Why Is There No Maclaurin Expansion for Log(x) Compared to Log(x+1)?

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Discussion Overview

The discussion centers around the absence of a Maclaurin expansion for the function log(x) compared to the presence of such an expansion for log(x+1). Participants explore the implications of the function's definition and the conditions under which Maclaurin series can be applied.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that the Maclaurin series requires the function and its derivatives to be evaluated at x=0, but log(x) is not defined at that point.
  • There is a question about why log(x+1) is used for expansion instead of other forms like log(x+1/2) or log(x+2), with some suggesting it is mostly for convenience.
  • One participant mentions that the radius of convergence and the speed of convergence are important considerations for the series expansion of log(x+1).
  • Another participant introduces the function log((x+1)/(x-1)), suggesting it can be expanded for any y=(x+1)/(x-1), but notes that expanding the Maclaurin series at x=0 would lead to evaluating the log at negative numbers.
  • There is a claim that the Maclaurin series at x=0 would result in minus infinity as the constant term.

Areas of Agreement / Disagreement

Participants generally agree that log(x) cannot be expanded using a Maclaurin series due to its definition at x=0. However, there are multiple competing views regarding the choice of log(x+1) for expansion and the implications of using other forms.

Contextual Notes

Limitations include the dependence on the definition of the logarithmic function and the specific conditions under which the Maclaurin series is applicable. The discussion does not resolve the implications of radius and speed of convergence for the series.

Mr Davis 97
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I am looking at examples of Maclaurin expansions for different functions, such as e^x, and sinx. But there is no expansion for log(x), only log(x+1). Why is that?
 
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The Maclaurin series uses the values of the function and it's derivatives at x=0. But log(x) is not defined at x=0.
 
FactChecker said:
The Maclaurin series uses the values of the function and it's derivatives at x=0. But log(x) is not defined at x=0.
So why does it have log(x+1), and not log(x+1/2) or log(x+2), for example?
 
Mr Davis 97 said:
So why does it have log(x+1), and not log(x+1/2) or log(x+2), for example?
Mostly convenience, I suppose. Each of the other two expressions could also be expanded as Maclaurin series.
 
Issues to consider are the radius of convergence and the speed of convergence of the series. An expansion of log(x+1) around x=0 will give values for log(y), y=x+1 from y=0 to y=2.
 
Log((x+1)/(x-1)) gives a series that can be used for any y=(x+1)/(x-1)
 
mathman said:
Log((x+1)/(x-1)) gives a series that can be used for any y=(x+1)/(x-1)
Expanding the Maclaurin series at x=0 would be trying to evaluate the log at negative numbers.
 
MacLaurin series at x = 0 has minus infinity as the constant term.
 

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