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Homework Help: Is this series convergent or divergent

  1. Nov 2, 2011 #1
    1. The problem statement, all variables and given/known data

    Me and my friend are debating on wether the follow seris is convergent or divergent. The seris is the sum of (-1)^n-1 * ln(n)/n.

    2. Relevant equations
    p test and comparision tests.

    And alternating series test

    3. The attempt at a solution
    My approach to this problem was that the ln(n) portion grows much slower than n portion. So I compared this function to 1/n. I know 1/n is divergent, so I concluded the above function was also divergent. My friend argues that the limit of ln (n)/n is zero and is greater than ln(n+1)/(n+1). So he says its convergent.
  2. jcsd
  3. Nov 2, 2011 #2


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    Science Advisor
    Homework Helper

    What about the (-1)^(n-1) factor? I think you'd better look at the alternating series test.
  4. Nov 2, 2011 #3


    Staff: Mentor

    What does the alternating series test say in this case? You seem to be ignoring the fact that this is an alternating series.
  5. Nov 2, 2011 #4
    So by using the alternating series test, this seris is convergent?
  6. Nov 2, 2011 #5


    Staff: Mentor

    Also, disregarding for the moment that the series is alternating, when you compare ln(n)/n with 1/n, what must happen for you to conclude that Ʃ(ln(n)/n) diverges?
  7. Nov 2, 2011 #6
    They would have to grow at the same rate?
  8. Nov 2, 2011 #7


    Staff: Mentor

    No, that doesn't have anything to do with the comparison test, which is one of the tests that you listed, and are apparently attempting to use.

    This test should be defined in your book. Specifically, there are different inequalities that come into play, depending on whether you are comparing to a convergent series or to a divergent series.
  9. Nov 2, 2011 #8
    Im confused. Im gonna go youtube this. Have a nice day.
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