My Math Forum result about arithmetic of convergent sequences
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June 30th, 2017, 06:23 AM   #1
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result about arithmetic of convergent sequences

Hello all,

I believe that, given a collection of increasing sequences whose total sum converges, each individual sequence will also converge. Attached is my short proof. Is it correct?

Regards,

Magnitude
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 June 30th, 2017, 06:49 AM #2 Math Team   Joined: Dec 2013 From: Colombia Posts: 7,309 Thanks: 2444 Math Focus: Mainly analysis and algebra Your first line seems to state that all the sequences diverge. Comments:You need only prove the result for two sequences. The result for $n$ sequences follows by induction. The given result then follows by considering that the sum is always greater than each of the series individually. Thus, if one diverges we use the comparison test with that series and the sum. Last edited by v8archie; June 30th, 2017 at 07:07 AM.
 June 30th, 2017, 07:31 AM #3 Member   Joined: Jan 2016 From: United Kingdom Posts: 33 Thanks: 0 V8archie, In my first line, I failed to mention the "collection" is a subset of the original bunch. Then I suppose such a collection could diverge, but then their sum diverges and adding all the rest in (which, having not been included in the collection are bounded above) gives that the total diverges, which is a contradiction. Is this sufficient? Meanwhile, I'm trying the induction approach now. Last edited by skipjack; June 30th, 2017 at 12:48 PM.

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