1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Value that this series converges to

  1. May 12, 2012 #1
    Ʃ4/(n^2-1) n=2

    Need help trying to solve it. Ive used other tests like the ratio test and they have all resulted inconclusive.

    It does converge because the Lim as n→∞ equals 0. Do not know where to go from here. Some help would be nice! Thank you!
     
  2. jcsd
  3. May 12, 2012 #2
    Could you tell me the name of the test that gives this conclusion?
     
  4. May 12, 2012 #3
    it was a test for divergence, i was told if the lim n→∞ of an ≠ 0 then it diverges
     
  5. May 12, 2012 #4
    Does this test ever tell you that a series converges?
     
  6. May 12, 2012 #5
    not explicitly, but if it does not diverge then i am assuming it must converge
     
  7. May 12, 2012 #6
    Well it's a common misconception, so don't feel bad. But the test for divergence is not always conclusive (in fact it often isn't). It only singles out series that don't meet the minimum requirement for convergence. It's kind of like an entrance exam; you gotta pass it to get in, but it's no guarantee that you're gonna be successful.

    So bottom line, the test for divergence only tells you that a series diverges. It never tells you when a series doesn't diverge (i.e. it never tells you that a series is convergent).

    Look at the harmonic series for a stock counterexample to the kind of reasoning you tried to use.
     
  8. May 12, 2012 #7
    As far as your problem goes, if I tell you that you've got yourself a telescoping series, is that enough to get you going?
     
  9. May 12, 2012 #8
    yes actually, thank you! i just needed a direction :)
     
  10. May 12, 2012 #9
    Good.

    Now as far as having a good strategy heading into these kinds of problems (so this isn't about math, really; it's about problems you run across in math classes):

    There are basically only two elementary ways that we can find the values of convergent series (remember the tests only tell you, when they're conclusive, whether or not a series converges/diverges. they don't tell you what it converges to). (1) Compute the limit of the partial sums and (2) get clever with some tricks with Power Series (don't worry if you haven't seen them). The kinds of series whose partial sums we can compute so that we can take a limit are essentially the geometric series (for which we have a formula) and telescoping series.

    So if you have a problem that asks you to compute the value of a convergent series, there's a real good chance that it's either a geometric series or a telescoping series. Geometric series are relatively easy to identify, so ...
     
  11. May 12, 2012 #10

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    Your notation is unclear: is your series supposed to be
    [tex] \sum \frac{4}{n^2-1} n = \sum \frac{4n}{n^2-1}, \text{ or is it }
    \sum \frac{4}{(n^2-1)n}?[/tex]
    If you mean the former, you have more-or-less written it correctly already (reading your expression according to standard priority rules), but if you mean the latter, you should use parentheses, like this: Ʃ4/[(n^2-1)n].

    Assuming you mean the latter: your argument about the series converging because the nth term --> 0 is patently *false* (that is, the argument is false, not necessarily the conclusion). Lots of divergent series have terms that --> 0; it is just that they do not --> 0 fast enough. That is the issue you must decide.

    RGV
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook