Proof of Convergence/Divergence Unaffected by Series Starting Point

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Discussion Overview

The discussion revolves around the convergence or divergence of infinite series and whether the starting point of the series affects this property. Participants explore definitions and provide examples to illustrate their points, with a focus on both general cases and specific series.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant asserts that if an infinite series is convergent from an integer T to infinity, it is also convergent from 1 to infinity, and vice versa.
  • Another participant suggests that this assertion is evident from the definition of a convergent series and encourages checking the definition.
  • A third participant provides a counterexample involving the series \(\sum_{x=10}^{\infty} \frac{1}{(x-7)^2}\), which converges, while \(\sum_{x=1}^{\infty} \frac{1}{(x-7)^2}\) does not converge, indicating that the starting point does affect convergence.
  • One participant questions whether the series in question pertains to power series, suggesting that the answer may vary based on the context.
  • Another participant argues that the example given by the third participant is not a valid series since the 7th term is undefined.

Areas of Agreement / Disagreement

Participants express disagreement regarding the impact of the starting point on convergence, with some supporting the idea that it does not affect convergence, while others provide counterexamples that suggest otherwise. The discussion remains unresolved.

Contextual Notes

Participants highlight the importance of definitions and the conditions under which convergence is assessed, noting that certain series may have undefined terms that complicate the discussion.

adamg
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if we know that an infinite series is convergent from an integer T, to infinity, then the series is convergent from 1 to infinity. conversely, if a series is convergent from 1 to infinity then it is convergent from T to infinity (i.e. starting point of the series does not affect convergence/divergence) This seems obvious but can anyone help me prove it please.
 
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This is obvious from the definition of a convergent series. So check the definition again.
 
I pulled my lecturer up on this recently, consider the series:

[tex]\sum_{x=10}^{\infty} \frac{1}{(x-7)^2}[/tex]

It converges and if you are interested to:

[tex]\frac{1}{12} \left(2 \pi^2 - 15\right)[/tex]

However:

[tex]\sum_{x=1}^{\infty} \frac{1}{(x-7)^2}[/tex]

Clearly does not converge, so be careful how you word it. Anyway, it's not too difficult to prove, just think of it like:

[tex]a_1 + a_2 + \ldots + a_{t-1} + \sum_{n=t}^{\infty} a_n[/tex]
 
Is the series adamg asking about a power series? If so, the answer to his question is yes. However in the more general case as Zurtex showed, it is not true.
 
I'd rather say his example

[tex]\sum_{x=1}^{\infty} \frac{1}{(x-7)^2}[/tex]

is not a series, since the 7'th term is not defined.
 

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