The recursive formula is This says: to compute , multiply by the factorial of . For example, , and , and so on, until you hit the base case.
Notice how each step reduces the problem size by . You're breaking a big problem into a smaller one, trusting that the recursive call will handle it. That trust feels strange at first. "How can I use the answer before I have it?" But you're not. You're defining a relationship between answers. The math guarantees it works.
