Solving Complex Integral Paths - Real Line Poles

In summary, when integrating a real integral between ##-\infty## and ##+\infty## with a holomorphic function that has a finite number of poles on the real line, the path chosen around the poles does matter. The sign of the integral will change depending on whether the path is clockwise or counterclockwise around the pole, and the result will also depend on whether the pole is inside or outside the contour. However, if both integrals are done properly, the final result should be the same.
  • #1
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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  • #2
The sign will change because one path is clockwise around the pole and the other is counter-clockwise.
 
  • #3
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?
 
  • #4
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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  • #5
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.
 

1. What is the purpose of solving complex integral paths with real line poles?

The purpose of solving complex integral paths with real line poles is to find the value of an integral along a path that passes through a pole on the real line. This is necessary because the integral becomes undefined at the pole, and by finding a complex path that avoids the pole, we can still calculate the value of the integral.

2. How do we determine the complex path for solving integrals with real line poles?

The complex path for solving integrals with real line poles is determined by using the Cauchy Residue Theorem. This theorem states that if a function has a pole of order n at a point c, the integral of the function along a closed curve can be calculated by summing the residues of the function at each of its poles within the curve.

3. What are some common techniques for solving complex integrals with real line poles?

There are several common techniques for solving complex integrals with real line poles, including using partial fractions, contour integration, and the method of residues. These methods involve breaking down the integral into smaller, more manageable parts and using known techniques to solve them.

4. Can complex integral paths with real line poles be solved numerically?

Yes, complex integral paths with real line poles can be solved numerically using computer software. This involves approximating the complex path and computing the value of the integral using numerical integration methods such as the trapezoidal rule or Simpson's rule.

5. Are there any real-world applications of solving complex integral paths with real line poles?

Yes, there are many real-world applications of solving complex integral paths with real line poles. Some examples include calculating electric fields in physics, evaluating complex integrals in engineering, and solving differential equations in mathematics. These techniques are also used in various areas of science, including fluid dynamics, quantum mechanics, and signal processing.

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