the same proof is discussed on math.stackexchange http://math.stackexchange.com/questions/61482/proving-the-identity-sum-k-1n-k3-big-sum-k-1n-k-big2-without-i

the same proof is discussed on math.stackexchange http://math.stackexchange.com/questions/61482/proving-the-identity-sum-k-1n-k3-big-sum-k-1n-k-big2-without-i http://math.stackexchange.com/questions/61482/proving-the-id...