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Vandermonde's identity : ウィキペディア英語版
Vandermonde's identity

In combinatorics, Vandermonde's identity, or Vandermonde's convolution, named after Alexandre-Théophile Vandermonde (1772), states that
:=\sum_^r,\qquad m,n,r\in\mathbb_0,
for binomial coefficients. This identity was given already in 1303 by the Chinese mathematician Zhu Shijie (Chu Shi-Chieh). See Askey 1975, pp. 59–60 for the history.
There is a q-analog to this theorem called the q-Vandermonde identity.
== Algebraic proof ==
In general, the product of two polynomials with degrees ''m'' and ''n'', respectively, is given by
:\biggl(\sum_^m a_ix^i\biggr) \biggl(\sum_^n b_jx^j\biggr)
= \sum_^\biggl(\sum_^r a_k b_\biggr) x^r,
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,
:(1+x)^ = \sum_^ x^r.
Using the binomial theorem also for the exponents ''m'' and ''n'', and then the above formula for the product of polynomials, we obtain
:\begin
\sum_^ x^r
&= (1+x)^\\
&= (1+x)^m (1+x)^n \\
&= \biggl(\sum_^m x^i\biggr)
\biggl(\sum_^n x^j\biggr)\\
&=\sum_^\biggl(\sum_^r \biggr) x^r,
\end

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.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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