An accessible category is a possibly large category which is however essentially determined by a small category, in a certain way.
A locally small category $C$ is $\kappa$-accessible for a regular cardinal $\kappa$ if:
the category has $\kappa$-directed colimits (or, equivalently, $\kappa$-filtered colimits), and
there is a set of $\kappa$-compact objects that generate the category under $\kappa$-directed colimits.
Then $C$ is an accessible category if there exists a $\kappa$ such that it is $\kappa$-accessible.
Unlike for locally presentable categories, it does not follow that if $C$ is $\kappa$-accessible and $\kappa\lt \lambda$ then $C$ is also $\lambda$-accessible. It is true, however, that for any accessible category, there are arbitrarily large cardinals $\lambda$ such that $C$ is $\lambda$-accessible.
Equivalent characterizations include that $C$ is accessible iff:
it is of the form $Ind_\kappa(S)$ for $S$ small, i.e. the $\kappa$-ind-completion of a small category, for some $\kappa$.
it is of the form $\kappa\,Flat(S)$ for $S$ small and some $\kappa$, i.e. the category of $\kappa$-flat functors from some small category to $Set$.
it is the category of models (in $Set$) of a suitable type of logical theory.
The relevant notion of functor between accessible categories is
A functor $F\colon C\to D$ between accessible categories is an accessible functor if there exists a $\kappa$ such that $C$ and $D$ are both $\kappa$-accessible and $F$ preserves $\kappa$-filtered colimits.
If $C$ is $\lambda$-accessible and $\lambda\unlhd\mu$ (see sharply smaller cardinal), then $C$ is $\mu$-accessible. Thus, any accessible category is $\mu$-accessible for arbitrarily large cardinals $\mu$.
If $\mathcal{C}$ is an accessible category and $K$ is a small category, then the category of presheaves $Func(K^{op}, \mathcal{C})$ is again accessible.
(preservation of accessibility under inverse images)
Let $F : C \to D$ be a functor between locally presentable categories which preserves $\kappa$-filtered colimits, and let $D_0 \subset D$ be an accessible subcategory. Then the inverse image $f^{-1}(D_0) \subset C$ is a $\kappa$-accessible subcategory.
This appears as HTT, corollary A.2.6.5.
(accessibility of fibrations and weak equivalences in a combinatorial model category)
Let $C$ be a combinatorial model category, $Arr(C)$ its arrow category, $W \subset Arr(C)$ the full subcategory on the weak equivalences and $F \subset Arr(C)$ the full subcategory on the fibrations. Then $F$, $W$ and $F \cap W$ are accessible subcategories of $Arr(C)$.
This appears as HTT, corollary A.2.6.6.
(closure under limits)
The 2-category $Acc$ of accessible categories, accessible functors, and natural transformations has all small 2-limits.
This can be found in Makkai-Paré. Some special cases are proven in Adámek-Rosický.
(directed unions)
The 2-category $Acc$ has directed colimits of systems of fully faithful functors. If there is a proper class of strongly compact cardinals, then it has directed colimits of systems of faithful functors.
See (Paré-Rosický).
(adjoint functors)
Every accessible functor satisfies the solution set condition, and every left or right adjoint between accessible categories is accessible. Therefore, the adjoint functor theorem takes an especially pleasing form for accessible categories that are complete and cocomplete (i.e. are locally presentable): a functor between such categories is a left (resp. right) adjoint iff it is accessible and preserves all small colimits (resp. limits).
A small category is accessible precisely when it is idempotent complete.
Makkai-Paré say that this means accessibility is an “almost pure smallness condition.”
For a category $\mathcal{M}$ the following are equivalent:
$\mathcal{M}$ is finitely accessible.
$\mathcal{M}\simeq \mathbb{T}\text{-}Mod(Set)$ for some theory $\mathbb{T}$ of presheaf type.
Moreover, one has the following result due to Christian Lair:
For a category $\mathcal{M}$ the following are equivalent:
$\mathcal{M}$ is accessible.
$\mathcal{M}$ is sketchable.
See also at categorical model theory.
Every accessible category $C$ is well-powered, since it has a small dense subcategory $A$, for which the restricted Yoneda embedding $C\to [A^{op},Set]$ is fully faithful and preserves monomorphisms, hence embeds the subobject posets of $C$ as sub-posets of those of $[A^{op},Set]$.
Every accessible category with pushouts is well-copowered. This is shown in Adamek-Rosicky, Proposition 1.57 and Theorem 2.49. Whether this is true for all accessible categories depends on what large cardinal properties hold: by Corollary 6.8 of Adamek-Rosicky, if Vopenka's principle holds then all accessible categories are well-copowered, while by Example A.19 of Adamek-Rosicky, if all accessible categories are well-copowered then there exist arbitrarily large measurable cardinals.
See at Functor category – Accessibility.
Locally presentable categories: Cocomplete possibly-large categories generated under filtered colimits by small generators under small relations. Equivalently, accessible reflective localizations of free cocompletions. Accessible categories omit the cocompleteness requirement; toposes add the requirement of a left exact localization.
The term accessible category is due to
A standard textbook on the theory of accessible categories:
See also
Robert Paré, Jiří Rosický, Colimits of accessible categories (arXiv:1110.0767)
Jiří Adámek, Francis Borceux, Stephen Lack, Jiří Rosický, A classification of accessible categories, Journal of Pure and Applied Algebra 175:7-30, 2002. abstract
which further stratifies the accessible categories in terms of sound doctrines.
The concept is studied in a 2-categorical setting in
A discussion of accessible (∞,1)-categories is in section 5.4, p. 341 of
Some recent developments in the theory of accessible categories can be found in a series of papers on categorical model theory and abstract elementary classes (many of which also contain some results about arbitrary accessible categories), such as:
Tibor Beke, Jiří Rosický, Abstract elementary classes and accessible categories, 2011, arxiv
Michael Lieberman, Jiří Rosický, Sebastien Vasey, Internal sizes in μ-abstract elementary classes, arxiv
Michael Lieberman, Jiří Rosický, Sebastien Vasey , Set-theoretic aspects of accessible categories, arxiv
See also: