Divergence and convergence question

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    Convergence Divergence
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Discussion Overview

The discussion centers around the behavior of sums and products of divergent series, specifically whether the sum of two divergent series is always divergent and the nature of the product of two divergent series. It explores theoretical implications and examples related to convergence and divergence.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether the sum of two divergent series, Ʃ(an±bn), is always divergent, suggesting it may not be.
  • Another participant provides an example where the sum is convergent, stating that if an = -bn, the series can converge.
  • A third participant seeks clarification on whether the sum of two divergent series is always divergent, asking for a definitive yes or no.
  • One participant asserts that the sum of two divergent series can be either convergent or divergent, providing their own example to illustrate this point.
  • Another participant argues that the product of two divergent series is also divergent, emphasizing the distinction between summing series and multiplying series, and referencing the rule that Ʃan*Ʃbn does not equal Ʃ(an*bn).
  • A later reply states that the product of two divergent series will always be divergent, but clarifies that this refers to a term-by-term product rather than simply multiplying the sums of the series.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of sums and products of divergent series. There is no consensus on whether the sum of two divergent series is always divergent, and opinions vary on the nature of the product of divergent series.

Contextual Notes

Participants reference specific examples and rules regarding series, but there are unresolved assumptions about the definitions and conditions under which these series operate. The discussion does not resolve the mathematical implications of these examples.

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

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