During an induction proof I came across an equality that I can't understand.
During the last step of the induction there is:
n∑k=1(k+1)(nk−1)=n−1∑k=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.
n∑k=1(k+1)(nk−1)=n∑h=1(h+1)(nh−1)(change dummy variable)=n−1∑k=0(k+2)(nk)h−1=kh+1=k+2h=1⟹k=0h=n⟹k=n−1
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