the simplest such functions are "closed formulas", which can be expressed as a composition of elementary functions such as factorials, powers, and so on. for instance, as shown below, the number of different possible orderings of a deck of "n" cards is "f"("n") = "n"!. the problem of finding a closed formula is known as algebraic enumeration, and frequently involves deriving a recurrence relation or generating function and using this to arrive at the desired closed form.