Friday, 23 September 2016

calculus - Does the series converge: sumi2nftyfraclognn(loglogn)2



Does the series 2lognn(loglogn)2
converge?



The root and ratio tests are inconclusive, the integral test may be too difficult to apply. I've tried the limit comparison test with logn/n but that is also inconclusive since lognn(loglogn)2lognn=1(loglogn)20.


Answer



Using the Cauchy condensation test, (cf Wikipedia), your series diverges.
The transformed series reads :
n2nn2nlog2n=nnlog2n




Remark : I interpreted log as a base two logarithm, which does not affect the result


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