Wednesday, 20 January 2016

real analysis - Series convergence/divergence



I'm trying to figure out whether the following series diverges or converges by using D'Alemberts (quotientcriteria), Cauchy (integral- and rootcriteria) and Leibniz convergence test for alternating series as well as direct comparisson tests.




k=21(ln(k!))2



I'm very unclear on how to parse this series. My guess is that I have to find a series which is smaller since my guess is that it does converge.



ln(k!)ln(k)=>1ln(k)1ln(k!)



But after trying the quotient criteria:



(ln(k))2(ln(k+1))2




I find it unclear how to continue.


Answer



Of course it converges. You just have to show that ln(k!)2 grows fast enough.



For example:
We have k!(k/2)k/2, so for k6,



ln(k!)2k24ln(k/2)2k24



and k=21k2 converges, so by comparison your series also does.



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