Complex Analysis 2nd Ed. by Stephen D. Fisher: Q&A

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Discussion Overview

The discussion revolves around questions related to complex analysis, specifically focusing on the variation of the maximum principle and the properties of harmonic functions. Participants are exploring theoretical aspects and proofs related to these concepts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses a desire to ask questions while studying complex analysis, indicating that they may have varying levels of understanding.
  • Another participant presents a variation of the maximum principle, stating that if u(x,y) is harmonic and nonconstant on a domain D, then |u(x,y)| has no local maximum in D, and seeks to prove this statement.
  • It is noted that if u is a complex-valued function, it is harmonic and thus analytic on D, leading to the conclusion that |u| has no local maximum in D based on the Maximum Modulus Principle.
  • A participant challenges the assertion that a harmonic function u implies the existence of another harmonic function v such that f(z) = u(z) + iv(z) is analytic on D, providing a counterexample involving the domain D = ℂ \ {0} and u(z) = log |z|.
  • Another participant acknowledges the assumption of a simply connected domain in the previous discussion.
  • Further discussion indicates that the mean-value property may be a useful approach to prove the statement regarding real-valued harmonic functions.

Areas of Agreement / Disagreement

Participants express differing views on the conditions under which harmonic functions can be combined to form analytic functions. There is no consensus on the implications of the maximum principle or the existence of harmonic functions in certain domains.

Contextual Notes

Some statements depend on the assumption of simply connected domains, and the discussion includes unresolved mathematical steps regarding the proof of the maximum principle for real-valued functions.

becu
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Hi,
I'm studying complex analysis right now, I would like to use this thread to ask questions when I read books. Many questions will be very stupid, so please bear with me.
Also, English is my second language.

text: Complex Analysis (2nd edition)
author: Stephen D. Fisher

[question deleted] this first question is very stupid. i figured it out. thank you. i will come back for other questions.

Thanks.
 
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Variation of Maximum principle: "If u(x,y) is harmonic and nonconstant on a domain D, then |u(x,y)| has no local maximum in D". the proof of this is left as an exercise. i want to prove it.

case 1: u(x,y) is complex-valued function, then since u is harmonic, it is analytic on D. By the Maximum Modulus Principle, |u| has no local max in D
case 2: u(x,y) is real-valued function, then u has no local max and no local min in D. How do you go from this to |u|?

Thanks.
 
If u is harmonic on D, then there exist another harmonic function, v(x,y), such that f(z)= u(z)+ iv(z) where z= x+ iy, is analytic on D.
 
HallsofIvy said:
If u is harmonic on D, then there exist another harmonic function, v(x,y), such that f(z)= u(z)+ iv(z) where z= x+ iy, is analytic on D.

Not true 100%. For example [itex]D=\mathbb{C}\backslash\{0\}[/itex] and [itex]u(z)=\log |z|[/itex].
 
True! I was assuming a simply connected domain.
 
HallsofIvy said:
If u is harmonic on D, then there exist another harmonic function, v(x,y), such that f(z)= u(z)+ iv(z) where z= x+ iy, is analytic on D.
then? we're using this fact to prove when the case u is complex-valued function right? I got that part down, put I'm stuck when u is real-valued function.

actually when I read further down, the book suggest to use mean-value property to prove above statement.
 

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