nLab Dwyer map



Category theory

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Basic facts




A Dwyer map is a type of cofibration in the category of small categories designed for the purpose of constructing homotopy invariant colimits with respect to weak homotopy equivalences, i.e. weak equivalences created from weak homotopy equivalences of simplicial sets by the nerve functor. It plays an essential role in the construction of the Thomason model structure. Its definition is inspired by neighborhood deformation retracts.

A suitable generalization serves a similar purpose in the homotopy theory of relative categories, see Barwick, Kan.


A functor of small categories i:CDi \colon C \to D is a Dwyer map if it is a sieve and factors as a composite of f:CCf \colon C \to C' and j:CDj \colon C' \to D such that

  • ff admits a deformation retraction, i.e. a functor r:CCr \colon C' \to C such that rf=id Cr f = \id_C toghether with a natural transformation h:frid Ch \colon f r \to \id_{C'} such that hf=id fh f = \id_f,
  • jj is a cosieve.

In the original definition rr was assumed to be a right adjoint of ff. The definition above is due to Cisinski who called this more general notion a pseudo-Dwyer map. Cisinski’s definition is now considered more useful and hence usually called by the simpler name “Dwyer map”.



Dwyer maps are closed under:

Moreover, the nerve functor preserves pushouts along Dwyer maps and transfinite composites of Dwyer maps up to weak homotopy equivalence.

The proofs can be found in Thomason, Proposition 4.3, Lemma 4.7 and Cisinski, Lemme 4.


Let cc denote the left adjoint of the nerve functor and let SdSd denote the barycentric subdivision functor. The composite cSd 2c Sd^2 carries injective simplicial maps to Dwyer maps.

This is Thomason, Proposition 4.2.

So among the Dwyer maps are all cofibrations in the Thomason model structure. But not all Dwyer maps are Thomason cofibrations – for example, the unique functor C\emptyset \to C is always a Dwyer map, but not every category is Thomason-cofibrant.


  • R. W. Thomason, Cat as a closed model category,

    Cahiers Topologie Géom. Différentielle 21, no. 3 (1980), pp. 305–324. MR0591388 (82b:18005) numdam scan

  • Denis-Charles Cisinski, Les morphisme de Dwyer ne sont pas stables par rétractes, Cahiers Topologie et Géom. Différentielle Catégoriques, 40 no. 3 (1999), pp. 227–231. (Numdam)
  • Clark Barwick, Daniel Kan, Relative categories: Another model for the homotopy theory of homotopy theories. Indagationes Mathematicae 23 (2012) pp. 42–68.

Last revised on March 4, 2016 at 00:24:40. See the history of this page for a list of all contributions to it.