Approximating different functions

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Discussion Overview

The discussion revolves around the approximation of two different functions, ##f## and ##f'##, which share the same domain but have different codomains. Participants explore whether the approximation ##f'(x') \approx f(x')## holds when ##x' = x + \sigma##, with ##|\sigma| << 1##. The scope includes conceptual reasoning and mathematical exploration.

Discussion Character

  • Exploratory, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant questions the validity of the approximation ##f'(x') \approx f(x')## under the given conditions, providing a counterexample with specific functions: ##f(x)=x^{-2}## and ##f'(x)=x^{-1}##.
  • Another participant suggests that the notation used for the functions could be misleading, as ##f'(x')## may imply a derivative rather than a distinct function.
  • A later reply agrees with the notation concern and mentions a preference for using different symbols like ##f## and ##g## to avoid confusion.

Areas of Agreement / Disagreement

Participants express disagreement regarding the generality of the approximation. There is no consensus on the validity of the approximation, and the discussion remains unresolved.

Contextual Notes

Participants highlight potential confusion in notation, which may affect the clarity of the discussion. The counterexample provided raises questions about the conditions under which the approximation might hold.

kent davidge
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I have a question regarding different functions. Suppose we have two functions ##f## and ##f'## with same domain, but different codomains.

Consider that ##f': x' \mapsto f'(x')## and ##f: x \mapsto f(x)##. If ##x' = x + \sigma##, with ##|\sigma| << 1##, can we say that ##f'(x') \approx f(x')##?
 
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No. Not in this generality. Let's say ##f(x)=x^{-2}\; , \;f\,'(x)=x^{-1}##. Then we have ##x'=-{10}^{-n}=x+\sigma = {10}^{-n}-2\cdot {10}^{-n}## and ##|\sigma|=|2\cdot {10}^{-n}| << 1## but ##-{10}^{n} =f\,'(x') \not \approx {10}^{2n}=f(x')##.
 
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kent davidge said:
I have a question regarding different functions. Suppose we have two functions ##f## and ##f'## with same domain, but different codomains.

Consider that ##f': x' \mapsto f'(x')## and ##f: x \mapsto f(x)##. If ##x' = x + \sigma##, with ##|\sigma| << 1##, can we say that ##f'(x') \approx f(x')##?
If you have two different functions, why not call them f and g? Or ##f_1## and ##f_2##? Your notation ##f'(x')## looks like we're evaluating the derivative of f at a number x'.
 
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Mark44 said:
If you have two different functions, why not call them f and g? Or ##f_1## and ##f_2##? Your notation ##f'(x')## looks like we're evaluating the derivative of f at a number x'.
Yea, I agree. Coincidentally I was going to write them down as ##f## and ##g## in the opening post, but I used the more abstract notation.
 

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