is a game-changer in complex analysis. It lets us find function values inside a using only the values on the boundary. This powerful tool simplifies complex integration and opens doors to evaluating tricky integrals.
Building on Cauchy's integral theorem, this formula connects analytic functions to their derivatives and integrals. It's the foundation for Taylor series expansions and calculations, making it essential for solving real-world problems in physics and engineering.
Cauchy's Integral Formula
Derivation from Cauchy's Integral Theorem
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Cauchy's integral theorem states that if f(z) is analytic in a simply connected domain D and C is a simple closed contour in D, then ∮Cf(z)dz=0
To derive Cauchy's integral formula, consider a circle C centered at z0 with radius r, and a point z inside C
Apply Cauchy's integral theorem to the function f(ζ)/(ζ−z) on the domain D−{z}
Use the parametrization ζ=z0+reiθ and the fact that ∮Cdζ/(ζ−z)=2πi
Obtain the integral formula f(z)=2πi1∮Cζ−zf(ζ)dζ
Cauchy's integral formula expresses the value of an at a point inside a contour in terms of the values of the function on the contour
Provides a powerful tool for evaluating complex integrals and derivatives
Relationship to Taylor Series
Cauchy's integral formula can be used to derive the of an analytic function f(z) around a point z0
Differentiate the integral formula n times to obtain f(n)(z0)=2πin!∮C(ζ−z0)n+1f(ζ)dζ
Substitute the Taylor series of f(ζ) around z0 into the integral and evaluate term by term
The resulting series is the Taylor series of f(z) around z0: f(z)=∑n=0∞n!f(n)(z0)(z−z0)n
The Taylor series provides a local approximation of an analytic function near a point
Useful for studying the behavior of functions and solving differential equations
Applications of Cauchy's Integral Formula
Evaluating Integrals and Derivatives
To evaluate the integral of an analytic function f(z) along a closed contour C, choose a point z0 inside C and apply Cauchy's integral formula: ∮Cf(z)dz=2πi⋅f(z0)
Simplifies the calculation of complex integrals by reducing them to evaluating the function at a single point
Cauchy's integral formula can be used to evaluate integrals of the form ∮CQ(z)P(z)dz, where P and Q are polynomials and Q has simple zeros inside C
The result is a sum of residues at the zeros of Q, given by ∑k=1nQ′(zk)P(zk), where zk are the zeros of Q inside C
The derivative of an analytic function f(z) can be computed using Cauchy's integral formula: f′(z0)=2πi1∮C(ζ−z0)2f(ζ)dζ
Higher-order derivatives can be obtained by differentiating under the integral sign: f(n)(z0)=2πin!∮C(ζ−z0)n+1f(ζ)dζ
Allows for the calculation of derivatives without the need for explicit differentiation
Residue Theorem
The is a powerful application of Cauchy's integral formula for evaluating integrals of the form ∮Cg(z)f(z)dz, where f and g are analytic functions and g has isolated zeros inside C
The residue of f/g at a zero z0 of g is defined as Res(f/g,z0)=(m−1)!1limz→z0dzm−1dm−1[(z−z0)mg(z)f(z)], where m is the multiplicity of the zero
The residue theorem states that ∮Cg(z)f(z)dz=2πi∑k=1nRes(f/g,zk), where zk are the zeros of g inside C
The residue theorem simplifies the evaluation of complex integrals by reducing them to the calculation of residues
Particularly useful for integrals involving rational functions, logarithms, and trigonometric functions
Finds applications in various fields, such as physics (Laplace transforms, Fourier analysis) and engineering (control theory, signal processing)
Properties of Analytic Functions
Mean Value Property
The mean value property states that for an analytic function f(z) in a disk D(z0,r), the value f(z0) is equal to the average of f(z) over any circle C centered at z0 with radius r: f(z0)=2π1∫02πf(z0+reiθ)dθ
To prove the mean value property, apply Cauchy's integral formula to the circle C and use the parametrization z=z0+reiθ
Demonstrates the smoothness and regularity of analytic functions
The mean value property can be generalized to higher dimensions (harmonic functions) and non-circular domains (convex domains)
Plays a crucial role in the study of partial differential equations and potential theory
Maximum Modulus Principle
Cauchy's integral formula can be used to prove the maximum modulus principle, which states that if f(z) is analytic and non-constant in a domain D, then ∣f(z)∣ cannot attain its maximum value at any interior point of D
If ∣f(z)∣ attains its maximum at an interior point z0, then by the mean value property, f(z) must be constant in a neighborhood of z0, contradicting the assumption that f is non-constant
Implies that the maximum value of ∣f(z)∣ must be attained on the boundary of D
The maximum modulus principle has important consequences for the behavior of analytic functions
: A bounded entire function must be constant
Fundamental theorem of algebra: Every non-constant polynomial has a root
Open mapping theorem: A non-constant analytic function maps open sets to open sets
Uniqueness and Continuity
Cauchy's integral formula implies the uniqueness of analytic functions: If two analytic functions f(z) and g(z) agree on a set with a limit point in a domain D, then they agree everywhere in D
Follows from the fact that the difference f(z)−g(z) is analytic and vanishes on a set with a limit point, so it must be identically zero by the identity theorem
Allows for the extension of local properties of analytic functions to global properties
Analytic functions are infinitely differentiable, and their derivatives are also analytic
The continuity and differentiability of analytic functions follow from the Cauchy-Riemann equations and the existence of the complex derivative
Higher-order derivatives can be computed using the generalized Cauchy integral formula: f(n)(z0)=2πin!∮C(ζ−z0)n+1f(ζ)dζ
Analytic functions possess a high degree of smoothness and regularity
Generalizations of Cauchy's Integral Formula
Higher-Order Derivatives and Integrals
The generalized Cauchy integral formula expresses the n-th order derivative of an analytic function f(z) at a point z0 in terms of an integral: f(n)(z0)=2πin!∮C(ζ−z0)n+1f(ζ)dζ
Derived by differentiating Cauchy's integral formula n times under the integral sign
Allows for the evaluation of higher-order derivatives without the need for explicit differentiation
The generalized Cauchy integral formula can also be extended to integrate analytic functions: If f(z) is analytic in a simply connected domain D and C is a simple closed contour in D, then ∫Cf(z)dz=2πi1∮Cζ−z∫ζf(ξ)dξdζ, where the inner integral is taken along a path from a fixed point to ζ
Allows for the of analytic functions using contour integration techniques
Useful for computing definite integrals of real-valued functions by converting them to complex contour integrals (e.g., using the residue theorem)
Cauchy's Integral Formula for Unbounded Domains
Cauchy's integral formula can be extended to unbounded domains by considering contours that extend to infinity
For an analytic function f(z) in an unbounded domain D, choose a contour C that consists of a large circle CR of radius R and a path γ connecting a point z0 inside CR to infinity
Apply Cauchy's integral theorem to the region bounded by CR and γ, and take the limit as R→∞
The resulting formula is f(z0)=2πi1∫γζ−z0f(ζ)dζ, where the integral is taken along the path γ from infinity to z0
The extension of Cauchy's integral formula to unbounded domains is particularly useful for studying the behavior of analytic functions at infinity
Allows for the classification of singularities (poles, essential singularities) and the computation of residues at infinity
Finds applications in complex analysis, such as the study of entire functions and meromorphic functions