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Determine the convergence of the series (1 – n sin (1/n)) / n

Consider the series

    \[ \sum_{n=1}^{\infty} \frac{1 - n \sin \frac{1}{n}}{n}. \]

Determine whether the series converges or diverges. If it converges, determine whether it converges conditionally or absolutely.


The series converges absolutely.

Proof. We use the comparison test,

    \[ \sum_{n=1}^{\infty} \frac{1-n \sin \frac{1}{n}}{n} \leq \sum_{n=1}^{\infty} \left( 1 - n \sin \frac{1}{n} \right). \]

The series on the right converges absolutely by the previous exercise (Section 10.20, Exercise #31). Hence, by the comparison test, we have established the convergence of the series

    \[ \sum_{n=1}^{\infty} \frac{1 - n \sin \frac{1}{n}}{n}. \qquad \blacksquare\]

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