Abstract. We study homotopy limits for 2-categories using the theory of Quillen model categories. In order to do so, we establish the existence of projective and injective model structures on diagram 2-categories. Using this result, we describe the homotopical behaviour not only of conical limits but also of weighted limits for 2-categories. Finally, homotopy limits are related to pseudo-limits.
Quillen model structures in 2-category theoryThe 2-category of groupoids, functors, and natural transformations admits a model structure in which the weak equivalences are the equivalence of categories and the fibrations are the Grothendieck fibrations [1,5,13]. Similarly, the 2-category of small categories, functors, and natural transformations admits a model structure in which the weak equivalences are the equivalence of categories and the fibrations are the isofibrations, which are functors satisfying a restricted version of the lifting condition for Grothendieck fibrations which involves only isomorphisms [13,19]. Steve Lack has vastly generalised these results by showing that every 2-category K with finite limits and colimits admits a model structure, called here the natural model structure on K, in which the weak equivalences and the fibrations are the equivalences and the isofibrations in K [17]. The notions of equivalence and isofibration for a map in a 2-category are obtained by suitably generalising the notions of equivalence and of isofibration for a functor. We take Lack's theorem as a starting point to study homotopy limits for 2-categories.Our first step is to show that for every small 2-category A and every 2-category K with finite limits and small colimits, the functor 2-category [A, K] admits a model structure in which the weak equivalences are the pointwise equivalences and the fibrations are the pointwise isofibrations. We refer to this model structure as the projective model structure. When K is assumed to be locally presentable, the existence of the projective model structure follows by a result on the lifting of the natural model structure on a 2-category K to 2-categories of algebras for a 2-monad with rank on K [17, Theorem 4.5]. The special form of the 2-category [A, K], however, allows us to avoid assuming that K is locally presentable, and to give a simple proof of the model category axioms for the projective model structure, which does not Date: April 9th, 2007.