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Suppose that $b_n>0$ for all $n$, that $$\sum_{n=1}^{\infty}b_n$$ and $$\sum_{n=1}^{\infty}nb_n$$ are all convergent. does that guarantee $$\lim_{n\to\infty}n\sum_{k=n}^{\infty}b_k$$ exists?

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For $k \ge n$ note that $nb_k \le k b_k$, so that you have $$\sum_{k=n}^{\infty} nb_k \le \sum_{k=n}^{\infty} k b_k.$$ Since you have assumed that $\sum_{k=1}^{\infty} kb_k$ converges, use what you know about limits of tail ends of convergent series.

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Aha, I see. Thank you! – asdgdffasd Nov 14 '12 at 1:42

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