Question about Taylor series and big Oh notation

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Discussion Overview

The discussion centers on the relationship between Taylor series and "big Oh" notation, specifically in the context of the Taylor expansion of the exponential function e^x. Participants explore the meaning and implications of the notation in this mathematical framework.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant asks for clarification on why the statement e^x = 1 + x + x^2/2 + O(x^3) is true, indicating a familiarity with big Oh notation but uncertainty about its application.
  • Another participant explains that in this context, big O notation signifies that the remaining terms in the Taylor expansion are of the order x^3 and higher.
  • A further contribution refines this understanding, suggesting that big Oh notation implies the absolute value of the difference between e^x and the Taylor series terms is bounded by a constant times |x^3| for sufficiently small x.
  • One participant reflects on their learning experience in computer science, noting that their understanding of big Oh notation was limited to "of order and higher."
  • A later reply asserts that the original statement is only true if x < 1, adding that for x > 1, a different notation (Ω(x^3)) would be appropriate, indicating a lower bound for the function.
  • This participant also references an external source for further clarification on big Oh notation.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the big Oh notation depending on the value of x, indicating that there is no consensus on the conditions under which the original statement holds true.

Contextual Notes

Some assumptions regarding the behavior of the function e^x and the applicability of big Oh notation are not fully explored, particularly in relation to the bounds for different ranges of x.

AxiomOfChoice
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Question about Taylor series and "big Oh" notation

Can someone please explain WHY it's true that

[tex] e^x = 1 + x + \frac{x^2}{2} + \mathcal{O}(x^3)[/tex]

I'm somewhat familiar with "big Oh" notation and what it stands for, but I'm not quite sure why the above statement is true (or statements like it). Thanks!
 
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I know big O notation goes beyond what it's used for here but in this case, it means the remaining terms in the taylor expansion of that exponential are of the order x^3 and higher.
 


Pengwuino said:
I know big O notation goes beyond what it's used for here but in this case, it means the remaining terms in the taylor expansion of that exponential are of the order x^3 and higher.
Given what "big Oh" notation usually means, I think that, more precisely, it means that the absolute value of the difference between [itex]e^x[/itex] and the first few terms of the Taylor series is bounded by some constant times [itex]|x^3|[/itex] for [itex]x[/itex] sufficiently close to zero.

I, too, used to think that [itex]\mathcal{O}(x^3)[/itex] was just a stand-in for the sentence "terms of order [itex]x^3[/itex] and higher." But that's not all it says, apparently.
 


Yah it says something more but I first learned it in our computer science courses which were simply god awful (god awful department as well) so we never knew what it meant beyond "of order and higher".
 


AxiomOfChoice said:
Can someone please explain WHY it's true that

[tex] e^x = 1 + x + \frac{x^2}{2} + \mathcal{O}(x^3)[/tex]

I'm somewhat familiar with "big Oh" notation and what it stands for, but I'm not quite sure why the above statement is true (or statements like it). Thanks!

This is only true if x<1. It means that e^x is 1+x+x/2 plus a function that is bounded above by a Kx^3 for some K, for sufficiently small x.

If x>1, you'd have to write [tex] e^x = 1 + x + \frac{x^2}{2} + \Omega(x^3)[/tex]

Which means the same except a function bounded BELOW by Kx^3, for sufficiently large x.

Check out http://en.wikipedia.org/wiki/Big-oh#The_family_of_Bachmann.E2.80.93Landau_notations
 
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