Provide a combinatorial proof that:

(2n choose n) = Sum from k=0 of n of [n choose k]^2

I have a couple starting points but I'm not sure if they qualify as a combinatorial proof. Any ideas on how to start this?

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- Feb 8th 2011, 06:53 PMdivinelogosCombinatorial Proof
Provide a combinatorial proof that:

(2n choose n) = Sum from k=0 of n of [n choose k]^2

I have a couple starting points but I'm not sure if they qualify as a combinatorial proof. Any ideas on how to start this? - Feb 10th 2011, 02:42 PMemakarov
See the end of this subsection of the Wikipedia article about the binomial coefficients.