Evaluating Complex Integral: Over |z|=4 Region

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Discussion Overview

The discussion revolves around evaluating the complex integral of the function \(\int \frac{e^z}{\sinh(z)} dz\) over the contour defined by the circle \(|z|=4\). Participants explore various methods for solving the integral, including the Residue Theorem and the argument principle, while also considering the context of an exam question.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant expresses difficulty in evaluating the integral and mentions an attempt to parameterize the contour.
  • Another participant suggests the use of the Residue Theorem, indicating it may be relevant for the problem.
  • A participant notes that the question is from an exam paper and questions whether the Residue Theorem is the intended method, given the context of the exam.
  • Some participants argue that if the problem provides information about the zeros of \(\sinh(z)\), it implies the use of the Residue Theorem is appropriate.
  • An alternative approach using the argument principle is proposed, detailing how to analyze the integral without relying on the Residue Theorem.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best method to evaluate the integral. Some advocate for the Residue Theorem, while others suggest alternative approaches like the argument principle. The discussion remains unresolved regarding the preferred method for this specific problem.

Contextual Notes

Participants reference specific results related to the function \(\sinh(z)\) and its zeros, indicating that the problem may have dependencies on these definitions. There is also uncertainty about the explicit requirements of the exam question.

fudgenstuff
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I've been given the problem of evaluating the integral

[tex]\int[/tex](exp^z)/Sinh(z) dz

Over the region C which is the circle |z|=4

I can't figure out how to do this,I tried parameterizing with z(t)=4e^i[tex]\theta[/tex] but the integrand just seems far too complicated. Any suggestions?

(Apologies for the terrible formatting)
 
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Welcome to PF fudgenstuff,

Have you come across the Residue Theorem before?

I'm assuming that this is homework, for future reference we have Homework & Coursework forums where such questions can be posted.
 
Ah sorry, if one of the mods could move this thread then that'd be great.

The question is from an exam paper,I'm not entirely sure they want us to use the residue theorem here as there's another question on it later on in the paper.
This question follows on from deducing that Sinh(x+iy) = sinh(x)cos(y) + icosh(x)sin(y), and then showing that Sinh(z)=0 only if z= i*pi*n (where n is an integer), so I'm not sure if I'm supposed to use that result somehow?
 
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fudgenstuff said:
The question is from an exam paper,I'm not entirely sure they want us to use the residue theorem here as there's another question on it later on in the paper.
If it doesn't explicitly say that you can't use the Residue Theorem then you'd be crazy not to!
 
If they tell you how to find the zeros of sinh(z), they want you to use the residue theorem.
 
Here's an alternative way, using the argument principle. We have
[tex]\frac{e^z}{sinh(z)} = \frac{cosh(z) + sinh(z)}{sinh(z)} = 1 + \frac{cosh(z)}{sinh(z)}[/tex]

Now the contour integral of the first is of course zero, and the contour integral of the second is of the form [tex]\frac{f'(z)}{f(z)}[/tex] which by the argument principle is [tex]2 \pi i[Z-P][/tex] where Z and P are the numbers of zeros and poles of sinh(z) enclosed by the contour, respectively. Since you already know the number of zeroes, and since sinh(z) has no poles, you're done.

If you haven't seen the residue theorem, you probably haven't seen this, but it's always nice to avoid computations if possible.
 
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