse was indeed true: in Hovey's book on model categories it is proved in Theorem 1.2.10, part (iv) that whenever a map $f: A \rightarrow B$ in a model category is an isomorphism when viewed as a morphism in the homotopy category, then it is a weak equivalence. Could you please explain, in your argument, how to apply the 2-out-of-3 property? Thanks in advance!
Mar 5, 2012 at 11:55 history edited Thomas Nikolaus CC BY-SA 3.0
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Mar 4, 2012 at 23:30 history answered Thomas Nikolaus CC BY-SA 3.0