How Can the Residue Theorem Be Applied to Prove This Integral?

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SUMMARY

The discussion centers on applying the Residue Theorem to evaluate the integral of a complex function \( f(z) \) that is analytic outside the unit circle. Given that the limit of \( zf(z) \) as \( z \) approaches infinity equals \( A \), it is established that for any circle \( C_{R} \) with radius \( R > 1 \), the integral \( \oint f(z)dz \) equals \( 2\pi iA \). The application of the Residue Theorem is confirmed by recognizing that the residue at infinity is \( A \), thereby validating the integral's evaluation.

PREREQUISITES
  • Understanding of complex analysis concepts, particularly the Residue Theorem.
  • Familiarity with analytic functions and their properties in the context of complex variables.
  • Knowledge of Cauchy's integral theorem and its applications.
  • Basic comprehension of the Riemann sphere and its implications for complex functions.
NEXT STEPS
  • Study the application of the Residue Theorem in various complex integrals.
  • Learn about the properties of analytic functions and their behavior at infinity.
  • Explore Cauchy's integral theorem in greater detail, including its proof and applications.
  • Investigate the concept of residues at poles and their significance in complex analysis.
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Students and professionals in mathematics, particularly those specializing in complex analysis, as well as educators seeking to enhance their understanding of integral evaluation techniques using the Residue Theorem.

de1irious
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Hi, so suppose f(z) is a complex function analytic on {z|1<|z|} (outside the unit circle). Also, we know that limit as z-->infinity of zf(z) = A. Now I need to show that for any circle [tex]C_{R}[/tex] centered at origin with radius R>1 and counterclockwise orientation, that

[tex]\oint f(z)dz = 2\pi iA[/tex]

Any ideas? I'm trying to use Cauchy integral theorem somehow but it's not working.
 
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Are you familiar with the Riemann sphere?

In this picture, every closed curve in C can be considered to go around infinity (which is just one point on the sphere). If you choose a circle P with radius larger than R then your function is analytic in the connected component of C\P which contains infinity.

The residue of your function at infinity is A so the Residue Theorem implies your assertion at once.
 
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