7.1 Residue theorem

The residue theorem is a powerful tool in complex analysis for evaluating integrals. It connects the value of a contour integral to the residues of a function's singularities inside the contour. This theorem simplifies complex calculations and has wide-ranging applications.

Residues are crucial for solving real-world problems in physics and engineering. By transforming difficult real integrals into complex ones, we can use the residue theorem to find solutions that would be challenging or impossible with traditional calculus methods.

Residues in Complex Analysis

Definition and Role of Residues

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• A residue is a complex number associated with an isolated singularity of a complex function
• Residues are used to evaluate complex integrals, especially those where the integral cannot be computed using standard methods (Fundamental Theorem of Calculus)
• The residue of a function $f(z)$ at an isolated singularity $z₀$ is defined as the coefficient $a₋₁$ in the Laurent series expansion of $f(z)$ around $z₀$
• Cauchy's residue theorem relates the value of a complex line integral around a closed contour to the sum of the residues at the singularities enclosed by the contour

Applications of Residues

• Residues provide a powerful tool for evaluating real definite integrals by transforming them into complex contour integrals
  • This technique is particularly useful when the original real integral is difficult or impossible to evaluate using standard methods (integration by parts, substitution)
  • By extending the integrand to the complex plane and choosing an appropriate contour, the residue theorem can be applied to simplify the evaluation process
• Residues also find applications in various fields of physics and engineering, such as:
  • Quantum mechanics: Calculating transition amplitudes and energy levels
  • Electromagnetism: Solving boundary value problems and determining field distributions
  • Signal processing: Analyzing the frequency response of systems and designing filters

Residue Theorem for Simple Poles

Statement of the Theorem

• The residue theorem for simple poles states that if $f(z)$ is analytic inside and on a simple closed contour $C$ except for a finite number of isolated singularities $z₁, z₂, ..., zₙ$ inside $C$, then: $\oint_C f(z) dz = 2\pi i \sum_{i=1}^n \text{Res}(f, z_i)$ where $\text{Res}(f, z_i)$ denotes the residue of $f$ at $z_i$
• A simple pole is an isolated singularity $z₀$ of a function $f(z)$ such that $\lim_{z \to z₀} (z - z₀)f(z)$ is finite and nonzero

Proof of the Residue Theorem for Simple Poles

1. Consider a simple closed contour $C$ and a function $f(z)$ analytic inside and on $C$ except for a finite number of isolated singularities $z₁, z₂, ..., zₙ$ inside $C$
2. Construct small circles $C_i$ around each singularity $z_i$ and connect them with line segments to form a new contour $C'$
3. Apply Cauchy's integral theorem to the function $f(z)$ on the contour $C'$, noting that the integral along $C'$ is equal to the integral along $C$
4. Evaluate the integrals along the small circles $C_i$ using the definition of residue for simple poles: $\text{Res}(f, z_i) = \lim_{z \to z_i} (z - z_i)f(z)$
5. Show that the integrals along the line segments connecting the small circles vanish as the radii of the circles approach zero
6. Combine the results to obtain the residue theorem: $\oint_C f(z) dz = 2\pi i \sum_{i=1}^n \text{Res}(f, z_i)$

Evaluating Complex Integrals with Residues

Steps for Applying the Residue Theorem

1. Identify the isolated singularities of the integrand within the contour
2. Classify the singularities as simple poles, poles of higher order, or essential singularities
3. Calculate the residues at each singularity using the appropriate method based on the type of singularity
   • For simple poles: $\text{Res}(f, z₀) = \lim_{z \to z₀} (z - z₀)f(z)$
   • For poles of order $m$: $\text{Res}(f, z₀) = \frac{1}{(m-1)!} \lim_{z \to z₀} \frac{d^{m-1}}{dz^{m-1}} [(z - z₀)^m f(z)]$
4. Apply the residue theorem by summing the residues multiplied by $2\pi i$ to obtain the value of the integral

Evaluating Real Definite Integrals

1. Express the real integral as a complex contour integral by extending the integrand to the complex plane
2. Choose an appropriate contour (semicircle, rectangle) that encloses the singularities of the integrand
3. Apply the residue theorem to evaluate the complex contour integral
4. Use the result to determine the value of the original real definite integral
   • Often, the real part of the complex integral corresponds to the value of the real definite integral
   • Techniques such as Jordan's lemma and the estimation lemma can be used to show that certain integrals along the contour vanish as the contour grows larger

Residue Theorem for Higher-Order Poles

Definition of Higher-Order Poles

• A pole of order $m$ is an isolated singularity $z₀$ of a function $f(z)$ such that:
  • $\lim_{z \to z₀} (z - z₀)^m f(z)$ is finite and nonzero
  • $\lim_{z \to z₀} (z - z₀)^k f(z) = 0$ for all $k < m$
• The residue of a function $f(z)$ at a pole of order $m$ is given by: $\text{Res}(f, z₀) = \frac{1}{(m-1)!} \lim_{z \to z₀} \frac{d^{m-1}}{dz^{m-1}} [(z - z₀)^m f(z)]$

Extending the Residue Theorem

1. Identify the poles of the integrand and their orders within the contour
2. Calculate the residues at each pole using the formula for higher-order poles
3. Apply the residue theorem by summing the residues multiplied by $2\pi i$ to obtain the value of the integral
   • The residue theorem remains valid for integrands with poles of any order, providing a general method for evaluating complex integrals in the presence of isolated singularities
4. When dealing with higher-order poles, carefully evaluate the limit and the derivatives to determine the residue accurately
   • The process may involve using L'Hôpital's rule or expanding the function using Taylor series