Testing the Factorial Function for Large Numbers

In the area of software testing, there are many untestable cases due to the lack of oracles. This is the Oracle Problem in the literature. By untestable, we mean that it is infeasible for testing within reasonable amount of time. Examples of untestable cases are finding a hotel with minimum price in a large set of choices, finding the sum or average of a large set of numbers of arbitrary pattern, performing simulation, etc. Oracle problem cannot be solved completely and we can only alleviate the problem by showing more evidence on the correctness of the corresponding function. Along with this, by testing an untestable function, we mean to check some typical, necessary, and testable, properties of the function. In this paper, we give some methods for testing the untestable Factorial function n!, for very large n.