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Homework Help: Root and Ratio tests Inconclusive

  1. Nov 11, 2006 #1
    Here is the question in the book:
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    Give an example of a divergent infinite series of positive numbers [itex]a_{n}[/itex] such that [itex]\lim_{n\rightarrow \infty}a_{n+1}/a_{n} = \lim_{n\rightarrow \infty}a_{n}^{1/n} = 1[/itex] and an example of a convergent infinite series of positive numbers with the same property.
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    For the divergent series I gave a somewhat trivial example. The series: 1+1+1+1+1+... which is certainly divergent and has the necessary limit properties.

    For the convergent series though I am a little clueless. What should I be looking for. I know that the nth root of the "last" term should be 1, and also the "last" term should be approaching 0, but I just can't seem to get anything to satisfy both. Please don't post such a series, but instead if you could give some hints as to how I should go about finding one, and maybe other things that I should know when looking for such a sequence. Thanks!
     
    Last edited: Nov 11, 2006
  2. jcsd
  3. Nov 11, 2006 #2

    quasar987

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    hint: n^(1/n)-->1
     
  4. Nov 11, 2006 #3
    [itex]\sum_{n=0}^{\infty} 1/n^2[/itex] converges and has the properties. Thanks.

    edit... Using your hint again, I could use [itex]\sum_{n=0}^{\infty} 1/n[/itex] as the divergent series with the desired properties which is probably a little nicer than my lame example :smile:
     
    Last edited: Nov 11, 2006
  5. Nov 11, 2006 #4

    quasar987

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    Nice.

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