Sunday, 21 April 2019

sequences and series - Compute limnto+inftynfrac12left(1+frac1nright)left(11cdot22cdot33cdotsnnright)frac1n2


How to compute
limn+n12(1+1n)(112233nn)1n2
I'm interested in more ways of computing limit for this expression





My proof:



Let u_nbe that sequence we've:



\begin{eqnarray*} \ln u_n &=& -\frac{n+1}{2n}\ln n + \frac{1}{n^2}\sum_{k=1}^n k\ln k\\ &=& -\frac{n+1}{2n}\ln n + \frac{1}{n^2}\sum_{k=1}^n k\ln \frac{k}{n}+\frac{1}{n^2}\sum_{k=1}^n k\ln n\\ &=& \frac{1}{n^2}\sum_{k=1}^n k\ln \frac{k}{n}\\ &=& \frac{1}{n}\sum_{k=1}^n \frac{k}{n}\ln \frac{k}{n}\\ &\to&\int_0^1 x\ln x\,dx = -1/4 \end{eqnarray*}



Therefore the limit is e^{-\frac{1}{4}}

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