Product (category theory)

In , the product of two (or more) in a is a notion designed to capture the essence behind constructions in other areas of mathematics such as the Cartesian product of sets, the direct product of groups or rings, and the product of topological spaces. Essentially, the product of a family of objects is the "most general" object which admits a morphism to each of the given objects. Fix a category Let and be objects of A product of and is an object typically denoted equipped with a pair of morphisms satisfying the following universal property: For every object and every pair of morphisms there exists a unique morphism such that the following diagram commutes: Whether a product exists may depend on or on and If it does exist, it is unique up to canonical isomorphism, because of the universal property, so one may speak of the product. This has the following meaning: let be another cartesian product, there exists a unique isomorphism such that and . The morphisms and are called the canonical projections or projection morphisms; the letter (pronounced pi) alliterates with projection. Given and the unique morphism is called the product of morphisms and and is denoted Instead of two objects, we can start with an arbitrary family of objects indexed by a set Given a family of objects, a product of the family is an object equipped with morphisms satisfying the following universal property: For every object and every -indexed family of morphisms there exists a unique morphism such that the following diagrams commute for all The product is denoted If then it is denoted and the product of morphisms is denoted Alternatively, the product may be defined through equations. So, for example, for the binary product: Existence of is guaranteed by existence of the operation Commutativity of the diagrams above is guaranteed by the equality: for all and all Uniqueness of is guaranteed by the equality: for all The product is a special case of a . This may be seen by using a (a family of objects without any morphisms, other than their identity morphisms) as the required for the definition of the limit.