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Thread: Question About Series

  1. #1
    Jan 2009

    Question About Series

    I have 2 general questions:

    #1: How do I find the sum of a harmonic series? My book uses the example of the sum of 1/n from n=1 to infinity.

    #2: So far, for all of the problems I've worked in which I'm asked to find if a given series converges/diverges and to find it's sum, the series has either been geometric or telescoping (sometimes after a bit of rearranging).

    What's the general method for finding the sum if the series isn't geometric, telescoping, or harmonic? I think there is another kind called factorial, but I haven't learned that one yet.

    Also, I'm aware of what the definition states; but if I could see an example problem worked in which the series cannot be transformed into any of the above, that would be wonderful. Thanks!
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  2. #2
    Jul 2009
    1. To my knowledge, there is only one Harmonic Series, \sum_{n=1}^\infty \frac{1}{n}, and it diverges (slowly). Therefore it has no sum.
    2. There are many types or classes of infinite series: geometric series, telescoping series, power series, Taylor series, E-Series, series of functions, Laurent series, and on and on. There are some guidelines for summing each kind of series, (there are even strategies for "summing" certain divergent series), but there is nothing like a general formula that simply spits out a sum for whatever series you input. One learns different classes/types of series, and some general strategies for solving them. In some cases, quite a bit of ingenuity is required to demonstrate that a given series sums to a given value.

    If you have a specific series in mind that you need to sum, please provide it and one of us may be able to point you in the needed direction.
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