Descente pour les n-champs (Descent for n-stacks)
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We develop the theory of n-stacks (or more generally Segal n-stacks which are $\infty$-stacks such that the morphisms are invertible above degree n). This is done by systematically using the theory of closed model categories (cmc). Our main results are: a definition of n-stacks in terms of limits, which should be perfectly general for stacks of any type of objects; several other characterizations of n-stacks in terms of ``effectivity of descent data''; construction of the stack associated to an n-prestack; a strictification result saying that any ``weak'' n-stack is equivalent to a (strict) n-stack; and a descent result saying that the (n+1)-prestack of n-stacks (on a site) is an (n+1)-stack. As for other examples, we start from a ``left Quillen presheaf'' of cmc's and introduce the associated Segal 1-prestack. For this situation, we prove a general descent result, giving sufficient conditions for this prestack to be a stack. This applies to the case of complexes, saying how complexes of sheaves of $\Oo$-modules can be glued together via quasi-isomorphisms. This was the problem that originally motivated us.
An error in Lemma 6.2 (pointed out by D. Dugger) is partially fixed and 6.1, 6.3, 10.11, 12.1, 14.4, 15.9, 19.4, 21.1 are modified accordingly; the descent 21.1 is now stated only for complexes bounded below
An error in Lemma 6.2 (pointed out by D. Dugger) is partially fixed and 6.1, 6.3, 10.11, 12.1, 14.4, 15.9, 19.4, 21.1 are modified accordingly; the descent 21.1 is now stated only for complexes bounded below