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Divergence and convergence question

  1. Dec 15, 2011 #1
    Is the sum of 2 divergent series Ʃ(an±bn) divergent? From what I have learned is that it is not always divergent. Is this true? I believe that is what the picture i included is saying, but i maybe miss interpreting it. Also, is the product of 2 divergent series divergent or convergent?
     

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  3. Dec 15, 2011 #2

    mathman

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    The simplest example for the sum is an = -bn.

    For the product case let an = bn = 1/n. ∑1/n diverges, ∑1/n2 converges.
     
  4. Dec 15, 2011 #3
    Sorry I don't quite understand the sum example. Is the sum of 2 divergent series always divergent? Yes or no? Thanks.
     
  5. Dec 16, 2011 #4

    mathman

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    The sum of two divergent series may be convergent or divergent. My example is a case of convergent. It is easy to construct other cases (an=bn) where it is divergent.
     
  6. Dec 16, 2011 #5
    Oh i understand the sum part, but I think the product of two divergent series is also divergent.
    Because when you say that Ʃ1/n*Ʃ1/n is convergent you are missing an important rule Ʃan*Ʃbn≠ Ʃ(an*bn) so you can't apply your second example to the product of two divergent series. For multiplication of series is are like multiplying infinite quantities
    such as (a_1+a_2+a_3)*(b_1+b_2+b_3) you have to distribute each term to each term and simplify. Thus Ʃan*Ʃbn≠ Ʃ(an*bn), which you made the assumption in your example.


    I could be wrong, but this is what I have read.
     
  7. Dec 17, 2011 #6

    mathman

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    If you are taking a product of two divergent series, it will always be divergent. However I thought you meant term by term product, not simply taking two numbers (the sums) and multiplying together.
     
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