function, (), is invertible if there is a function, (), which is its inverse. In this case, there is only one possible inverse, and we denote it by ). This inverse is characterized by the equations, ()) = and ()) = .
A function is invertible (with a unique inverse) if the output uniquely determines the input (i.e., it is one-to-one) and the set of legal outputs is equal to the set of legal inputs.