I was thinking of using a mathematical induction argument on this, but I only got as far as the base cases for n = 1, 2 and 3. However, I don't know how to get the inductive step to work algebraically. That is given that the statement is true for n, how can I algebraically manipulate the factorials so that it has to be true for n + 1?
So, I looked up the problem. Here is a combinatorial solution that I am not totally convinced of, but I assume is correct:
http://math.stackexchange.com/questi...isible-by-nn-1
What I liked about that solution was the way the person who answered it rewrote the problem as a fraction and then reinterpreted it as a combinatorial problem that must have an integer as a solution. Hence the numerator is divisible by the denominator.
One might also be able to use the gamma function which would allow one to rewrite the problem as integrals. But I didn't see how that would make the inductive step any easier to calculate.