Absolute Convergence Proof: The Relationship Between a_k and b_k

In summary, if a_k is less than or equal to b_k for all natural numbers k, and the sum of b_k from k=1 to infinity is absolutely convergent, then the sum of a_k from k=1 to infinity also converges. A counterexample to prove this statement false would be to let a_k=-1 and b_k=0, which satisfies the conditions but results in the sum of a_k diverging. However, this counterexample may not hold if the terms are positive or written in magnitudes.
  • #1
DEMJ
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Homework Statement



if [itex]a_k \le b_k[/itex] for all [itex]k \in \mathbb{N}[/itex] and [itex]\sum_{k=1}^{\infty} b_k[/itex] is absolutely convergent, then [itex]\sum_{k=1}^{\infty} a_k[/itex] converges.



Homework Equations


It's either true or false.


The Attempt at a Solution


I think a counterexample to prove it's false is if we let [itex]a_k=-1, b_k = 0[/itex] which satisfies [itex]a_k \le b_k[/itex] and [itex]b_k[/itex] is abs. convergent but [itex]\sum_{k=1}^{\infty} a_k[/itex] diverges.
Is this a correct counterexample?
 
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  • #2
that looks reasonable, if the ak & bk were positive numbers or it was written for maginutudes it would be true
 

Related to Absolute Convergence Proof: The Relationship Between a_k and b_k

1. What is the definition of absolute convergence?

Absolute convergence is a mathematical concept that refers to the convergence of an infinite series, where the sum of the absolute values of the terms in the series is finite.

2. How is absolute convergence different from conditional convergence?

Conditional convergence refers to the convergence of an infinite series when the sum of the terms in the series is finite, but the sum of the absolute values of the terms is infinite. This means that the series may converge to different values depending on the order in which the terms are added, unlike absolute convergence.

3. What is the significance of proving absolute convergence?

Proving absolute convergence is important because it guarantees that the infinite series will have a well-defined sum, regardless of the order in which the terms are added. This allows for more accurate calculations and predictions in various mathematical and scientific fields.

4. What are some common methods used to prove absolute convergence?

There are several methods that can be used to prove absolute convergence, including the Ratio Test, the Comparison Test, and the Root Test. These tests involve evaluating the limit or ratio of the terms in the series to determine if the series converges absolutely.

5. Can a series be absolutely convergent but not convergent?

No, if a series is absolutely convergent, it must also be convergent. This is because absolute convergence is a stronger condition than convergence, and if a series satisfies the criteria for absolute convergence, it will also satisfy the criteria for convergence.

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