Saturday, 26 November 2016

complex analysis - Series of a sub sequence which converges in mathbbC



I came across an old question in the analysis course I am studying which goes like this:



Assume that (xn)nN is a sequence which converges to 0 in C. Is there always a sub sequence (xnk)nN such that the series kxnk converges absolutely? If yes prove, if not give a counter example.



My first intuition looking at the questions is to that that the answer is yes, but I'm not really sure how to go about proving it. I know that in R if a sequence has a limit and converges, then every sub sequence has the same limit and converges as well. Thus given that the original sequence converges to 0 then each sub sequence must also converge to 0 and the sum of any of these sub sequences would also converge.




Is this correct or am I wrong from the start? Any help is appreaciated


Answer



Since xn converges towards zero, for every k there exists nk such that |xnk|<12k. xnk converges absolutely.


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