How Does the Sequence Convergence to 1/2 Work?

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    Convergence Sequence
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Discussion Overview

The discussion revolves around the convergence of a sequence defined by the sum of fractions involving the index n, specifically the sequence given by \(\left\{\frac{1}{n^2}+\frac{2}{n^2}+...+\frac{n-1}{n^2}\right\}\). Participants explore the limit of this sequence and whether it converges to 1/2 or zero, delving into the nature of sequences versus series.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant notes that the sequence converges to 1/2 according to their textbook but expresses confusion about how this is possible since the individual fractions approach zero as n increases.
  • Another participant suggests that the question hinges on whether the fractions tend to zero faster than their count increases.
  • There is a request for an analytical method to find the limit of the sequence.
  • A participant provides a partial sum representation, indicating that the sum can be expressed using Gauss's formula, which prompts admiration from others.
  • Some participants clarify that the expression is a sequence rather than a series, emphasizing the role of the index n in each term.
  • One participant asserts that the limit of the sequence is zero, while another claims the limit of the sequence of partial sums is 1/2, highlighting a potential disagreement.
  • Another participant discusses the relationship between sequences and series, suggesting a method to express the sequence as a series but acknowledges it may complicate the understanding.
  • A participant expresses frustration and requests to be ignored, indicating a personal struggle with the topic.

Areas of Agreement / Disagreement

Participants express differing views on the limit of the sequence, with some asserting it approaches zero and others claiming it converges to 1/2. The discussion remains unresolved regarding the correct interpretation of the sequence and its limit.

Contextual Notes

There is ambiguity regarding the definitions of sequences and series in this context, as well as the mathematical steps involved in determining the limit. The discussion also reflects varying levels of understanding among participants.

quasar987
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Apparently (according to my textbook), the sequence defined by

[tex]\left\{\frac{1}{n^2}+\frac{2}{n^2}+...+\frac{n-1}{n^2}\right\}[/tex]

converges towards 1/2, i.e. has 1/2 as a limit.

How could that be?! It seems to me that as n approaches infinity, all the fractions fall to zero. What is it I'm missing?
 
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The question is if they tend to zero faster than their number grow towards infinity.
 
:eek:

Is there a way to find this analytically?
 
quasar987 said:
:eek:

Is there a way to find this analytically?
Sure; you may write the partial sum as:
[tex]\frac{1}{n^{2}}(1+2+++n-1)=\frac{1}{n^{2}}\frac{n(n-1)}{2}[/tex]
 
arildno said:
Sure; you may write the partial sum as:
[tex]\frac{1}{n^{2}}(1+2+++n-1)=\frac{1}{n^{2}}\frac{n(n-1)}{2}[/tex]

Gauss's sum again! Damn! You guys are smart, are you all doctors in mathematics or physics?
 
Some of them are. :) Don't worry about it, I feel the same way you do all the time.
 
quasar987 said:
Apparently (according to my textbook), the sequence defined by

[tex]\left\{\frac{1}{n^2}+\frac{2}{n^2}+...+\frac{n-1}{n^2}\right\}[/tex]

converges towards 1/2, i.e. has 1/2 as a limit.

Hold on a second. How is it that the index appears in every term when you list out the series?

Also, the above is a series, not a sequence.

How could that be?! It seems to me that as n approaches infinity, all the fractions fall to zero. What is it I'm missing?

The limit of the sequence is zero.
The limit of the sequence of partial sums is 1/2.
 
Tom Mattson said:
Hold on a second. How is it that the index appears in every term when you list out the series?

Also, the above is a series, not a sequence.

Tom, it's precisely the fact that the index "n" appears in each of the terms that makes this a sequence, and not a series, as it's given.

[tex]a_n=\sum_{i=1}^{n-1}\frac{i}{n^2}[/tex]

It's the limit of [tex]a_n[/tex] he's after. Since each of the terms in the sum is dependent on n, you can't break it into a series as I suspect you are thinking of doing.


You can of course think of any sequence as a series, by setting [tex]b_1=a_1, b_n=a_n-a_{n-1}[/tex], then [tex]a_n=\sum_{i=1}^{n}b_i[/tex], but that can be an awkward thing to do. In this case we'd find [tex]b_n=\frac{1}{2n(n+1)}[/tex], but I don't think that's what you were getting at?
 
Last edited:
  • #10
I really do know better than that...

Do me a favor and just ignore me for the rest of the night...
 
Last edited:

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