Thursday, 4 January 2018

calculus - Convergence of sumin=1nftysqrt[n]21



I'm trying to determine whether n=1(n21)

converges or diverges. Ratio, root, nth term, etc tests are either inconclusive or too difficult to simplify. I feel like there must be something I can bound this series by but I can't think what.



In this question the answerer very smartly (I have no idea how he/ she thought to do that) used the fact that (1+1n)ne3 to bound n31 by 1n but that only worked because 3e. So even though that question looks very similar I don't think I can apply that idea here.



Edit: This is in the section before power/ Taylor series so I don't think I'm allowed to use that.


Answer



Hint:




n21=elog2n1=log2n+O(n2)



as n.



ADDENDUM



To address your specific problem, consider instead



(1+log2n)n




which you should be able to show is less than 2. I that case, then you can show that n21>(log2)/n and draw your conclusion about the sum.


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