Expanding Gamma function around poles

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Discussion Overview

The discussion revolves around expanding the gamma function around its poles, specifically focusing on the expression \(\Gamma[(1/2) \pm (ε/2)]\) where \(ε = d-4\). The scope includes mathematical reasoning and technical explanations related to the properties of the gamma function and its derivatives.

Discussion Character

  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant requests assistance in expanding the gamma function around the pole \(ε\) at first order.
  • Another participant proposes an approximation for \(\Gamma(½ ± ε/2)\) and mentions the need to use the digamma function, providing specific values for \(\Gamma(½)\) and \(\psi(½)\).
  • A further contribution provides detailed expansions for \(\Gamma(\frac{1}{2} - \frac{\epsilon}{2})\) and \(\Gamma(\frac{1}{2} + \frac{\epsilon}{2})\), including terms involving Euler's constant and logarithmic functions.
  • A participant expresses gratitude for the help received and indicates their novice status in the subject.

Areas of Agreement / Disagreement

Participants appear to agree on the need to use the digamma function and provide expansions, but there is no explicit consensus on the final forms of the expansions or the implications of the results.

Contextual Notes

Limitations include potential dependencies on the definitions of the gamma and digamma functions, as well as unresolved mathematical steps in the expansions provided.

DMESONS
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Can someone help me to expand the following gamma functions around the pole ε, at fisrt order in ε

\Gamma[(1/2) \pm (ε/2)]

where ε= d-4
 
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Γ(½ ± ε/2) ≈ Γ(½) ± ε/2 Γ'(½)

No, seriously.. :smile:

Well, you also need to use the digamma function, ψ(x) = Γ'(x)/Γ(x). And the values Γ(½) = √π and ψ(½) = - γ - 2 ln 2 where γ is Euler's constant.
 
<br /> \Gamma(\frac{1}{2} - \frac{\epsilon}{2}) = \sqrt{\pi }+\frac{1}{2} \sqrt{\pi } \epsilon (\gamma_E +\log (4))+O\left(\epsilon ^2\right)<br />

<br /> \Gamma(\frac{1}{2} + \frac{\epsilon}{2}) = \sqrt{\pi }+\frac{\sqrt{\pi } \epsilon \psi ^{(0)}\left(\frac{1}{2}\right)}{2}+O\left(\epsilon ^2\right)<br />
 
Bill_K and Hepth, I am so grateful for your help

I am new in this subject

:smile:
 

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