E^(-i * x) not well-defined. Why?

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Discussion Overview

The discussion revolves around the definition and properties of the function e^(-i * x), particularly in the context of complex variables and holomorphic functions. Participants explore why this expression might be considered ill-defined in certain scenarios, especially when evaluated on the complex plane.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions the claim that e^(-i * x) is ill-defined, noting that it is well-defined according to many experts.
  • Another participant argues that while e^(-i * x) is well-defined as a function of a real variable, it encounters issues when considered as a function of a complex variable, particularly at z=0.
  • A participant discusses the standard definition of a function for real numbers and suggests that this definition is too restrictive for complex variables, leading to the need for a more flexible approach.
  • One participant introduces the concept of Principal Value to address the definition of certain complex functions, noting that this can alter expected properties, such as the logarithm's behavior.
  • There is a mention of the restriction of the argument in the logarithm to a specific range to maintain consistency in complex analysis.

Areas of Agreement / Disagreement

Participants express differing views on the definition and behavior of e^(-i * x) in complex analysis. There is no consensus on whether it is ill-defined, and multiple perspectives on the implications of its definition are presented.

Contextual Notes

Some participants highlight limitations in the standard definitions of functions when applied to complex variables, indicating that these definitions may not capture the full behavior of certain functions.

nigels
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e^(-i * x) not well-defined. Why??

Hi, Just saw this as a step in an example that demonstrates the differentiability of holomorphic function. But I can't for the life of me figure out why e^(-2iθ) is ill-defined.
 
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nigels said:
Hi, Just saw this as a step in an example that demonstrates the differentiability of holomorphic function. But I can't for the life of me figure out why e^(-2iθ) is ill-defined.

What do you mean it is ill-defined. Why do you say that? It's well defined in the opinions of many smart people. I can't go beyond that until I know what objection there is to the conventional definition there is.
 


It is well defined as a function of a real variable theta. But as a function on the plane, considering theta=theta(z), it has a problem at z=0. Does this answer your question? It is hard to tell without more context.

As a function of z, this function is the complex conjugate of (z/|z|)^2
 


The standard definition of "function" for real numbers requires that "if x= y, then f(x)= f(y)"- i.e. that f is "well-defined". For functions of complex variables, that is simply too restrictive. "Functions" that we would like to be able to use, such as e^x would no longer be "functions". So we drop that requirement.
 


Some functions are given the requirement of Principal Value to make some functions of a complex variables become actual functions.

I.e , let z=Re^(ix), and restrict x to be in
(-pi,pi].

This restriction works since e^(ix)=e^(ix+i*2n*pi) for all integers n.

On the other hand, it makes functions behave less like we would wabt them to.

That is, Log(xy)=/=Log(x)+Log(y) generally for principal value logarithm. On the other hand, log(xy)=log(x)+log(y).

(correct me if I'm wrong as I am just typing from memory)

Edit: x,y is a complex number for the logarithm examples.
 

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