Dual (category theory)
（重定向自Dual category）
In category theory, a branch of mathematics, duality is a correspondence between properties of a category C and so-called dual properties of the opposite category C. Given a statement regarding the category C, by interchanging the source and target of each morphism as well as interchanging the order of composing two morphisms, a corresponding dual statement is obtained regarding the opposite category C. Duality, as such, is the assertion that truth is invariant under this operation on statements. In other words, if a statement is true about C, then its dual statement is true about C. Also, if a statement is false about C, then its dual has to be false about C.