In combinatorics, Vandermonde's identity is the following identity for binomial coefficients: for any nonnegative integersr, m, n. The identity is named after Alexandre-Théophile Vandermonde, although it was already known in 1303 by the Chinese mathematicianZhu Shijie. There is a q-analog to this theorem called the q-Vandermonde identity. Vandermonde's identity can be generalized in numerous ways, including to the identity
In general, the product of two polynomials with degrees m and n, respectively, is given by where we use the convention that ai = 0 for all integers i > m and bj = 0 for all integers j > n. By the binomial theorem, Using the binomial theorem also for the exponents m and n, and then the above formula for the product of polynomials, we obtain where the above convention for the coefficients of the polynomials agrees with the definition of the binomial coefficients, because both give zero for all i > m and j > n, respectively. By comparing coefficients of xr, Vandermonde's identity follows for all integers r with 0 ≤ r ≤ m + n. For larger integers r, both sides of Vandermonde's identity are zero due to the definition of binomial coefficients.
Vandermonde's identity also admits a combinatorial double counting proof, as follows. Suppose a committee consists of m men and n women. In how many ways can a subcommittee of r members be formed? The answer is The answer is also the sum over all possible values of k, of the number of subcommittees consisting ofk men and r − k women:
Geometrical proof
Take a rectangular grid of r x squares. There are paths that start on the bottom left vertex and, moving only upwards or rightwards, end at the top right vertex. Call the bottom left vertex. There are paths starting at that end at, as k right moves and m−kupward moves must be made. Similarly, there are paths starting at that end at, as a total of r−k right moves and − upward moves must be made and the path length must be r−k + − = n. Thus there are paths that start at, end at, and go through. This is a subset of all paths that start at and end at, so sum from k = 0 to k = r to obtain the total number of paths that start at and end at.
Generalizations
Generalized Vandermonde's identity
One can generalize Vandermonde's identity as follows: This identity can be obtained through the algebraic derivation above when more than two polynomials are used, or through a simple double counting argument. On the one hand, one chooses elements out of a first set of elements; then out of another set, and so on, through such sets, until a total of elements have been chosen from the sets. One therefore chooses elements out of in the left-hand side, which is also exactly what is done in the right-hand side.
Chu–Vandermonde identity
The identity generalizes to non-integer arguments. In this case, it is known as the Chu-Vandermonde identity and takes the form for general complex-valueds and t and any non-negative integern. It can be proved along the lines of the algebraic proof above by multiplying the binomial series for and and comparing terms with the binomial series for. This identity may be rewritten in terms of the falling Pochhammer symbols as in which form it is clearly recognizable as an umbral variant of the binomial theorem. The Chu-Vandermonde identity can also be seen to be a special case of Gauss's hypergeometric theorem, which states that where is the hypergeometric function and is the gamma function. One regains the Chu-Vandermonde identity by taking a = −n and applying the identity liberally. The Rothe–Hagen identity is a further generalization of this identity.
When both sides have been divided by the expression on the left, so that the sum is 1, then the terms of the sum may be interpreted as probabilities. The resulting probability distribution is the hypergeometric distribution. That is the probability distribution of the number of red marbles in r draws without replacement from an urn containing n red and m blue marbles.
