Complex anaylsis, winding number question.

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    Complex Winding
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Discussion Overview

The discussion revolves around the concept of the winding number in complex analysis, specifically focusing on the integral representation of the winding number and the interpretation of the variable ζ within that context. Participants explore the mathematical formulation and seek clarification on the integration process involved.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Homework-related

Main Points Raised

  • One participant asks for clarification on what ζ represents in the winding number equation, suggesting it may be a bound or dummy variable for points on the curve γ.
  • Another participant confirms that ζ is indeed an arbitrary point on the curve γ and explains its role as the variable of integration.
  • A third participant questions whether the integration is taken from the interval [a, b] and requests an example of the equation in use.
  • A later reply affirms that the integral is taken over the interval [a, b] after parametrizing the curve γ and provides an example involving the unit circle.

Areas of Agreement / Disagreement

Participants generally agree on the interpretation of ζ as a variable of integration and the process of integrating over the interval [a, b]. However, there is no consensus on the specifics of the example or its implications for understanding the winding number.

Contextual Notes

Limitations include the need for clearer definitions of the curve γ and the parametrization process, as well as the potential for confusion regarding the example provided.

Who May Find This Useful

Readers interested in complex analysis, particularly those studying winding numbers and integration in the context of curves in the complex plane.

screwyshrew
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So the in the equation for the winding number/index of a curve

I(\gamma, z) = \frac{1}{2i\pi} \int\gamma \frac{1}{ζ-z}dζ

where \gamma : [a, b] → ℂ and z is an arbitrary point not on \gamma, what exactly does ζ represent?
 
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It represents an arbitrary point of the curve gamma. I.e. it is a bound or "dummy" variable for any point of gamma. a fuller notation would be the integral over all zeta on gamma. But the fact that it appears after the "d" also tells you it is the variable of integration, and is usual.
 
This is possibly a dumb question (please bear with me): the integration would be taken from a to be, correct? Could you post an example of this equation in use, please?
 
yes, after parametrizing the curve gamma by an interval [a.b], then the integral is pulled back to that interval and THEN taken from a to b.

e.g. just integrate dw/w over the unit circle, parametrizing it by cos(t) +i sin(t) for the interval [0,1]. try it. that's the winding number about z = 0 of the unit circle. (times 2pi.i)
 

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