# What is the combinatorial meaning of $\sum\limits_{R = 0}^{N}\binom{N}{r}\binom{N-R}{n-r} = \binom{N+1}{n+1}$?

What is the combinatorial meaning of $$\sum_{R = 0}^{N}\binom{N}{r}\binom{N-R}{n-r} = \binom{N+1}{n+1} \quad?$$ If the summing is performed with respect to $r$, i.e. $$\sum_{r = 0}^{n}\binom{N}{r}\binom{N-R}{n-r} = \binom{N}{n},$$ then it has clear combinatorial meaning: the number of combinations by taking $n$ balls from an urn containing $R$ red balls and $N-R$ white balls.

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Could you provide an example? Also, could you please give more context to this question eg. stating a distribution (hyper-geo; negative-binomial etc.) – bryansis2010 Feb 4 '13 at 8:01

You are asking about an identity that reads, for $k,m,n\in\mathbf N$: $$\sum_{i=0}^k\binom im\binom{k-i}n=\binom{k+1}{m+n+1}.$$ It is a variation of the Vandermonde identity, which reads, for $k,l,m\in\mathbf N$: $$\sum_{i=0}^m\binom ki\binom l{m-i}=\binom{k+l}m,$$ and which is combinatorially explained by counting the possibilities to choose a team of $m$ children among $k$ girls and $l$ boys.
Here is a cominatorial explaination for the variation asked about. Choose a subset of $m+n+1$ numbers $a_j$ from the $k+1$-set $\{0,\ldots,k\}$, and arrange them in increasing order $0\leq a_0<\cdots<a_{m+n}\leq k$. Put $i=a_m$, then there are $\binom im$ choices left for $a_0,\ldots,a_{m-1}$, and $\binom{k-i}n$ choices for $a_{m+1},\ldots,a_{m+n}$.