Solving Sequences & Series: Limits & Sums

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Discussion Overview

The discussion revolves around the relationship between sequences and their corresponding infinite series, particularly focusing on limits and sums. Participants explore concepts related to convergence, the significance of partial sums, and the conditions under which a series converges or diverges.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether the limit of a sequence as its terms approach infinity is the same as the sum of the infinite series formed by those terms.
  • Another participant expresses confusion about the purpose of introducing the sequence of partial sums when finding the sum of an infinite series, seeking clarification on its significance.
  • A participant provides an example of a converging sequence (1/2, 1/4, 1/8, ...) that converges to 0, but argues that the sum of this series does not converge to 0.
  • It is noted that for a series to converge, the corresponding sequence of terms must converge to zero, but this does not guarantee that the series itself converges, as illustrated by the harmonic series example.
  • Another participant explains that the concept of infinite sums is defined through the limit of a sequence and the formation of partial sums, stating that a series converges if the sequence of partial sums converges.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between sequences and series, particularly regarding convergence. There is no consensus on the implications of the limit of a sequence versus the sum of an infinite series, and the discussion remains unresolved.

Contextual Notes

Participants highlight limitations in understanding the definitions and implications of convergence, particularly regarding the harmonic series and the nature of infinite sums.

Bashyboy
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Hello,

I am curious to know that if we take some seqence, [itex]a_n[/itex], and take the limit as the the terms of the sequence goes to infinity, will the sequence head towards the same value that the the sum of the infinite amount of terms added together? (I hope I worded that properly...)
 
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I have another question, regarding infinite series. The book I am reading says if we want to find the sum of an infinite series, we have to consider the following sequence of partial sums. (I attached the picture.) What is the point of introducing this idea, does it elucidate anything? because it seems to just confuse me--and they don't even elaborate on why they are doing this. Could someone please help by explaining it to me?
 

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Take the sequence 1/2, 1/4/ 1/8, 1/16, ... (1/2)^n. The sequence converges to 0, but does the sum converge to 0? We can clearly see that it does not.

To answer your second post. It seems you are studying geometric series at the moment which I gave you an example of. One can derive a formula for finding the sum of a geometric series by considering partial sums.
 
Bashyboy said:
Hello,

I am curious to know that if we take some seqence, [itex]a_n[/itex], and take the limit as the the terms of the sequence goes to infinity, will the sequence head towards the same value that the the sum of the infinite amount of terms added together? (I hope I worded that properly...)

If a series converges at all, it's necessary for the corresponding sequence of terms to converge to zero. Mmm_Pasta gave an example.

Note that a sequence converging to zero does not necessarily mean that the series converges. The famous harmonic series 1/2 + 1/3 + 1/4 + 1/5 + ... diverges to infinity; yet the corresponding sequence of terms goes to zero.
Bashyboy said:
I have another question, regarding infinite series. The book I am reading says if we want to find the sum of an infinite series, we have to consider the following sequence of partial sums. (I attached the picture.) What is the point of introducing this idea, does it elucidate anything? because it seems to just confuse me--and they don't even elaborate on why they are doing this. Could someone please help by explaining it to me?

The axioms of the real numbers tell us that we can add two numbers and get a third. And we can extend that to work with any finite sum of terms.

But how can we even assign meaning to an infinite sum? We do that by first defining the limit of a sequence. Then, given an infinite sum, we form the sequence of partial sums; and we say that the series converges if the sequence of partial sums does.
 
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