Explanation:
Let f be a function having domain in the set X, and range in the set Y. Then f would be invertible if there exists a function g having domain Y and range X, such as:

In other words, the function
is applied to an input x, and gives an outcome of y, then its inverse function, let say
, can be applied to y to give the result of x.
In other words,
y = f(x) if and only if x = g(y)
For example, considering the function

the inverse is:
