Continuity of complex functions

Click For Summary

Discussion Overview

The discussion revolves around the continuity of complex functions, specifically exploring whether there exist functions that are continuous on the real line but discontinuous on the complex plane. Participants consider examples and the underlying reasons for such behavior.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant inquires about the existence of functions that are continuous on the real line but discontinuous in the complex plane, questioning if such functions can exist.
  • Another participant proposes a specific function defined piecewise, suggesting that it is an example of a function that meets the criteria of being continuous on the real line but discontinuous in the complex plane.
  • A third participant mentions the function Log(1+x^2) as a potential example, though further context is not provided.
  • A later reply introduces the function 1/p(z), where p(z) is a polynomial without zeroes on the real line, suggesting that this construction can yield discontinuous functions in the complex plane while remaining continuous on the real line.
  • Participants discuss the possibility of extending these ideas to more complex forms, such as Laurent series, by manipulating polynomials and their properties.

Areas of Agreement / Disagreement

Participants present multiple competing views and examples regarding the existence of such functions, indicating that the discussion remains unresolved with no consensus reached.

Contextual Notes

Some examples provided depend on specific definitions of continuity and the behavior of functions in the complex plane, which may not be universally agreed upon. The discussion also involves assumptions about the properties of polynomials and their roots.

variety
Messages
20
Reaction score
0
Do you guys know of any functions which are continuous on the real line, but discontinuous on the complex plane? If not, is there a reason why this can never happen?
 
Physics news on Phys.org
Well, you can construct one, if you want...

Let
f(z) = \begin{cases} |z| & -1 \le \text{Im}(z)| \le 1 \\ 0 & \text{ otherwise} \end{cases}.

Why?
 
Log(1+x^2)
 
Or, inspired by lurflurf's example, 1/p(z) where p(z) is any polynomial in z without zeroes on the real line like x2 + 1 or in general
p(z) = \prod_{j = 1}^N (z - a_j - b_j i)
with all the bi not equal to zero.

And you can even multiply that by any polynomial (as long as it doesn't cancel out all the singular points of p(z)) and get another one.
And you can let N go to infinity to get a Laurent series for some function.
 

Similar threads

  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 1 ·
Replies
1
Views
4K
  • · Replies 9 ·
Replies
9
Views
2K
  • · Replies 7 ·
Replies
7
Views
3K
  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 21 ·
Replies
21
Views
6K
  • · Replies 1 ·
Replies
1
Views
1K
  • · Replies 3 ·
Replies
3
Views
2K
Replies
3
Views
2K