Forgetful functor

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A forgetful functor is a type of functor in mathematics. The nomenclature is suggestive of such a functor's behaviour: given some object with structure as input, some or all of the object's structure or properties is 'forgotten' in the output. For an algebraic structure of a given signature, this may be expressed by curtailing the signature in some way: the new signature is an edited form of the old one. If the signature is left as an empty list, the functor is simply to take the underlying set of a structure; this is in fact the most common case.

For example, there are several forgetful functors from the category of commutative rings. Recall that a (unital) ring, described in the language of universal algebra, is an ordered tuple (R,+,*,a,m,0,1) satisfying certain axioms, where "+" and "*" are binary functions on the set R, a and m are unary operations corresponding to additive and multiplicative inverse, and 0 and 1 are nullary operations giving the identities of the two binary operations. Deleting the 1 gives a forgetful functor to the category of rings without unit; it simply "forgets" the unit. Deleting "*", m, and 1 yields a functor to the category of abelian groups which assigns to each ring R the underlying additive abelian group of R. To each morphism of rings is assigned the same function considered merely as a morphism of addition between the underlying groups. Deleting all the operations gives the functor to the underlying set R.

It is beneficial to distinguish between forgetful functors which "forget structure" versus those which "forget properties". For example, in the above example of commutative rings, in addition to those functors which delete some of the operations, there are functors which forget some of the axioms. There is a functor from the category CRng to Rng which forgets the axiom of commutativity, but keeps all the operations. Occasionally the object may include extra sets not defined strictly in terms of the underlying set (in this case, it is a matter of taste which part to consider the underlying set, though this is rarely ambiguous in practice). For these objects, there are forgetful functors which forget the extra sets which are more general.

Most common objects studied in mathematics are constructed as underlying sets along with extra sets of structure on those sets (operations on the underlying set, privileged subsets of the underlying set, etc) which may satisfy some axioms. For these objects, a commonly considered forgetful functor is as follows. Let \mathcal{C} be any category based on sets, e.g. groups - sets of elements - or topological spaces - sets of 'points'. As usual, write \operatorname{Ob}(\mathcal{C}) for the objects of \mathcal{C} and write \operatorname{Fl}(\mathcal{C}) for the morphisms of the same. Consider the rule:

A in \operatorname{Ob}(\mathcal{C}),\quad A\mapsto |A|= the underlying set of A,
u in \operatorname{Fl}(\mathcal{C}),\quad u\mapsto |u|= the morphism, u, as a map of sets.

The functor |\;\;| is then the forgetful functor from \mathcal{C} to \mathbf{Set}, the category of sets.

Forgetful functors are almost always faithful. Concrete categories have forgetful functors to the category of sets -- indeed they may be defined as those categories which admit a faithful functor to that category.

Forgetful functors which only forget axioms are always fully faithful; every morphism which respects the structure between objects which satisfy the axioms automatically also respects the axioms. Forgetful functors which forget structures need not be full; there are morphisms which don't respect the structure. These functors are still faithful though; distinct morphisms which do respect the structure are still distinct when the structure is forgotten. Functors which forget the extra sets need not be faithful; distinct morphisms respecting the structure of those extra sets may be indistinguishable on the underlying set.

In the language of formal logic, a functor of the first kind is one which removes axioms. One of the second kind are those which remove predicates. The third kind are those which remove types.

An example of the first kind of the forgetful functor Ab → Grp. One of the second kind is the forgetful functor Ab → Set. A functor of the third kind is the functor Mod → Ab, where Mod is the fibred category of all modules over arbitrary ring. To see this, just choose a ring homomorphism between the underlying rings which does not change the ring action. Under the forgetful functor, this morphism yields the identity. Note that an object in Mod is a tuple which includes a ring and an abelian group, so it is a matter of taste which to forget.

Forgetful functors tend to have left adjoints which are 'free' constructions. For example, the forgetful functor from \mathbf{Mod}(R) (the category of R-module) to \mathbf{Set} has left adjoint F, with X\mapsto F(X), the free R-module with basis X. For a more extensive list, see (Mac Lane 1997). Fld, the category of fields, furnishes an example of a forgetful functor with no adjoint. There is no field satisfying a free universal property for a given set.

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