Wednesday, 9 January 2013

elementary number theory - Division of Factorials



I have a partition of a positive integer $(p)$. How can I prove that the factorial of $p$ can always be divided by the product of the factorials of the parts?



As a quick example $\frac{9!}{(2!3!4!)} = 1260$ (no remainder), where $9=2+3+4$.




I can nearly see it by looking at factors, but I can't see a way to guarantee it.


Answer



The key observation is that the product of $n$ consecutive integers is divisible by $n!$. This can be proved by induction.


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