Ackermann Function

In computability theory, theAckermann function, named after Wilhelm Ackermann, is one of the simplest and earliest-discovered examples of a totalcomputable function that is not primitive recursive. All primitive recursive functions are total and computable, but the Ackermann function illustrates that not all total computable functions are primitive recursive. This function has a value ackermann(n) < 5, for any value of n that can be written in this physical universe. So essentially it's a constant time operation