In category theory, an abstract branch of mathematics, the dual of a category is the category formed by reversing all the morphisms of . That is, we take to be the category with objects those of , but with the morphisms from X to Y in being the morphisms from Y to X in . The dual of a dual of a category is itself.
It is also often called the opposite category. Examples come from reversing the direction of inequalities in a partial order. In logic, this then looks like a very general description of negation (that is, proofs run in the opposite direction). Inverse limits and direct limits are interchanged. One way in which the concept is used is to remove the distinction between covariant and contravariant functors: a contravariant functor to is equally a functor to the opposite of .
In some cases one can identify the opposite category: for example the category of affine schemes is the opposite of the category of commutative rings. The Pontryagin duality restricts to the duality between the category of compact Hausdorff abelian topological groups and that of (discrete) abelian groups. The category of Stone spaces and continuous functions is the opposite of the category of Boolean algebras and homomorphisms.
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