Generator (category theory)

From Wikipedia, the free encyclopedia
Jump to: navigation, search

In category theory in mathematics a family of generators (or family of separators) of a category is a collection of objects, indexed by some set I, such that for any two morphisms in , if then there is some i∈I and morphism , such that the compositions . If the family consists of a single object G, we say it is a generator (or separator).

Generators are central to the definition of Grothendieck categories.

The dual concept is called a cogenerator or coseparator.


  • In the category of abelian groups, the group of integers is a generator: If f and g are different, then there is an element , such that . Hence the map suffices.
  • Similarly, the one-point set is a generator for the category of sets. In fact, any nonempty set is a generator.
  • In the category of sets, any set with at least two objects is a cogenerator.


External links[edit]