Complex analysis: I have to find sequence of C^inf functions that

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Homework Help Overview

The discussion revolves around a problem in complex analysis concerning the behavior of sequences of functions. The original poster is tasked with demonstrating that a certain property holds for holomorphic functions but fails when the functions are merely infinitely differentiable.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to find examples of functions that are infinitely differentiable but do not satisfy the uniform convergence property for their derivatives. They explore various functions, including simple ones and more complex forms like ln(z), but face challenges in demonstrating the required properties.
  • Another participant suggests a specific function, f_n, and questions whether it can be proven to be infinitely differentiable, prompting further exploration of the limit as n approaches infinity.
  • Concerns are raised about the definition of infinitely differentiable in the context of complex analysis, particularly regarding the continuity of partial derivatives of the real and imaginary parts.

Discussion Status

The discussion is active, with participants exchanging ideas and clarifying concepts. Some guidance has been offered regarding the nature of infinitely differentiable functions in complex analysis, and there is an ongoing exploration of how to extend real functions into the complex plane. The original poster expresses uncertainty about their progress, indicating a lack of consensus but a willingness to engage with the problem.

Contextual Notes

Participants are navigating the specific definitions and requirements of complex analysis, particularly the distinction between holomorphic and infinitely differentiable functions. There is an acknowledgment of the complexity involved in the problem, and some participants express concern about the adequacy of their approaches.

QIsReluctant
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Homework Statement


... if fj are holomorphic on an open set U and fj \stackrel{uniformly}{\rightarrow} f on compact subsets of U then δ/δz(fj) \stackrel{uniformly}{\rightarrow} δ/δz(f) on compact subsets of U. Give an example to show that if the word "holomorphic" is replaced by "infinitely differentiable" then the result is false.

Homework Equations


The above.


The Attempt at a Solution


I've used the disk D(0, 1) and all of the obvious choices: |z|, \overline{z}, etc. None of them work. It seems that their derivatives exhibit similar behaviour to the functions themselves, i.e., converge uniformly on that disk. I've considered some wackier functions like ln(z) but finding the real and imaginary parts of the function and doing partial derivatives is, shall we put it mildly, a chore. Can anyone hrlp me please?
 
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Think about the real function ##f_n(x)=e^{-\frac{1}{nx}}## for x>0 and ##f_n(x)=0## for x<=0. Can you prove that's infinitely differentiable? What the limit function like as n->infinity?
 
Thanks for the response, Dick.

The problem is that in complex analysis "infinitely differentiable" means something different: if a function is of the form f(z) = u + iv then the two "parts" have to have continuous partial derivatives w/r to x (real part) and y (imaginary part).

Unfortunately your example takes a while to put into the above format, and I fear that all my work will be for nothing ...
 
Apologies if you know all of that, by the way, and I'm just not seeing what's obvious!
 
QIsReluctant said:
Thanks for the response, Dick.

The problem is that in complex analysis "infinitely differentiable" means something different: if a function is of the form f(z) = u + iv then the two "parts" have to have continuous partial derivatives w/r to x (real part) and y (imaginary part).

Unfortunately your example takes a while to put into the above format, and I fear that all my work will be for nothing ...

I guess I don't see the problem. Just extend the real function to the complex plane. Define u to be my ##f_n## and v to be 0.
 
... I am officially an idiot. Thank you.
 

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