Thursday, December 8, 2016

divisibility - What is the highest power of a prime that divides nPr?

I know that the highest power of a prime which divides n! is given by
[np]+[np2]+[np3]...


Where [x] is the greatest integer function.



For nPr, which equals n!(nr)!, I can find the highest power of p dividing n! and (nr)!, and then subtract the two to get the highest power of p dividing nPr.



Now is there a better method to do this, where we don't need to find the highest power of p dividing (nr)!? Can we directly find the power of p dividing the product of r consecutive integers?

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