Sunday, 15 January 2017

sequences and series - Convergence of suminftyn=2frac1nlog(logn)s



Does this series converge?
n=21nlog(logn)s
I wrote it as

n=21nslog(logn)
and I dont know how to deal with the double logarithm. This has to be shown with Cauchy's condensation test.


Answer



Use Cauchy's Condensation Text , assuming s>0 (otherwise divergence is almost trivial) :



2n2nlog(log2n)s=1slog(nlog2)=1slogn+sloglog2



and since the last term's series clearly diverges also ours does diverge.


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