Proving the Divergence of A+B Series

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Discussion Overview

The discussion revolves around proving that the sum of a convergent series (A) and a divergent series (B) results in a divergent series (A + B). The scope includes theoretical aspects of series convergence and divergence, as well as mathematical reasoning related to infinite series.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant asks how to prove that if series A converges and series B diverges, then the sum of these series diverges.
  • Another participant states that the limit of a sum is the sum of the limits, suggesting that if both series converge, their sum will also converge.
  • A different participant introduces a relationship involving the sums of the series, implying that if both sums converge, it affects the sum of series B.
  • One participant expresses doubt about the simplicity of the proof, emphasizing that the series are defined from n=1 to infinity and that normal rules may not apply in this context.
  • Another participant counters that the rules of convergence apply, asserting that if the sums of A and B exist, then the sum of A + B also exists and is equal to the sum of A plus the sum of B.
  • A participant clarifies that they assumed the discussion was about infinite series, as the terms "convergent" and "divergent" are relevant in that context.

Areas of Agreement / Disagreement

Participants express differing views on the application of convergence rules for infinite series. There is no consensus on the proof or the validity of the claims made regarding the sums of the series.

Contextual Notes

Participants highlight the importance of the definitions and properties of series when discussing convergence and divergence, particularly in the context of infinite series. There are unresolved assumptions about the behavior of sums at infinity.

mrnoll
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hi.

how can I prove that if A is a converges and B diverges

that the Sum of these series (A +B) diverges..



( A = a1 + a2 + a3 + ...
B = b1 + b2 + b3 + ...) if the series start from n=1
 
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Limit of a sum is the sum of the limits.
The convergence of the series is determined by the row of partial sums.
Therefore, if both rows converge, the sum of both will too.
 
In order to apply Dimitri Terryn's suggestion, you will also have to note that if C=(A+ B), then Sum(B)= Sum(C- A). Assume both Sum(C) and Sum(A) converge. What does that tell you about Sum(B)?
 
Hi,

I don't think it´s that easy actually.
Because what I forgot to say was that these series go from
n=1 to n=infinity.

And in infinity normal rules don't always apply like SumA + SumB = Sum(A+B)

I need a proof... :)
 
it is that easy because what the infinite series converges to is defined to be the limit of the sequence of partial sums.
 
I don't think it´s that easy actually.
Because what I forgot to say was that these series go from
n=1 to n=infinity.

And in infinity normal rules don't always apply like SumA + SumB = Sum(A+B)

In this case they do. If Sum(A) and Sum(B) exist then
Sum(A+ B) exists and is equal to Sum(A)+ Sum(B).

I had assumed you were talking about infinite series since otherwise it wouldn't make sense to talk about "convergent" and "divergent".
 

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