Can Cauchy's residue theorem be used for functions with poles at infinity?

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    Cauchy Residue Theorem
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Discussion Overview

The discussion revolves around the applicability of Cauchy's residue theorem for complex functions that have poles at infinity. Participants explore whether the theorem can be utilized in such cases and provide examples and clarifications related to the topic.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a complex integral involving a simple pole at infinity and suggests that the residue theorem applies, stating that the integral equals \(2\pi i \text{Res}(z,\infty,f(z))\).
  • Another participant questions the clarity of the initial post and interprets it as asking whether the residue theorem applies in the case of a pole at infinity, affirming that it does.
  • A participant clarifies their intent, reiterating the question about the use of Cauchy's residue theorem for functions with poles at infinity.
  • Another participant agrees that the theorem can be applied, providing an example involving a non-meromorphic function and a specific contour integral, explaining the transformation that allows the function to be meromorphic inside the contour.

Areas of Agreement / Disagreement

There appears to be a general agreement among participants that Cauchy's residue theorem can be applied to functions with poles at infinity, though the discussion includes clarifications and examples that highlight different aspects of the theorem's application.

Contextual Notes

Participants discuss the conditions under which the residue theorem can be applied, particularly in relation to the meromorphic nature of functions and the choice of contours. There are references to specific examples that may depend on the definitions and assumptions regarding branch cuts and contour integration.

zetafunction
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my question is , let us have the following complex integral

[tex]\oint f(z)dz[/tex] where f(z) has a simple pole at [tex]z=\infty[/tex]

then by Residue theorem [tex]\oint f(z)dz =2\pi i Res(z,\infty,f(z)[/tex]

or equal to the limit [tex](z-\infty )f(z)[/tex] with 'z' tending to infinity
 
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I'm sorry, I don't see a question here! Are you asking if it is one or the other of those?

If so, it is the first. The second, the "limit [tex](z-\infty )f(z)[/tex] with z tending to infinity", is the residue itself.
 
sorry i am not from US or England so my english could be a little mistaken

the idea of the post is: can we use 'Cauchy's residue theorem' even in the case the function f(z) has a pole at infinity ??
 
zetafunction said:
sorry i am not from US or England so my english could be a little mistaken

the idea of the post is: can we use 'Cauchy's residue theorem' even in the case the function f(z) has a pole at infinity ??

Yes. This is useful if the function is not meromorphic in the interior of the contour. Take e.g. the real integral (x^2 - x^3)^(1/3)dx from
x = 0 to 1. You can evaluate this by considering the so-called "dogbone" contour that goes from zero to 1 just below the real axis, encircles the branch point at z = 1 and then goes to zero just above the real axis and then encicles the branch point at z = 0.

Clearly the function is not meromophic inside the contour, but it is outside the contour (if you choose the branch cuts so that they cancel out outside the interval from zero to 1).

If you then perform the conformal transformation z ---> 1/z, you see that what was outside the contour is now inside the contour and what was inside is now outside. You also see that the function is now meromorphic inside the the contour and has a pole at zero.
 

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