Integrating a function of the complex conjugate of x with respect to dx

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Discussion Overview

The discussion revolves around the integration of a function involving the complex conjugate of a variable, specifically in the context of quantum mechanics and expectation values of operators. Participants explore the implications of treating the complex conjugate in integration, particularly focusing on the function sin(x*), where x* denotes the complex conjugate of x.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant raises a question about integrating sin(x*) and expresses uncertainty about treating x* as a constant or variable.
  • Another participant confirms that x is a complex variable.
  • It is suggested that if x is real or imaginary, integration can be straightforward, but sin(conjugate(z)) may not be holomorphic, indicating that numerical integration might be necessary.
  • A question is posed regarding whether the integral is definite or indefinite, and the importance of the path for complex integrals is highlighted.
  • Further clarification is provided that holomorphic functions depend on the derivative with respect to z* being zero, and a reference to the Cauchy-Riemann equations is made.
  • A participant proposes considering the integral over a specific path in the complex plane to potentially gain insight into the problem.

Areas of Agreement / Disagreement

Participants express differing views on the treatment of the complex conjugate in integration, with some suggesting numerical methods while others discuss the implications of holomorphic functions. The discussion remains unresolved regarding the best approach to the integration problem.

Contextual Notes

There are limitations related to the assumptions about the nature of x (real vs. complex) and the implications of holomorphicity on the integration process. The discussion also touches on the need for specific paths in complex integration, which may affect the outcome.

HilbertSpace
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The reason I ask the aforementioned question is because I came across the expectation values of operators in Quantum Mechanics. And part of the computation involves integrating a function of the complex conjugate of x with respect to dx.

So as an example let's say I have:

∫ sin (x*) dx where the * denotes the complex conjugate.

How would I do it? (I would post an attempt to solve it but I have no idea whether to even treat the x* as a constant, or variable or...something).Thank you in advance for any help you may provide.
 
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Is x a complex variable, then?
 
Yes, x is a complex variable.
 
If your x is just real or imaginary conjugate(x) = x or conjugate(x) = -x and so integration can be performed pretty easily.

I don't believe the function sin(conjugate(z)) is holomorphic so it won't end up having a complex anti-derivative.

This mean that numerical integration will be needed.
 
Is it a definite or indefinite integral?

If it is definite, what path would you like to use? A complex integral can be looked at as a path integral in the plane, for which a parametrization might make it feasible to calculate exactly.

But like Skrew said, this does not look amenable to methods from complex analysis like computing residues.
 
Skrew said:
If your x is just real or imaginary conjugate(x) = x or conjugate(x) = -x and so integration can be performed pretty easily.

I don't believe the function sin(conjugate(z)) is holomorphic so it won't end up having a complex anti-derivative.

This mean that numerical integration will be needed.

IN Fact holomorphic depends on the derivative wrt z* to be zero
 
Mandlebra said:
IN Fact holomorphic depends on the derivative wrt z* to be zero

I'm not well versed on different approaches but my book defines d/dz and d/d(conjugate(z)) to be such that the cauchy Riemman equations are satisified IFF d/d(conjugate(z)) = 0, at which point the definition of the derivative can be applied and the limit exists.

Is there another way to look at this using the difference quotient of conjugate(z)?
 
Some insight may be gained by considering the integral over a path P consisting of a straight line segment at angle θ to the real axis:
\int_{P}f(z).d\overline{z} = e^{2i\theta} \int_{P}f(z).dz
 

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