Wednesday, 19 March 2014

limits - For what value of p would the series sumin=2nftyfrac1npln(n) converge?





For what value of p would the series n=21npln(n) converge?



I have tried using the series convergence test, and limit comparison test, which does't help. So i am considering using the divergence test, and finding all values for p for which the limit is not 0


Answer



Interestingly,
n=21npln(n)
converges exactly when
n=21np
converges:
converges for
p>1 and
diverges for
p1.




The comparison test does it
for p>1
and the fact that
ln(n)nc0
for any c>0
does it for
p<1.



The only problem is when

p=1
and this can be handled by
either the
integral test
or Cauchy's
condensation test.


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