Friday, 22 May 2015

summation - Can't understand an equality between sums



During an induction proof I came across an equality that I can't understand.

During the last step of the induction there is:
nk=1(k+1)(nk1)=n1k=0(k+2)(nk)



My question is which sum and combination identities were used to achieve this transformation. Using simply the index shift of sums I couldn't work out the same result.
Edit: I understand that the equality stands, I can't understand how I can produce the RHS from the LHS. Which identities do I have to use?


Answer



If you're unfamiliar with index shifts, change the variable first.
nk=1(k+1)(nk1)=nh=1(h+1)(nh1)(change dummy variable)=n1k=0(k+2)(nk)h1=kh+1=k+2h=1k=0h=nk=n1



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