Friday, 24 July 2015

calculus - Convergence of series of the form sum1/nx



Why the solution says n=11n1.5 converges? Does every series n=11nx converges to 0 except 1/n (harmonic Series)?



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I found that after verifying a series with series convergence test, especially for comparison test and limit comparison test, I do not have a clear mind on verifying the series that I comparing is converge or diverge. Do I need to use the partial sum to test the convergence of series that I comparing every time?


Answer



When discussing series, avoid saying "series converges to ..."; this kind of statement is almost always misguided.



When discussing sequences, we talk about what their terms converge to. For example, 1/n converges to 0 as n.



When discussing series, we could still think about what happens to individual terms, but this is not the main thing: the convergence of a series is a matter of their partial sums. The series n=11/2n converges, but it would be wrong to say that it "converges to 0". Rather, the sequence of its terms 1/2n converges to zero. The series itself converges and has sum equal to 1.



In your examples: the sequence of terms 1/nx converges to 0 for any x>0.




But the series n=11/nx converges only when x>1. And its sum is never 0; it's some positive number which we don't necessarily know or perhaps even care about.


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