I would like to obtain the algebraic proof for the following identity. I already know the combinatorial proof but the algebraic proof is evading me.
n∑r=0(nr)(2nn−r)=(3nn)
Thanks.
Answer
We make use of the Binomial Theorem. Observe that:
3n∑k=0(3nk)xk=(1+x)3n=(1+x)n(1+x)2n=[n∑i=0(ni)xi][2n∑j=0(2nj)xj]=3n∑k=0[n∑r=0(nr)(2nk−r)]xk
Hence, by setting k=n, we compare the coefficients of xn of both sides to obtain:
(3nn)=n∑r=0(nr)(2nn−r)
as desired.
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