Is this Complex Function Continuous?

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Homework Help Overview

The discussion revolves around the continuity of a complex function defined as f(z) = (e^z - z^e)/(z^3-1). Participants explore the nature of discontinuities in the function, particularly focusing on the denominator and the implications of the numerator's behavior.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the discontinuities arising from the denominator z^3 - 1 and question the implications of the numerator e^z - z^e being potentially discontinuous. There is also exploration of the definition and continuity of z^e, particularly in relation to branch cuts.

Discussion Status

The discussion is active, with participants questioning the continuity of both the numerator and denominator. Some have provided insights into the nature of discontinuities, while others express uncertainty about the implications of these discontinuities on the overall function.

Contextual Notes

Participants note that z^e is not well-defined across the complex plane due to the nature of the logarithm and its branches, which may affect the continuity of the function. There is also mention of the nth roots of unity as relevant to the discussion of z^3 = 1.

desaila
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Homework Statement


f:Complex Plane ->Complex Plane by f(z) = (e^z - z^e)/(z^3-1) continuous? (Hint: it
has more than one discontinuity.)






The Attempt at a Solution



My attempt at a solution was thus, initially I expanded z^3 and tried to find where it equaled 1. That wasn't very straightforward, and then I figured I could just say the function is discontinuous when z^3 = 1, but I really don't think that's sufficient. Is there something I'm missing here?

Thanks in advance.
 
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z^e is not terribly well defined either. It's e^(log(z)*e) and log(z) has branches.
 
Dick said:
z^e is not terribly well defined either. It's e^(log(z)*e) and log(z) has branches.

Well, if z^e isn't continuous is that really a problem? I thought that things in the numerator weren't really an issue when dealing with continuity.
 
Are you familiar with the nth "roots of unity"?
 
desaila said:
Well, if z^e isn't continuous is that really a problem? I thought that things in the numerator weren't really an issue when dealing with continuity.

Not true. A discontinuous numerator is just as bad as a zero in the denominator.
 
jhicks said:
Are you familiar with the nth "roots of unity"?

No. I'll wiki this, but I'd be interested in your explanation as well.

Dick said:
Not true. A discontinuous numerator is just as bad as a zero in the denominator.

Really. So z^3 = 1 wouldn't be so bad, I'd just have to find discontinuities in e^z and z^e then?

EDIT: After reading about the nth roots of unity I could provide them for z^3 = 1. They're given on the wiki entry( http://en.wikipedia.org/wiki/Roots_of_unity )
 
Last edited:
No, z^3=1 is bad too. That's three discontinuous points. e^z doesn't have any. log(z) has a whole line of discontinuities. So that goes for z^e as well.
 
Dick said:
No, z^3=1 is bad too. That's three discontinuous points. e^z doesn't have any. log(z) has a whole line of discontinuities. So that goes for z^e as well.

What I meant was showing the discontinuities of z^3 = 1 wouldn't be too bad. Sorry about the misunderstanding.
 
Also, Dick, is there somewhere I can read up on z^e? Why it's defined the way you say it is, etc.
 
  • #10
There's no other way to define it. z=e^(log(z)) so z^e=e^(log(z)*e). But there is no continuous definition of log(z) in the whole complex plane. You have to remove part of the domain to get a continuous function. Look up "branch cuts". It's not just z^e. sqrt(z) has a very similar problem.
 
  • #11
What exactly is the question? It appears to ask whether or not the function is continuous but then immediately tells you that it is not! Is the problem to find the points of discontinuity?
 
  • #12
generally the numerator also matters..for example. Sinz/z doesn't REALLY have a discontinuity at the origin (i.e the origin is NOT a pole) whereas 1/z DOES have a legitimate discontinuity at the origin(it's a simple pole).

you need to understand the concept of branch cuts. also computing complex roots (wen the numbers are simple of course) is straightforward and can and probably should be done by hand.
 

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