Monday, 16 February 2015

convergence divergence - Is suminftyn=2frac1nlog2n convergent or divergent?



Cauchy ratio test yields 1 (so it's inconclusive). I have tried this:



1nlog2n=1nlognlogn=1lognnlogn1lognnn1logn!



Now, since 1/logn! diverges, the original series must diverge too. But Wolfram Alpha says it's convergent. How did I go wrong and how can I solve this?


Answer




You might find it easier to apply the Cauchy condensation test:



n=21nlog2nn=12n2nlog22n=n=11n2log22


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