In this paper we obtain several model structures on
,
the category of small double categories. Our model structures
have three sources. We first transfer across a categorification-nerve
adjunction. Secondly, we view double categories as internal categories in
and take
as our weak equivalences various internal equivalences defined via Grothendieck topologies.
Thirdly,
inherits a model structure as a category of algebras over a
–monad.
Some of these model structures coincide and the different points of view give us
further results about cofibrant replacements and cofibrant objects. As part of this
program we give explicit descriptions for and discuss properties of free double
categories, quotient double categories, colimits of double categories, horizontal nerve
and horizontal categorification.
Keywords
categorification, colimit, double category, fundamental
category, fundamental double category, horizontal
categorification, internal category, model structure,
transfer of model structure, $2$–category, $2$–monad
Department of Mathematics
University of Chicago
5734 South University
Chicago, IL 60637
USA
and Departament de Matemàtiques
Universitat Autònoma de Barcelona
08193 Bellaterra (Barcelona)
Spain