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I have $N$ indistinguishable objects and $K$ distinguishable boxes and I need to calculate ways how i can put this objects to boxes that no box has more than $X$ and not less than $Y$ objects.

I found topic How many ways to distribute $n$ objects into $r$ boxes so that each box have at least $1$ (but no more than $k$) objects? with recurrent formula but it's too slow and takes too much time with not little numbers.

Is there another way to calculate it?

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Vladislav
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1 Answers1

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Notice that what you want eventually reduces to distributing $N-((K-1)X) $ balls over $K $ boxes, with no box having more than $Y-X+1 $ balls, so it is reduced to the same problem you linked.

Distributing $N $ balls over $K $ boxes with a minimum of 1 ball and a maximum of $M $ is the same as distributing $N-M $ balls over $K $ boxes with each box having, at most, $M-1$ balls.

This means your problem is equivalent to distributing $N-(KX) $ balls over $K $ boxes with no box having more than $Y-X $ balls.

If you find a better solution to that problem then you find a better solution for yours.

I guess you could use the Principle of Inclusion-Exclusion to impose your restrictions but it is also not a closed formula. And although you can make it non-recursive, it is recursive in nature.

RGS
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  • Thank you! Can you also tell or link how to count boxes with no box having more than $X$ objects? – Vladislav Nov 20 '16 at 18:41
  • P.S. We can calc it with this formula http://math.stackexchange.com/questions/89417/number-of-ways-to-put-n-unlabeled-balls-in-k-bins-with-a-max-of-m-balls-in But it takes too much time again ;( – Vladislav Nov 20 '16 at 18:57