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Question

Test the series for convergence.

i=21lni\sum_{i=2}^{\infty} \frac{1}{\ln i}

Solution

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Answered 1 year ago
Answered 1 year ago
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This problem can be easily solved if we notice the following relation

ln(x)x,x0.\ln(x)\le x,\quad x\ge0\,.

Since the function ln(x)\ln(x) is positive for x2x\ge 2, it follows

ln(x)=ln(x)x1lnx1x,x2.\begin{align*} \ln(x)=|\ln(x)|&\le x\\ \frac{1}{\ln x}&\ge\frac{1}{x},\quad x\ge2\,. \end{align*}

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