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This is the Functors chapter of Category Theory.
A functor is a morphism between categories. Given categories and , a functor has domain and codomain , and consists of two suitably related functions:
- The object function , which assigns to each object in , an object in .
- The arrow function (also ), which assigns to each arrow in , an arrow in , such that it satisfies and where is defined.
- The power set functor is a functor . Its object function assigns to every set , its power set and its arrow function assigns to each map , the map .
- The inclusion functor sends every object in a subcategory to itself (in ).
Types of functors 
- A functor is an isomorphism of categories if it is a bijection on both objects and arrows.
- A functor is called full if, for every pair of objects in and every arrow in , there exists an arrow in with . In other words, is surjective on arrows given objects .
- A functor is called faithful if, for every pair of objects in and every pair of parallel arrows in , the equality implies that . In other words, is injective on arrows given objects . The inclusion functor is faithful.
- A functor is called forgetful if if "forgets" some or all aspects of the structure of .