Understanding little-o notation

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Discussion Overview

The discussion centers around the understanding of little-o notation as presented in Apostol's calculus text. Participants express confusion regarding specific examples and proofs related to this notation, exploring its implications and definitions within the context of mathematical analysis.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant expresses difficulty in understanding the proof for theorem 7.8.e and specific examples involving little-o notation, questioning the reasoning behind certain equalities as functions approach zero.
  • Another participant proposes that -1/2x^2 is of order o(x^2) and suggests that the sum of smaller orders, o(x^2) + o(x^3), equals o(x^2), indicating a potential understanding of the notation.
  • Some participants express uncertainty about the necessity of understanding little-o notation for future sections of Apostol's work, questioning whether it can be skipped in the context of the overall material.
  • A participant seeks clarification on the definition of little-o notation, specifically whether it is defined as the limit of h(x)/x approaching zero.
  • Another participant clarifies that little-o notation can apply as x approaches any limit, not just zero or infinity, defining f(x) = o(g(x)) as f(x)/g(x) approaching zero.
  • A later reply provides a reference to a rigorous definition of little-o notation from another calculus text, suggesting it is a vector space of functions that approach zero.

Areas of Agreement / Disagreement

Participants generally express confusion and uncertainty regarding little-o notation, with no consensus on its understanding or necessity for future study. Multiple competing views on its definition and application are present.

Contextual Notes

Some participants mention specific examples and definitions that may depend on interpretations or assumptions not fully explored in the discussion. The applicability of little-o notation in relation to other mathematical concepts remains unresolved.

Who May Find This Useful

Readers interested in advanced calculus, real analysis, or those studying Apostol's work may find this discussion relevant, particularly if they are grappling with similar concepts in little-o notation.

zonk
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I am currently reading Apostol volume 1, and I'm having trouble understanding little-o notation. I have searched the web for free resources, but can't find any. I don't quite understand his proof for theorem 7.8.e, and I don't understand his examples.

For instance, why does \frac{1}{1 + g(x)} = 1 - g(x) + g(x)\frac{g(x)}{1 + g(x)} = 1+ g(x) + o(g(x)) as g(x) approaches 0?

Also for his example, why does o(-\frac{1}{2}x^2 + o(x^3)) = o(x^2)?

Anyone know of any free resource that explains this lucidly?
 
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I think I understand it better. -1/2x^2 is of order o(x^2), and since we are considering smaller orders o(x^2) + o(x^3) = o(x^2). Also the third term approaches zero and 1 - g(x) is of order o(g(x)). Is this understanding correct?
 
No, I still don't understand little-o. I tried some exercises and failed miseraby. Is it something we need to understand a future section in Apostol, or is it something books on real analysis present and can safely be skipped with respect to understanding the rest of Apostol Calculus volume 1 and 2?
 
zonk said:
No, I still don't understand little-o. I tried some exercises and failed miseraby. Is it something we need to understand a future section in Apostol, or is it something books on real analysis present and can safely be skipped with respect to understanding the rest of Apostol Calculus volume 1 and 2?

Just to confirm, is your little o(h(x)) defined to be lim (x->0) h(x)/x -> zero? If not what is it?
 
Yeah, except x can approach any limit, not just 0 or infinity. So f(x) = o(g(x)) means that f(x)/g(x) -> 0.
 
zonk said:
I am currently reading Apostol volume 1, and I'm having trouble understanding little-o notation. I have searched the web for free resources, but can't find any. I don't quite understand his proof for theorem 7.8.e, and I don't understand his examples.

For instance, why does \frac{1}{1 + g(x)} = 1 - g(x) + g(x)\frac{g(x)}{1 + g(x)} = 1+ g(x) + o(g(x)) as g(x) approaches 0?

Also for his example, why does o(-\frac{1}{2}x^2 + o(x^3)) = o(x^2)?

Anyone know of any free resource that explains this lucidly?

You can find the rigorous definition of little-o in Section 5 of Chapter 3 from Advanced Calculus by Loomis http://www.math.harvard.edu/~shlomo/

A function f \in \mathbb{o} if f(0)=0 and \left\|f(\alpha)\right\|/\left\| \alpha \right\| \rightarrow 0 as \alpha \rightarrow 0

All functions in little-oh constitute a vector space
 

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