Find harmonic conj of u(x,y)=ln(x^2+y^2)

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Homework Help Overview

The discussion revolves around finding the harmonic conjugate of the function u(x,y) = ln(x^2 + y^2) and determining the region where it is defined. Participants explore the implications of the function's properties in the context of complex analysis, particularly regarding the existence of a harmonic conjugate in the punctured complex plane C\{0}.

Discussion Character

  • Conceptual clarification, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss converting the function to polar coordinates and identify v(r,Θ) = Θ as a potential harmonic conjugate. Questions arise about the continuity of v and its implications for the Cauchy-Riemann equations. There is exploration of the behavior of Θ around the unit circle and the transition from 2π to 0.

Discussion Status

The discussion is active, with participants questioning the continuity of v and its relationship to the existence of a harmonic conjugate. There is a focus on understanding why the lack of continuity affects the satisfaction of the Cauchy-Riemann equations, with multiple interpretations being explored.

Contextual Notes

Participants note that the function u has no harmonic conjugate on C\{0}, raising questions about the implications of multivaluedness and continuity in the context of complex functions.

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Homework Statement


Find harmonic conjugate of u(x,y)=ln(x^2+y^2) and specify the region it is defined
then show u has no harm conj on C\{0}


Homework Equations





The Attempt at a Solution


Ok so i found the harmonic conj by converting to polar and found it to be v(r,Θ) = Θ.

I am having trouble finding out where v is defined. Also it seems our analytic function f = u + iv
is log(z) since u(r,Θ) = ln|r|. Isn't that defined everywhere but zero? Why would u not have a harmonic conj at C\{0}?
 
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The complex log can be defined everywhere but zero, but then you cannot make it continuous.
The same problem comes up with v=Θ.
 
mfb said:
The complex log can be defined everywhere but zero, but then you cannot make it continuous.
The same problem comes up with v=Θ.

If you don't mind, could you explain where the problem with v = Θ comes in?

Also, since v = Θ is not continuously defined on C\{0} would that mean that it doesn't have a partial, and therefore cannot satisfy cauchy riemann, which in turn is the reason why u cannot have a harm conj on C\{0}?
 
DotKite said:
If you don't mind, could you explain where the problem with v = Θ comes in?
What happens if go around the unit circle, for example?

Also, since v = Θ is not continuously defined on C\{0} would that mean that it doesn't have a partial
I don't know what you mean here.
 
mfb said:
What happens if go around the unit circle, for example?
Θ will go from 0 to 2pi


I don't know what you mean here.

I am trying to find the reason why v(r,Θ) not continuous on C\{0} implies u does not have a harmonic conjugate on C\{0}. Is it because if it is not continuous then it's partial derivative will not exist on C\{0}, thus implying that that Cauchy Riemann equations will not be satisfied on C\{0}?
 
DotKite said:
Θ will go from 0 to 2pi
And what happens at the transition "close to 2 pi -> 0"?

I am trying to find the reason why v(r,Θ) not continuous on C\{0} implies u does not have a harmonic conjugate on C\{0}. Is it because if it is not continuous then it's partial derivative will not exist on C\{0}, thus implying that that Cauchy Riemann equations will not be satisfied on C\{0}?
At least not everywhere. Right.
 
When theta gets to 2pi it repeats. So at 0 and 2pi the function is multivalued thus not continuous?
 
DotKite said:
When theta gets to 2pi it repeats. So at 0 and 2pi the function is multivalued thus not continuous?
Right.
 
Thanks for all the help these last couple of days mfb
 

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