Summing a Series: Grouping Terms Legit?

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Discussion Overview

The discussion revolves around the legitimacy of summing a series by grouping terms, particularly in the context of alternating series and the implications for convergence. It explores both finite and infinite series, addressing concepts such as convergence and absolute convergence.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant questions whether grouping terms in a series is legitimate, particularly in the case of an alternating series, suggesting that the sum could vary based on how terms are grouped.
  • Another participant asserts that for finite series, grouping terms is acceptable, but for infinite series, it raises concerns about convergence.
  • A further contribution clarifies that the sum of an infinite series is defined as the limit of its partial sums, providing an example of the oscillating nature of the partial sums in the case of an alternating series.
  • Another participant notes that demonstrating different groupings can yield different sums is a technique to show that some series are not absolutely convergent, while also stating that showing some groupings yield the same sum does not imply absolute convergence.

Areas of Agreement / Disagreement

Participants express differing views on the legitimacy of grouping terms in infinite series, with some arguing it is problematic due to convergence issues, while others provide examples and techniques related to convergence and absolute convergence. The discussion remains unresolved regarding the implications of grouping terms.

Contextual Notes

Participants reference concepts such as convergent series and absolutely convergent series, indicating that the discussion may depend on these definitions and the specific nature of the series being considered.

pivoxa15
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If one was to sum a series by grouping each two terms would that be legtimate?

i.e if you have an alternating series than grouping two terms would not be would it? i.e 1,-1,1,-1... would sum to 0 if you grouped them or 1 depending on how you group it.

Sometimes the only way to write a series under a summation sign is to group two elements in the series together.
 
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For a finite series, you would be fine.

For an infinite series, not so much. The relevant things to look up are:

Convergent series
Absolutely convergent series

Basically, a convergent series has a limit of its partial sums (meaning you can't just skip by twos) that exists. If a series is absolutely convergent, the sum of the absolute values of the terms converges... and it turns out if it's absolutely convergent, you can re-arrange the terms (otherwise you can't).
 
The sum of an infinite series, if it is convergent, is defined to be the limit of the partial sum S_n as n tends to infinity.

In this case, the partial sums take the form 1, 0, 1, 0...,1,0,...
 
They say the series does not converge because it oscillates.

In fact showing that groups terms in certain ways leads to different answers is a technique used to show some series are not absolutely convergent. The converse is not true though, showing certain groupings give the same answer does not mean it has absolute convergence, since there would be an infinite number of groupings possible.
 

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