Condition for finite series: sum of squares finite + ?

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SUMMARY

The discussion centers on the conditions under which the series of positive real numbers, represented as \( \sum_{n=1}^{\infty} u_n \), converges given that \( \sum_{n=1}^{\infty} u_n^{2} \) is finite. A key finding is that if the limit \( \lim (u_{n+1}/u_n)^2 < 1 \), then \( \sum_{n=1}^{\infty} u_n \) converges. However, it is established that \( \lim (u_{n+1}/u_n)^2 \le 1 \) is a sufficient condition for convergence, but not a necessary one.

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Let u_n be a sequence of positive real number.
If \sum_{n=1}^{\infty}u_n^{2} finite + (condition??) then \sum_{n=1}^{\infty}u_n finite.
I want to find the condition.Please help me.
 
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Any obvious condition would be that (u_{n+1}/u_n)^2 not go to 1 as n goes to infinity. The only way \sum_{n=1}^{\infty}u_n^{2} can converge is if lim (u_{n+1}/u_n)^2\le 1. If lim (u_{n+1}/u_n)^2&lt; 1 then lim u_{n+1}/u_n&lt; 1 also and so \sum_{n=1}^{\infty}u_n converges. Of course, that is a sufficient condition, not a necessary condition. It is still possible that a sequence for which lim u_{n+1}/u_n\le 1 will converge.
 
Thank you so much,HallsofIvy. In my first post. you know, this is my ploblem which I've tried to do it for 1 month. Thank you again.
 

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