What Are the Key Differences Between These Summation Notations?

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Discussion Overview

The discussion focuses on the differences between two summation notations, specifically examining the clarity and intent behind the representation of terms in series. Participants explore the implications of using ellipses in mathematical expressions and how they can lead to different interpretations.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why the first summation explicitly shows two terms while the second shows three, indicating a lack of understanding of the notation.
  • Another participant suggests that the number of terms included is at the discretion of the writer to convey clarity.
  • Some participants argue that the notation 1/1 + 1/2 + ... does not establish a clear pattern, as it could imply different series depending on the context.
  • There is a suggestion that the use of ellipses should provide a clear continuation of the series, but interpretations may vary among readers.
  • Participants note that while the first series may lead to the obvious next term, the second series could mislead readers into guessing an incorrect next term without additional context.

Areas of Agreement / Disagreement

Participants express differing views on the clarity of the summation notations and the implications of using ellipses. There is no consensus on the best practices for representing series or the clarity of the examples provided.

Contextual Notes

Participants highlight that the clarity of summation notation can depend on the context and the conventions used, which may not be universally understood. The discussion reflects varying interpretations of mathematical notation.

Noxide
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Please explain the difference between these two statements.

1 + 2 + ... + n = 1/2n(n+1) for all n in the natural numbers

1 + 1/2 + 1/4 + ... + 1/2^n = 2 - 1/2^n for all n in the natural numbers


Why does the first explicitly show two terms being summed whereas the second shows 3 terms being summed...

I don't think I have a good understanding of how to work with these things...
 
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Hi Noxide! :smile:
Noxide said:
Why does the first explicitly show two terms being summed whereas the second shows 3 terms being summed...

No particular reason …

the writer just puts in as many terms as he thinks makes it clear what he means. :wink:
 
Just to expand on what tiny tim said
1/1 + 1/2 + … doesn’t establish a clear pattern:

It could mean 1/1 + 1/2 + 1/3 + 1/4 + …
or 1/1 + 1/2 + 1/3! + 1/4! + …
or 1/1 + 1/2 + 1/2^2 + 1/2^3 + …

It’s always helpful to give the reader a definite idea of what the series means before the ellipse (…) and the general term.
 
JonF said:
Just to expand on what tiny tim said
1/1 + 1/2 + … doesn’t establish a clear pattern:

It seems no less clear than 1 + 2 + ...
 
One would most likely guess that 3 comes after 1 and 2, which is the writer's intent. For the geometric series 1/2^n however the writer wants to make sure the reader doesn't guess 1/3 for the next term and provides 1/4 instead.
 
The ellipse means to continue in the obvious way. It's ultimately up to the reader to decide the clarity of the intents of the writer. The convention is to give the first three terms, but there are exceptions.
 
Rasalhague said:
It seems no less clear than 1 + 2 + ...

Oh, sorry, I wan't paying attention! Yes, 1+2+... the most obvious guess is 3. But in the second example, the next term is not the most obvious 1/3, but rather 1/4.
 

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