Monday, 16 May 2016

combinatorics - Algebraic proof of combinatorial identity



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.



nr=0(nr)(2nnr)=(3nn)



Thanks.



Answer



We make use of the Binomial Theorem. Observe that:
3nk=0(3nk)xk=(1+x)3n=(1+x)n(1+x)2n=[ni=0(ni)xi][2nj=0(2nj)xj]=3nk=0[nr=0(nr)(2nkr)]xk



Hence, by setting k=n, we compare the coefficients of xn of both sides to obtain:

(3nn)=nr=0(nr)(2nnr)


as desired.


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