Subfunctor

In category theory, a branch of mathematics, a subfunctor is a special type of functor that is an analogue of a subset.
Definition.
Let C be a category, and let "F" be a contravariant functor from C to the category of sets Set. A contravariant functor "G" from C to Set is a subfunctor of "F" if
This relation is often written as "G" ⊆ "F".
For example, let 1 be the category with a single object and a single arrow. A functor "F": 1 → Set maps the unique object of 1 to some set "S" and the unique identity arrow of 1 to the identity function 1"S" on "S". A subfunctor "G" of "F" maps the unique object of 1 to a subset "T" of "S" and maps the unique identity arrow to the identity function 1"T" on "T". Notice that 1"T" is the restriction of 1"S" to "T". Consequently, subfunctors of "F" correspond to subsets of "S".
Remarks.
Subfunctors in general are like global versions of subsets. For example, if one imagines the objects of some category C to be analogous to the open sets of a topological space, then a contravariant functor from C to the category of sets gives a set-valued presheaf on C, that is, it associates sets to the objects of C in a way that is compatible with the arrows of C. A subfunctor then associates a subset to each set, again in a compatible way.
The most important examples of subfunctors are subfunctors of the Hom functor. Let "c" be an object of the category C, and consider the functor . This functor takes an object "c"′ of C and gives back all of the morphisms "c"′ → "c". A subfunctor of gives back only some of the morphisms. Such a subfunctor is called a sieve, and it is usually used when defining Grothendieck topologies.
Open subfunctors.
Subfunctors are also used in the construction of representable functors on the category of ringed spaces. Let "F" be a contravariant functor from the category of ringed spaces to the category of sets, and let "G" ⊆ "F". Suppose that this inclusion morphism "G" → "F" is representable by open immersions, i.e., for any representable functor and any morphism , the fibered product is a representable functor and the morphism "Y" → "X" defined by the Yoneda lemma is an open immersion. Then "G" is called an open subfunctor of "F". If "F" is covered by representable open subfunctors, then, under certain conditions, it can be shown that "F" is representable. This is a useful technique for the construction of ringed spaces. It was discovered and exploited heavily by Alexander Grothendieck, who applied it especially to the case of schemes. For a formal statement and proof, see Grothendieck, "Éléments de géométrie algébrique", vol. 1, 2nd ed., chapter 0, section 4.5.