An inverse function is a concept of mathematics. A function will calculate some output \displaystyle{ y }, given some input \displaystyle{ x }. This is usually written \displaystyle{ f(x) = y }. The inverse function does the reverse. Let's say \displaystyle{ g } is the inverse function of \displaystyle{ f }, then \displaystyle{ g(y) = x }. Or otherwise put, \displaystyle{ g(f(x)) = x }. An inverse function to \displaystyle{ f } is usually called \displaystyle{ f^{-1} }. It is not to be confused with \displaystyle{ 1/f }, which is a reciprocal function.