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A Reedy category is a category $R$ equipped with a structure enabling the inductive construction of diagrams and natural transformations of shape $R$. It is named after Christopher Reedy.
The most important consequence of a Reedy structure on $R$ is the existence of a certain model structure on the functor category $M^R$ whenever $M$ is a model category (no extra hypotheses on $M$ are required): the Reedy model structure.
A Reedy category is a category $R$ equipped with two wide subcategories $R_+$ and $R_-$ and a total ordering on the objects of $R$, defined by a degree function $d:Ob(R) \to \alpha$, where $\alpha$ is an ordinal number, such that:
Any ordinal $\alpha$, considered as a poset and hence a category, is a Reedy category with $\alpha_+=\alpha$, $\alpha_-$ the discrete category on $Ob(\alpha)$, and $d$ the identity.
The opposite of any Reedy category is a Reedy category: use the same degree function, and exchange $R_+$ and $R_-$.
The integers regarded as a poset is NOT a Reedy category, since it is not well-founded in either direction.
Joyal's category, $\Theta$, is also a Reedy category.
Many very small categories of diagram shapes are Reedy categories, such as $(\cdot\to\cdot\to \dots)$, or $(\cdot\leftarrow \cdot\rightarrow\cdot)$, or $(\cdot\rightrightarrows\cdot)$. This is of importance for the construction of homotopy limits and colimits over such diagram shapes.
The prototypical examples of Reedy categories are the simplex category $\Delta$ and its opposite $\Delta^{op}$. More generally, for any simplicial set $X$, its category of simplices $\Delta/X$ is a Reedy category.
The Reedy category structure on $\Delta$ is defined by:
The degree function $d: Ob(\Delta) \to \mathbb{N}$ is defined by $[k] \mapsto k$.
a map $[k] \to [n]$ is in $\Delta_+$ precisely if it is injective;
a map $[n] \to [k]$ is in $\Delta_-$ precisely if it is surjective.
And the Reedy category structure on $\Delta^{op}$ is defined by switching $\Delta_+$ and $\Delta_-$.
A Reedy category in which $R_-$ contains only identities is called a direct category; the factorization axiom then says simply that $R=R_+$. Similarly, if $R_+$ contains only identities it is said to be an inverse category.
Any ordinal is of course a direct category, and so is the subcategory $R_+$ of any Reedy category considered as a category in its own right. This amounts to “discarding the degeneracies” in a shape category. In some examples there are no degeneracies to begin with, such as the category of opetopes; thus these are naturally direct categories.
Dually, so is the subcategory $R_-$ of any Reedy category considered as a category in its own right. This amounts to “discarding the faces” in a shape category.
One problem with the notion of Reedy category is that it violates the principle of equivalence: it is not invariant under equivalence of categories. It’s not hard to see that any Reedy category is necessarily skeletal. In fact, it’s even worse: no Reedy category can have any nonidentity isomorphisms!
Proof: Take any isomorphism $f$, let $f = g h$ and $h f^{-1} = g' h'$ be the unique factorizations. Then $id = g h f^{-1} = (g g') h'$, so $h' = id$ and $g g' = id$, whence $g = id$ and $g' = id$ since $g, g' \in R_+$. Thus $f = h \in R_-$. The same argument applied to $f^{-1}$ shows that $f$ preserves the degree, hence $f = id$. $\qed$
This is problematic for many $\Delta$-like categories such as the category of cycles, Segal’s category $\Gamma$, the tree category $\Omega$, and so on. The concept of
due to Clemens Berger and Ieke Moerdijk, avoids these problems. There is a similar notion (which however does not comprise all Reedy categories) due to Denis-Charles Cisinski. A further generalization which allows noninvertible level morphisms is a
The notion of elegant Reedy category, introduced by Julie Bergner and Charles Rezk, is a restriction of the notion which captures the property that the Reedy model structure and injective model structure coincide. Several important Reedy categories are elegant, such as the $\Delta$ and $\Theta$.
Eilenberg-Zilber categories are a special sort of generalized Reedy category that behave rather like an elegant strict Reedy category.
There is also a generalization of the notion of Reedy category to the context of enriched category theory: this is an enriched Reedy category.
If $R$ is a direct category, then for any model category $M$ the colimit functor $\colim_R \colon M^R \to M$ is a left Quillen functor. However, there are non-direct Reedy categories with the same property, they are called Reedy categories with fibrant constants.
See the references at Reedy model structure
Last revised on October 4, 2021 at 13:26:00. See the history of this page for a list of all contributions to it.