What Is a Continuous Partial Derivative in Two Variables?

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Discussion Overview

The discussion centers around the concept of continuous partial derivatives in two variables, particularly in the context of a specific function that exhibits discontinuity at a point despite having existing partial derivatives. Participants explore the implications of this discontinuity on differentiability and the visualization of continuous partial derivatives.

Discussion Character

  • Conceptual clarification, Debate/contested, Technical explanation

Main Points Raised

  • One participant describes a function that has partial derivatives at a point but lacks continuity, questioning the meaning of 'continuous partial derivative' in this context.
  • Another participant asserts that a partial derivative is a function, thus continuity is a relevant concept.
  • A participant queries whether the lack of continuity at a point implies that the function is not differentiable at that point.
  • It is noted that differentiability at a point requires the existence of a unique tangent plane, and while differentiability implies the existence of partial derivatives, the reverse is not true.

Areas of Agreement / Disagreement

Participants generally agree on the relationship between differentiability and the existence of partial derivatives, but there is ongoing discussion regarding the implications of continuity on differentiability, indicating some level of disagreement or uncertainty.

Contextual Notes

The discussion highlights the nuances of differentiability and continuity in the context of partial derivatives, with specific reference to a function that behaves differently at a critical point.

Shaybay92
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My textbook describes how some functions are not well approximated by tangent planes at a particular point. For example

f(x)= xy / (x^2 + y^2) for x /= 0
0 for x = 0

at (0,0) the partial derivatives exist and are zero but they are not continuous at 0. What exactly is a 'continuous partial derivative' in two variables? How do you visualize this?
 
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A partial derivative is a function, so speaking of continuity makes perfect sense.

This should help:

http://www.math.tamu.edu/~tvogel/gallery/node14.html
 
In the case of that example, is it not differentiable at zero because its not continuous there?
 
Shaybay92 said:
In the case of that example, is it not differentiable at zero because its not continuous there?

Correct.

A function is only differentiable at zero if a unique tangent plane can be assigned there.

Differentiability IMPLIES existence of partial derivatives, but the converse does not hold.
 
Thanks! By the way, nice job on the 9,999 posts :)
 

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