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Question

Show that the series j=1(12j)/j\sum_{j=1}^{\infty}\left(1-2^{-j}\right) / j diverges.

Solution

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Answered 1 year ago
Answered 1 year ago
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Firstly, test the series

j=1(12j)j\sum_{j=1}^{\infty}\frac{(1-2^{-j})}{j}

to the neccesary convergence condition:

limn(12n)n=limn(1n1n2n),=()limn(1n)limn(1j2j),=00,=0.\begin{align*} \lim_{n\rightarrow\infty} \frac{(1-2^{-n})}{n}&=\lim_{n\rightarrow\infty}\left(\frac{1}{n}-\frac{1}{n\cdot2^n} \right) \,,\\ &\overset{(\ast)}{=}\lim_{n\rightarrow\infty}\left(\frac{1}{n}\right)-\lim_{n\rightarrow\infty}\left(\frac{1}{j\cdot2^j}\right)\,,\\ &=0-0\,,\\ &=\underline{0}\,.\quad\color{#c34632}\checkmark \end{align*}

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