Monday, 18 June 2018

sequences and series - Is sum+inftyn=1frac(1)nlognn! a positive sum?




The below series is convergent series by the ratio test but i'm no able to know if this series have a positive sum , and i don't succeed to check if it has a closed form ,Then my question here is :




Question:
Is this : +n=1(1)nlognn! a positive sum ?



Answer



For n2, the expression lognn! is strictly decreasing, as log(n+1)(n+1)!<lognn!log(n+1)<(n+1)lognlogn(n+1)<n+1, which is true, because logn(n+1)<logn(n2)=2. Therefore, n=1(1)nlognn! is an alternating series with terms that strictly decrease in magnitude after the first nonzero term, so the series has the same sign as the first nonzero term, which is positive.


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