related by the Dold-Kan correspondence
A model category is a context in which we can do homotopy theory or some generalization thereof; two model categories are ‘the same’ for this purpose if they are Quillen equivalent. For example, the classic version of homotopy theory can be done using either topological spaces or simplicial sets. There is a model category of topological spaces, and a model category of simplicial sets, and they are Quillen equivalent.
In short, Quillen equivalence is the right notion of equivalence for model categories — and most importantly, this notion is weaker than equivalence of categories. The work of Dwyer–Kan, Bergner and others has shown that Quillen equivalent model categories present equivalent (infinity,1)-categories.
Let and be model categories and let
Write and for the corresponding homotopy categories.
Notice that may be regarded as obtained by first passing to the full subcategory on cofibrant objects and then inverting weak equivalences, and (being a left Quillen adjoint) preserves weak equivalences between cofibrant objects. Thus, induces a functor
The Quillen adjunction is a Quillen equivalence if the following equivalent conditions are satisfied.
For every cofibrant object and every fibrant object , a morphism is a weak equivalence in precisely when the adjunct morphism is a weak equivalence in .
For every cofibrant object , the composite is a weak equivalence in , and for every fibrant object , the composite is a weak equivalence in , where and denote fibrant and cofibrant resolutions, respectively.
Not every equivalence between homotopy categories of model categories lifts to a Quillen equivalence. An interesting counterexample is given for instance in (Dugger-Shipley 09).
sSet-enriched Quillen equivalences between combinatorial model categories present equivalences between the corresponding locally presentable (infinity,1)-categories. And every equivalence between these is presented by a Zig-Zag of Quillen equivalences. See there for more details.
For standard references see at model category.
An exmaple of an equivalence of homotopy categories of model categories which does not lift to a Quillen equivalence is in