Derivatives of a Function: General Case

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Discussion Overview

The discussion revolves around the properties of partial derivatives of a function of multiple variables, specifically examining the relationship between evaluating a partial derivative at a point and substituting variable labels. Participants explore whether the equality between two expressions involving partial derivatives holds true in general for any function of multiple variables.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant proposes that the expression f(a,b) = (\partial / \partial x [g(x,y)])_{x = a, y = b} is equivalent to \partial / \partial a [g(a,b], questioning if this holds true in general.
  • Another participant suggests that the first expression is simply a substitution of variables, implying that the equality is valid.
  • A different participant reiterates that the expression represents the partial derivative of g with respect to x at the point (a,b), emphasizing the importance of the equality for their purposes.
  • One participant notes that the difference between the two expressions lies in the order of differentiation and substitution, agreeing that they are equal.

Areas of Agreement / Disagreement

Participants generally agree on the validity of the equality between the two expressions, although the emphasis on the importance of this equality varies among them. There is some nuance in how they articulate their understanding of the relationship between differentiation and substitution.

Contextual Notes

The discussion does not resolve whether the equality holds for all functions or under specific conditions, leaving open the possibility of exceptions based on function behavior or definitions.

Physics_wiz
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Say I have a function [tex]g = g(x,y)[/tex]. Now, I have another function defined as:
[tex]f(x,y) = \partial / \partial x [g(x,y)][/tex]. Is the following true:

[tex]f(a,b) = (\partial / \partial x [g(x,y)])_{x = a, y = b} = \partial / \partial a [g(a,b)][/tex]

a, b, x, y are all variables. Is this true in general for any function of any number of variables?
 
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It looks like you just substituted a for x and b for y. So I guess the answer is yes, you can do that :).
 
Physics_wiz said:
[tex]f(a,b) = (\partial / \partial x [g(x,y)])_{x = a, y = b}[/tex]

To me that looks like the partial derivative of g with respect to x, at the point (a,b). It's better without that middle part, if you just want to give the variables a different set of 'labels'.
 
neutrino said:
To me that looks like the partial derivative of g with respect to x, at the point (a,b). It's better without that middle part, if you just want to give the variables a different set of 'labels'.

Yes, you're right in your interpretation. However, the equality between the middle part and the last part is the most crucial step for my purposes, so I can't take it out.
 
The only difference between the "middle" and "last" parts of you statement are that in the middle part, you differentiate using the "symbols" x and y, then replace them by a and b, while, in the last part, you replace x and y by a and b, then differentiate. Yes, those are equal.
 

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