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Example: How many ways are there to distribute 15 fruits to 6 people so that each person has at least 1 fruit but no more than 3?

I understand how to do it when we need to make sure that at least 1 object goes to 1 box. This is the same as the number of surjections from a set of size $n$ to a set of size $r$. However, I am lost on finding the way to restrict the number of maximum number of objects that can be placed at each box.

Camilo Celis Guzman
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  • Distributing $15$ fruits to $6$ people so that everyone gets between $1$ and $3$ is the same as distributing $9$ fruits to $6$ people so that everyone gets between $0$ and $2$, because you start by giving one fruit to everyone before you worry about distributing the rest. – Arthur Jun 10 '15 at 14:42
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    Distinguishable people? Indistinguishable fruit? – Henry Jun 10 '15 at 14:43
  • @Arthur I still don't understand how can I go from that fact you suggest to solving a general problem of this type. – Camilo Celis Guzman Jun 10 '15 at 14:55
  • @Henry yes. The objects (fruits) are indistinguishable and the boxes (people) are distinguishable. – Camilo Celis Guzman Jun 10 '15 at 14:55

2 Answers2

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You can do the calculation with recurrences.

Supposing you have $f$ fruit, $p$ people, $x$ maximum to each person, $y$ minimum to each person, want you want to count $w(f,p,x,y)$ ways of distributing the fruit. You can use:

  • for distinguishable people and distinguishable fruit $$w(f,p,x,y)=\sum_{i=y}^{x} {f\choose i}w(f-i,p-1,x,y)$$
  • for distinguishable people and indistinguishable fruit $$w(f,p,x,y)=\sum_{i=y}^{x} w(f-i,p-1,x,y)$$
  • for indistinguishable people and indistinguishable fruit $$w(f,p,x,y)=\sum_{i=y}^{x} w(f-i,p-1,\max(x,i),y)$$

in each case starting from $w(0,0,x,y)=1$ and $w(f,0,x,y)=0$ when $f\not = 1$.

Henry
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  • Thank you very much. Also, I assumed that people are distinguishable because each one is a different person, and the fruits are of the same kind so they are indistinguishable. Am I correctly assuming those facts based on the definitions of indistinguishable or distinguishable objects? – Camilo Celis Guzman Jun 10 '15 at 15:07
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    I cannot tell you whether the fruit or people are distinguishable or indistinguishable - that must come from the question. – Henry Jun 10 '15 at 15:12
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    For what it is worth, I get the answers $30270240000$, $50$ and $2$; the last two agreeing with Barry (with the $2$ being patterns like $3+3+3+3+2+1$ or $3+3+3+2+2+2$) – Henry Jun 10 '15 at 15:21
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How many ways are there to distribute 15 fruits to 6 people so that each person has at least 1 fruit but no more than 3?

Well, if everybody gets 1 fruit, that reduces you down to 9 fruit that you need to distribute. So then it's just a matter of how you can distribute those 9 such that nobody gets more than 2 more.

You can't have two or fewer people getting 2 fruit, since there aren't enough people, so that's only two terms you have to add:

$$\begin{split} N &= \sum_{i=0}^4N(\text{i people get 2 fruit}) \\ &= N(\text{3 people get 2}) + N(\text{4 people get 2}) \\ &= \binom{6}{3} + \binom{6}{4}\cdot2 \\ &= 20 + 30 \\ &= 50 \end{split}$$

Barry
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