Solving Complex Integral Paths - Real Line Poles

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Discussion Overview

The discussion revolves around the evaluation of complex integrals along paths that encircle poles located on the real line. Participants explore the implications of choosing different paths around these poles, particularly focusing on the effects of clockwise and counter-clockwise traversal on the integral's value.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether the choice of path around real line poles affects the integral's result, specifically asking about the implications of integrating around a pole above or below the real axis.
  • Another participant asserts that the sign of the integral will change depending on whether the path around the pole is clockwise or counter-clockwise.
  • A subsequent reply clarifies that the integral's value depends on whether the pole is inside or outside the contour, referencing the Residue theorem and Cauchy's theorem.
  • Further elaboration suggests that both paths, if executed correctly, should yield the same result, despite the differing contributions from the semicircles around the poles.
  • One participant acknowledges the complexity of the situation and agrees with a previous response that provided a more comprehensive explanation.

Areas of Agreement / Disagreement

Participants express differing views on the impact of the path choice around poles on the integral's value. While some suggest that the results will be the same if both integrals are performed correctly, others emphasize the importance of the pole's position relative to the contour.

Contextual Notes

Participants note the need to consider the contributions from semicircles around poles and the application of the Residue theorem versus Cauchy's theorem, indicating that assumptions about the path and pole locations are critical to the discussion.

Silviu
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Hello! If I have a real integral between ##-\infty## and ##+\infty## and the function to be integrated is holomorphic in the whole complex plane except for a finite number of points on the real line does it matter how I make the path around the poles on the real line? I.e. if I integrate on the semicircle in the upper plane and I have a pole at 0 let's say, do I get the same result if I go around the pole above or below the real axis? Thank you!
 
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The sign will change because one path is clockwise around the pole and the other is counter-clockwise.
 
FactChecker said:
The sign will change because one path is clockwise around the pole and the other is counter-clockwise.
Do you mean the sign of the semicircle around the pole (whose radius t=you take to go to 0 in the end)? But you also have to consider that in one case the pole is inside the contour (so you must use Residue theorem) and in the other is outside (SO by Cauchy theorem the integral around everything would be 0), right?
 
Silviu said:
[...] does it matter how I make the path around the poles on the real line?
You'll get the same result IF you do both integrals properly.

In the first case, the pole is outside the contour, so no residue contribution. But there's potentially a contribution from the small semicircle over the pole, traversed clockwise.

In the second case, the pole is inside the contour so there's a residue contribution. But there's also a contribution from the small semicircle under the pole, this time traversed anticlockwise.

You should end up with the same result either way.
 
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Silviu said:
Do you mean the sign of the semicircle around the pole (whose radius t=you take to go to 0 in the end)? But you also have to consider that in one case the pole is inside the contour (so you must use Residue theorem) and in the other is outside (SO by Cauchy theorem the integral around everything would be 0), right?
Yes. Sorry. My post was too brief. @strangerep gave a better answer.
 

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