related by the Dold-Kan correspondence
an adjunction with right adjoint, under certain conditions it is possible to transfer the model structure from to a model structure on by declaring the fibrations and weak equivalences in to be precisely those morphisms whose image under are fibrations or weak equivalences, respectively, in .
Typically this arises in situations where consist of the “same” objects as but equipped with extra stuff, structure, property, and is the corresponding forgetful functor sending objects in to their underlying objects in . Then is the corresponding free functor.
Let be a cofibrantly generated model category and
Say a morphism in is a fibration or weak equivalence precisely if its image under is, respectively, in .
Sufficient conditions for this to define a cofibrantly generated model category structure on are
the functor preserves small objects
this is the case in particular when preserves filtered colimits;
this is the case in particular if
If these conditions are met, then for (resp. ) the set of generating (acyclic) cofibrations in , the image set (resp. ) forms the set of generating (acyclic) cofibrations in .
One uses the small object argument repeatedly.
If carries the structure of a right proper model category, then also the transferred model structure on is right proper.
be a pullback diagram in , with the bottom morphism a weak equivalence and the right morphism a fibration. We need to show that then also the top morphism is a weak equivalence. By definition of transfer, this is equivalent to being a weak equivalence in .
Since is a right adjoint it preserves pullbacks, so that also
is a pullback diagram in . Since by definition of the transferred model strucure this is still the pullback of a weak equivalence along a fibration, and since is assumed to be right proper, it follows that is a weak equivalence in , hence that is a weak equivalence in .
Assume the adjunction
satisfies the conditions of the above proposition so that the model structure on is transferred to . Consider the case that is moreover an -enriched model category and that can be equipped with the structure of a -enriched category that is also -powered and copowered.
Assume now that the -powering of is taken by to the -powering of , in that .
Then the transferred model structure and the -enrichment on are compatible and make an -enriched model category.
By the axioms of enriched model category one sufficient condition to be checked is that for any cofibration in and for any fibration in , we have that the induced morphism
is a fibration, which is a weak equivalence if at least one of the two input morphisms is. By the induced model structure, this is checked by applying . But by assumption commutes with the powering, and since is a right adjoint it commutes with taking the pullback, so that under the morphism is
which is the morphism induced from . That this is indeed an (acyclic) fibration follows now from the fact that is an -enriched model category.
The model structure on algebraic fibrant objects is transferred from the underlying model category by forgetting the choice of fillers.
If is an accessible strict 2-monad on a locally finitely presentable 2-category . then the category of strict -algebras admits a transferred model structure from the 2-trivial model structure on . (This is proven directly, rather than by appeal to the acyclicity as above.)
Mike Shulman: In addition to lots of examples, I think it would be also nice to include here a non example, of a case where the putative transferred model structure provably doesn’t exist.
The arguments for transfer of model structures go back to
Proofs can be found in
The explicit study of transfer of model structures (on categories of sheaves) is apperently originally due to
See also prop. 1.4.23 of
A summary of the result is on p. 20 of
and on p. 6 of