Inverse relation
（重定向自Converse relation）
In mathematics, the inverse relation of a binary relation is the relation that occurs when the order of the elements is switched in the relation. For example, the inverse of the relation 'child of' is the relation 'parent of'. In formal terms, if X and Y are sets and L ⊆ X × Y is a relation from X to Y, then L is the relation defined so that y L x if and only if x L y. In set-builder notation, L = {(y, x) ∈ Y × X | (x, y) ∈ L}.