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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