Page "Proportionality (mathematics)" Paragraph 31

Formally, two variables are inversely proportional ( or varying inversely, or in inverse variation, or in inverse proportion or in reciprocal proportion ) if one of the variables is directly proportional with the multiplicative inverse ( reciprocal ) of the other, or equivalently if their product is a constant.

It follows that the variable y is inversely proportional to the variable x if there exists a non-zero constant k such that