Gamma Function Complex Argument: Problems in Stat Phys & How to Calculate

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Discussion Overview

The discussion revolves around the gamma function with complex arguments, particularly its application in statistical physics and the calculation of specific values such as \(\Gamma(i)\). Participants explore the definition, convergence issues, and methods for calculating the gamma function in the complex plane.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents the integral definition of the gamma function, but there is a typographical error in the formula.
  • Another participant corrects the formula, emphasizing the need for the correct variable in the integral.
  • There is a discussion about the convergence of the integral definition, noting that it does not converge for \(z=i\) and suggesting the use of analytic continuation.
  • A method for calculating \(\Gamma(i)\) is proposed, involving the functional equation \(\Gamma(z+1) = z\Gamma(z)\) to find \(\Gamma(1+i)\) first.
  • A numerical approximation for \(\Gamma(i)\) is provided, indicating that a closed-form answer is not expected.

Areas of Agreement / Disagreement

Participants express disagreement regarding the initial formula presented, with corrections made. There is also a consensus on the need for analytic continuation for calculating \(\Gamma(i)\), but no consensus on the broader implications or applications in statistical physics.

Contextual Notes

The discussion highlights limitations related to the convergence of the gamma function's integral definition and the necessity of analytic continuation for complex arguments. The specific conditions under which the integral converges are not fully resolved.

Who May Find This Useful

This discussion may be useful for those interested in complex analysis, statistical physics, and the properties of special functions like the gamma function.

Petar Mali
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\Gamma(z)=\int^{\infty}_0x^{z-1}e^{-x}dz
z\in\mathhad{C}

In which problems in statistical physics we need gamma functions of complex argument?

I don't know how to calculate \Gamma(i) for exaple?
 
Last edited:
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Petar Mali said:
\Gamma(z)=\int^{\infty}_0x^{z-1}e^{-z}dz
z\in\mathhad{C}

In which problems in statistical physics we need gamma functions of complex argument?

I don't know how to calculate \Gamma(i) for exaple?
Your formula is wrong (typo) Should be

\Gamma(z)=\int^{\infty}_0x^{z-1}e^{-x}dx
 
mathman said:
Your formula is wrong (typo) Should be

\Gamma(z)=\int^{\infty}_0x^{z-1}e^{-x}dx

Yes mistake. I make corrcection!
 
Petar Mali said:
Yes mistake. I make corrcection!

Not quite: you still have dz when it should be dx.
 
\Gamma(i)

Well, the integral definition converges only if \Re z > 0, so in particular it does not converge at z=i. So you need to use analytic continuation. But fortunately that is very easy for the \Gamma function, unlike most other functions. Use the functional equation \Gamma(z+1) = z\Gamma(z). So to compute \Gamma(i) we can compute \Gamma(1+i) then apply the formula.

You cannot expect a closed-form answer. \Gamma(1+i) \approx .4980156681-.1549498283i so divide by i to get \Gamma(i) \approx -.1549498283-.4980156681i.
 

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