Domain and range of complex functions

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Discussion Overview

The discussion revolves around determining the domain and range of complex functions, specifically in the context of functions expressed in terms of real and imaginary components. Participants explore theoretical aspects, mathematical reasoning, and seek resources for further learning.

Discussion Character

  • Exploratory
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about the process for finding the domain and range of a complex function defined as f(z) = w(x,y) = u(x,y) + iv(x,y), expressing confusion about the necessary steps.
  • Another participant suggests that the domain can be determined by the intersection of the domains of the real and imaginary parts, u and v, indicating that f is defined at (x, y) if both u and v are defined.
  • A participant notes that while the ranges of u and v provide an upper bound for the range of f, there may be combinations of values that cannot occur, highlighting the complexity of determining the range.
  • One participant proposes a boundary formula for the range involving the Jacobian determinant, stating that it is zero when ux vy = uy vx, and provides an example function to illustrate this point.
  • Another participant mentions that the domain and range of a complex function represent 2D regions, with many important functions having the entire complex plane as their domain and range, except for singular points.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the determination of the range, with some proposing specific methods and formulas while others acknowledge the complexity and potential limitations of these approaches. No consensus is reached on a definitive method for finding the range.

Contextual Notes

Participants note that the determination of the range may depend on the differentiability of the functions involved and that there may be unresolved mathematical steps in deriving the boundary conditions.

Avatrin
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If I am given a function:
f(z) = w(x,y) = u(x,y) + iv(x,y)

How do I find the domain and range of that function? Are there any good lectures online on this?

I know I have to use two planes; One for u and v, and one for x and y. Besides that, I am lost.
 
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The domain is easy - just the intersection of the domains of u and v. The function f is defined at (x, y) if and only if both u and v are.
The range is tougher. Knowing the ranges of u and iv separately gives you an 'upper bound' on the range (namely, the direct product of the two separate ranges) but there will likely be many combinations of values that can't arise.
If u and v are differentiable then there will be an interesting equation for the boundary of the range. Can't come up with it immediately, and couldn't find anything online... will try to derive it later.
 
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OK, I think I have the range boundary formula. It's where the Jacobian determinant is zero:
ux vy = uy vx
E.g. consider f = (1+x2) (cos(y) + i sin(y)). The formula collapses to x = 0, i.e. f = cos(y) + i sin(y), the unit circle.
 
The domain and the range of a complex function is a 2D region each. For example, a circle. For many important functions, the domain and the range is the entire complex plane, with a finite or countable number of singular points.
 

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