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The probability of choosing $(n_1, \space n_2, \space \cdots \space n_k)$ randomly from a set of $(N_1, \space N_2, \space \cdots \space N_k)$ such that for $1\le i\le k, \space i\in \mathbb N$

  • $n_i \le N_i$
  • $\displaystyle \sum_{1\le i\le k} n_i = n$
  • $\displaystyle \sum_{1\le i\le k} N_i = N$
  • $p_i = \cfrac {N_i}{n_i}$

is $$\cfrac {n!} {n_1!\cdots n_k!}p_1^{n_1}\cdots p_k^{n_k}$$ The number of possible outcome is $N^n$ while the number of required outcome is $\cfrac {n!} {n_1!\cdots n_k!} N_1^{n_1}\cdots N_k^{n_k}$.

My question breaks down to proving that $$\displaystyle \prod_{1\le i\le k} \left(\! \begin{array}{c} n - (n_1 + n_2 + \cdots +n_{i-1}) \\ n_i \end{array} \!\right) = \cfrac {n!} {n_1!\cdots n_k!}$$

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up vote 1 down vote accepted

Note that you have $\binom nk = \frac{n!}{k!(n-k)!}$ and hence \begin{align*} \prod_{i=1}^k \binom{n - (n_1 + \cdots + n_{i-1})}{n_i} &= \prod_{i=1}^k \frac{\bigl(n - (n_1 + \cdots + n_{i-1})\bigr)!}{n_i!\cdot \bigl(n - (n_1 + \cdots + n_{i-1} + n_i)\bigr)!}\\ &= \prod_{i=1}^k \frac 1{n_i!} \prod_{i=1}^k \frac{\bigl(n - (n_1 + \cdots + n_{i-1})\bigr)!}{\bigl(n - (n_1 + \cdots + n_{i-1} + n_i)\bigr)!}\\ &= \prod_{i=1}^k \frac 1{n_i!} \cdot \frac{n!}{\bigl(n-(n_1 + \cdots + n_k)\bigr)!}\\ &= \frac{n!}{n_1! \cdots n_k!} \end{align*} so its just your term in another form.

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why do you have double product there and how did $n!$ appear? – user31280 Oct 25 '12 at 11:27
$n!$ is the numerator of the first product for $i = 1$ (then there is no $n_j$ to be subtracted) and the "double" product arises from $\prod_i a_i b_i = \prod_i a_i \cdot \prod_i b_i$ – martini Oct 25 '12 at 11:45
i didn't see the telescoping product before! thanks!! – user31280 Oct 26 '12 at 23:07

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