Friday, 27 December 2019

real analysis - Does the series sumnge1fraclnleft(fracn+1nright)sqrtn converge?



Could you please give me some hint how to decide about convergence of the series
n1ln(n+1n)n ?




I tried using comparison test:
ln(n+1n)nln(1n)n=ln(n)n.
Series n1ln(n)n diverges by integral test, but for comparison test all compared sequenced must be non-negative and ln(1n)0 for all n.



Thanks.


Answer



Yes it does using the asymptotic comparison:



ln(n+1n)n1nn




Remark For the comparison test the general term of the series must have an unchanged sign i.e. not alternating series (no matter positive or negative).


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