What does equal mixed partial derivatives indicate about a function?

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Discussion Overview

The discussion revolves around the implications of equal mixed partial derivatives for a scalar-valued function f=f(x,y). Participants explore whether this condition indicates continuity, differentiability, or other properties of the function.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions what the equality of mixed partial derivatives implies about the function f, specifically regarding continuity and smoothness.
  • Another participant suggests that continuity is a necessary condition for the function f.
  • A third participant agrees, adding that the partial derivatives up to that order must exist and be continuous at the point in question.
  • Additionally, a later reply posits that the condition implies more than continuity, asserting that it indicates the function f is differentiable and belongs to the class C¹.

Areas of Agreement / Disagreement

Participants express differing views on the implications of equal mixed partial derivatives, with some asserting continuity is sufficient, while others argue for differentiability as a stronger requirement. The discussion remains unresolved regarding the exact conditions needed.

Contextual Notes

Participants do not clarify the specific definitions of continuity or differentiability they are using, nor do they address potential limitations or assumptions in their arguments.

kuba
Given a scalar-valued function [tex]f=f(x,y)[/tex], if it's true that [tex]\frac{\partial^2 f}{\partial x \partial y}=\frac{\partial^2 f}{\partial y \partial x}[/tex], what does that tell about function [tex]f[/tex]? Does it mean that it's continuous, or does it need to be smooth, or...?
 
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I'm presuming that the correct answer is that the function [tex]f[/tex] must be continuous.
 
You are right. The partial derivatives upto that order should exist and be continuous at the point under consideration.
 
Its more than continous, actually it tells you that the function f is differentiable.

[tex]f(x,y)\in C^{1}[/tex]
 

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