In mathematics, a complete category is a category in which all small limits exist. That is, a category C is complete if every diagram F : J → C where J is small has a limit in C. Dually, a cocomplete category is one in which all small colimits exist.
The existence of all limits (even when J is a proper class) is too strong to be practically relevant. Any category with this property is necessarily a thin category: for any two objects there can be at most one morphism from one object to the other.
A weaker form of completeness is that of finite completeness. A category is finitely complete if all finite limits exists (i.e. limits of diagrams indexed by a finite category J). Dually, a category is finitely cocomplete if all finite colimits exist.
It follows from the existence theorem for limits that a category is complete if and only if it has equalizers (of all pairs of morphisms) and all (small) products. Since equalizers may be constructed from pullbacks and binary products (consider the pullback of (f, g) along the diagonal Δ), a category is complete if and only if it has pullbacks and products.
Dually, a category is cocomplete if and only if it has coequalizers and all (small) coproducts, or, equivalently, pushouts and coproducts.
Finite completeness can be characterized in several ways. For a category C, the following are all equivalent:
- C is finitely complete,
- C has equalizers and all finite products,
- C has equalizers, binary products, and a terminal object,
- C has pullbacks and a terminal object.
The dual statements are also equivalent.
A small category C is complete if and only if its dual Cop is cocomplete. A small complete category is necessarily thin.
Examples and counterexamples
- The following categories are both complete and cocomplete:
- The following categories are finitely complete and finitely cocomplete but neither complete nor cocomplete:
- Any (pre)abelian category is finitely complete and finitely cocomplete.
- The category of complete lattices is complete but not cocomplete.
- The category of metric spaces, Met, is finitely complete but has neither binary coproducts nor infinite products.
- The category of fields, Field, is neither finitely complete nor finitely cocomplete.
- A poset, considered as a small category, is complete (and cocomplete) if and only if it is a complete lattice.
- The partially ordered class of all ordinal numbers is cocomplete but not complete (since it has no terminal object).
- A group, considered as a category with a single object, is complete if and only if it is trivial. A nontrivial group has pullbacks and pushouts, but not products, coproducts, equalizers, coequalizers, terminal objects, or initial objects.
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