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I need to prove that if $(a_n)$ is decreasing and $\displaystyle \sum a_n = + \infty$. Then $$\lim {\frac{a_1 + a_3 + \dotsb + a_{2n-1}}{a_2 + a_4 + \dotsb + a_{2n}}} = 1$$

I proved that $\displaystyle \lim {\frac{a_1 + a_3 + \dotsb + a_{2n-1}}{a_2 + a_4 + \dotsb + a_{2n}}} \geq 1$. Someone has any idea to finish the demonstration?

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Consider $\frac{a_1}{a_2 + \cdots + a_{2n}} + \frac{a_3 + \cdots + a_{2n-1}}{a_2 + \cdots + a_{2n}}$. Show that the second term is $\leq 1$. What can you say about the first term?

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  • $\begingroup$ hmm. It tends to zero. Ok. I will try to prove your assertion. Tks $\endgroup$ – jon jones Oct 23 '12 at 0:29

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