Wednesday, March 6, 2019

calculus - Can I show the series convergence by this relabeling method?



The series is



21(log(n))log(n)



Can I simply relabel the log(n) with the variable y and use the ratio test on 1yy? I'd get 0 in the ratio test limit and I conclude that the series converges.



Is this method valid?




Or should I really stick with the integral test instead? I tried the integral test too, but the integral is hard to evaluate.



Also, the Cauchy Condensation Test is usually helpful for series of logarithms but it doesn't seem useful in this example.



Any hints or solutions are welcome.



Thanks,


Answer



Note that n21(log(n))log(n)=n21exp(log((log(n))log(n)))=n21nlog(log(n))

and so, for example, for a sufficient large N we have nN that nlog(log(n))>n2 hence nN1nlog(log(n))nN1n2<.


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