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#### Alexmahone

##### Active member

- Jan 26, 2012

- 268

**:**

__My attempt__$\sum |\ a_n|$ is convergent.

$b_n=\left\{ \begin{array}{rcl}|a_{n_i}|\ &\text{for}& \ n=n_i \\ 0\ &\text{for}& \ n\neq n_i\end{array} \right.$

$0\le b_n\le\ |a_n|$ for all $n$.

Since $\sum |\ a_n|$ converges, $\sum b_n$ converges.

So, $\sum|\ a_{n_i}|$ converges. ($\because\sum_{n=0}^{n_i}b_n=\sum_{i=0}^i|\ a_{n_i}|$)

Is that okay?