We can see the fact that:
If a series $\sum_{n=1}^{\infty} a_{n}$ converges then:
$\displaystyle \lim_{n \rightarrow \infty} (a_n + a_{n+1} +···+ a_{n+r} )=0 $
This is my proof:
$\displaystyle \lim_{n \rightarrow \infty} (a_n + a_{n+1} +···+ a_{n+r} )$
$=\displaystyle \lim_{n \rightarrow \infty} a_n + \displaystyle \lim_{n \rightarrow \infty}a_{n+1} +···+\displaystyle \lim_{n \rightarrow \infty} a_{n+r} $
$=0+0+...+0=0$
Is it correct?
Also I want to ask:Does the converse of the implication holds:
That it: Does $\displaystyle \lim_{n \rightarrow \infty} (a_n + a_{n+1} +···+ a_{n+r} )=0 $
imply the series $\sum_{n=1}^{\infty} a_{n}$ convergent?
Whether it is true or not. I am searching for a proof and a justification. Could someone help to prove or disprove the statement?
Thanks so much !
Answer
As long as $r$ is finite, I believe your answer is correct. The converse is not true. Let's, for example, let $a_n = 1/n.$
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