nLab model structure on sections

Model structure on sections


Model category theory

model category, model \infty -category



Universal constructions


Producing new model structures

Presentation of (,1)(\infty,1)-categories

Model structures

for \infty-groupoids

for ∞-groupoids

for equivariant \infty-groupoids

for rational \infty-groupoids

for rational equivariant \infty-groupoids

for nn-groupoids

for \infty-groups

for \infty-algebras

general \infty-algebras

specific \infty-algebras

for stable/spectrum objects

for (,1)(\infty,1)-categories

for stable (,1)(\infty,1)-categories

for (,1)(\infty,1)-operads

for (n,r)(n,r)-categories

for (,1)(\infty,1)-sheaves / \infty-stacks

Model structure on sections



Let KK be a small category and F:KModelCatF:K\to ModelCat a pseudofunctor, where ModelCatModelCat is the 2-category of model categories, Quillen adjunctions pointing in the direction of their left adjoints, and mate-pairs of natural isomorphisms. This is sometimes called a Quillen presheaf (with “right” or “left” attached according to whether one is thinking about the left adjoints or the right adjoints in the Quillen adjunctions); we denote the Quillen adjunction F(f)F(f) by f !f *f_! \dashv f^*.

The category of sections of FF is the category of sections KFK \to \int F of its Grothendieck construction. Explictly, a section of FF consists of, for each kKk\in K an object X kF kX_k\in F_k, and for each f:kf:\ell\to k in KK a morphism X f *X kX_\ell \to f^* X_k (or equivalently f !X X kf_! X_\ell \to X_k) satisfying a functoriality condition. The category Sect(F)Sect(F) is the lax limit of the diagram KCatK\to Cat consisting of the left Quillen functors f !f_!, and also the colax limit of the analogous diagram consisting of the right Quillen functors. Accordingly, it is locally presentable if each category F kF_k is.

Projective and injective model structures


If each F kF_k is a combinatorial model category, accessible model category, or tractable model category, then Sect(F)Sect(F) has two model structures that are also combinatorial, accessible, or tractable respectively:

  1. A projective model structure in which a morphism XYX\to Y is a fibration or weak equivalence if each f k:X kY kf_k : X_k\to Y_k is such in F kF_k.
  2. An injective model structure in which a morphism XYX\to Y is a cofibration or weak equivalence if each f k:X kY kf_k : X_k\to Y_k is such in F kF_k.


The combinatorial and tractable cases are Barwick, 2.28 and 2.30. They are a straightforward application of transferred model structures, generalizing the usual constructions of model structures on functors. The accessible case follows similarly using the accessible version of transfer.

These model structures are a presentation of the (,1)(\infty,1)-categorical lax limit of FF. To present the pseudo (homotopy) limit of FF we have to localize them:


The above projective model structure has a left Bousfield localization, called the (projective) homotopy limit model structure, in which the fibrant objects are the projectively fibrant ones that are homotopy cartesian, i.e. for each f:kf:\ell\to k in KK the induced map X f *(X k)f *(RX k)X_\ell \to f^*(X_k) \to f^*(R X_k) is a weak equivalence, where RR denotes a fibrant replacement.


This is Barwick, 4.38.

Reedy model structures

If each F kF_k is a model category (not even necessarily cofibrantly generated) and KK is a Reedy category, then there is also a Reedy model structure on Sect(F)Sect(F) defined analogously to the ordinary case. This is in Balzin18, together with a generalization to any “admissible model semifibration” over a Reedy category.


  • These model structures are presentations of the (,1)(\infty,1)-category of sections (i.e. (co)lax limit) of the induced functor of (,1)(\infty,1)-categories. For the projective model structure this is shown in Harpaz18 (hence also for the injective model structure, since they have the same weak equivalences), which considers more generally sections defined on a simplicial category. For the Reedy model structure it is shown in Balzin18, with the special case of an inverse category being in Spitzweck10.

  • A subcategory of the injective model structure analogous to the homotopy limit model structure is shown in Bergner10 to present the homotopy limit of the corresponding diagram of complete Segal spaces.


  • Clark Barwick, On left and right model categories and left and right Bousfield localizations. Homology, Homotopy and Applications, 12.2, 2010, p. 245-320. pdf, related arxiv preprint

  • Julia E. Bergner, Homotopy limits of model categories and more general homotopy theories, arXiv:1010.0717

  • Markus Spitzweck, Homotopy limits of model categories over inverse index categories, 2010, doi

  • Johnson, Mark W. On modified Reedy and modified projective model structures. Theory Appl. Categ. 24 (2010), No. 8, 179–208.

  • Yonatan Harpaz, Lax limits of model categories, pdf

  • Edouard Balzin, Reedy Model Structures in Families, arXiv:1803.00681

Last revised on April 20, 2023 at 16:45:16. See the history of this page for a list of all contributions to it.