An object X in a category C which admits all filtered colimits is called compact if the functor commutes with filtered colimits, i.e., if the natural map is a bijection for any filtered system of objects in C. Since elements in the filtered colimit at the left are represented by maps, for some i, the surjectivity of the above map amounts to requiring that a map factors over some. The terminology is motivated by an example arising from topology mentioned below. Several authors also use a terminology which is more closely related to algebraic categories: use the terminology finitely presented object instead of compact object. call these the objects of finite presentation.
Compactness in ∞-categories
The same definition also applies if C is an ∞-category, provided that the above set of morphisms gets replaced by the mapping space in C.
For a triangulated categoryC which admits all coproducts, defines an object to be compact if commutes with coproducts. The relation of this notion and the above is as follows: suppose C arises as the homotopy category of a stable ∞-category admitting all filtered colimits. Then an object in C is compact in Neeman's sense if and only if it is compact in the ∞-categorical sense. The reason is that in a stable ∞-category, always commutes with finite colimits since these are limits. Then, one uses a presentation of filtered colimits as a coequalizer of an infinite coproduct.
Examples
The compact objects in the category of sets are precisely the finite sets. For a ringR, the compact objects in the category of R-modules are precisely the finitely presented R-modules. In particular, if R is a field, then compact objects are finite-dimensional vector spaces. Similar results hold for any category of algebraic structures given by operations on a set obeying equational laws. Such categories, called varieties, can be studied systematically using Lawvere theories. For any Lawvere theoryT, there is a category Mod of models of T, and the compact objects in Mod are precisely the finitely presented models. For example: suppose T is the theory of groups. Then Mod is the category of groups, and the compact objects in Mod are the finitely presented groups. The compact objects in the derived categoryR-modules are precisely the perfect complexes. Compact topological spaces are not the compact objects in the category of topological spaces. Instead these are precisely the finite sets endowed with the discrete topology. The link between compactness in topology and the above categorical notion of compactness is as follows: for a fixed topological spaceX, there is the category O whose objects are the open subsets of X. Then, X is a compact topological space if and only if X is compact as an object in O. If C is any category, the category of presheaves has all colimits. The original category C is connected to P by the Yoneda embedding. For any object X of C, j is a compact object. In a similar vein, any category C can be regarded as a full subcategory of the category Ind of ind-objects in C. Regarded as an object of this larger category, any object of C is compact. In fact, the compact objects of Ind are precisely the objects of C.
In most categories, the condition of being compact is quite strong, so that most objects are not compact. A category C is compactly generated if any object can be expressed as a filtered colimit of compact objects in C. For example, any vector spaceV is the filtered colimit of its finite-dimensional subspaces. Hence the category of vector spaces is compactly generated. Categories which are compactly generated and also admit all colimits are called accessible categories.
For categories C with a well-behaved tensor product, there is another condition imposing some kind of finiteness, namely the condition that an object is dualizable. If the monoidal unit in C is compact, then any dualizable object is compact as well. For example, R is compact as an R-module, so this observation can be applied. Indeed, in the category of R-modules the dualizable objects are the finitely presented projective modules, which are in particular compact. In the context of ∞-categories, dualizable and compact objects tend to be more closely linked, for example in the ∞-category of complexes of R-modules, compact and dualizable objects agree. This and more general example where dualizable and compact objects agree are discussed in.
