Hence,
gof ≠ fog Definition 9 A function f : X → Y is defined to be invertible, if there exists a function
g : Y → X such that  gof = IX and fog = IY The function g is called the inverse of f  and
is denoted by f –1 Thus, if f is invertible, then f must be one-one and onto and conversely, if f is
one-one and onto, then f must be invertible