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 k≥6,
ln(k!)2≥k24ln(k/2)2≥k24
and ∞∑k=21k2 converges, so by comparison your series also does.
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