8.1 Conformal mappings and their properties

Conformal mappings are complex functions that preserve angles locally. They're a powerful tool in complex analysis, allowing us to transform complicated problems into simpler ones while maintaining key geometric properties.

These mappings have unique characteristics that set them apart. They preserve angles and local shapes, but not necessarily distances or global shapes. Understanding these properties is crucial for applying conformal mappings effectively in various mathematical and physical problems.

Conformal Mappings: Definition and Properties

Definition and Key Characteristics

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• A conformal mapping is a complex function that preserves angles locally between curves in the complex plane
• Conformal mappings are angle-preserving transformations but not necessarily distance-preserving (e.g., a mapping that doubles distances while preserving angles)
• The real and imaginary parts of a conformal mapping satisfy the Cauchy-Riemann equations
• Conformal mappings are analytic functions meaning they are differentiable at every point in their domain
• The composition of two conformal mappings is also a conformal mapping (e.g., if $f(z)$ and $g(z)$ are conformal, then $f(g(z))$ is also conformal)
• The inverse of a conformal mapping, if it exists, is also a conformal mapping (e.g., if $f(z)$ is conformal and has an inverse, then $f^{-1}(z)$ is also conformal)

Properties of Conformal Mappings

• Conformal mappings preserve the local shape of infinitesimal figures such as small circles or squares
• Conformal mappings preserve the orientation of angles meaning that if a curve is mapped to its image, the orientation of the angle between the tangent vectors at corresponding points remains the same
• Conformal mappings preserve the ratio of the magnitudes of tangent vectors at corresponding points
• Conformal mappings do not necessarily preserve distances, areas, or global shapes of figures (e.g., a conformal mapping may distort the shape of a large circle into an ellipse)

Geometric Preservation under Conformal Mappings

Angle Preservation

• Conformal mappings preserve angles between curves at their point of intersection
• If two curves intersect at an angle $\theta$ in the domain, their images under a conformal mapping will also intersect at the same angle $\theta$
• This angle preservation property holds for any number of intersecting curves at a point

Local Shape Preservation

• Conformal mappings preserve the local shape of infinitesimal figures such as small circles or squares
• An infinitesimal circle in the domain will be mapped to an infinitesimal circle in the codomain, although the size may change
• An infinitesimal square in the domain will be mapped to an infinitesimal square in the codomain, with the same orientation and possibly a different size

Non-Preservation of Global Properties

• Conformal mappings do not necessarily preserve distances between points (e.g., a mapping that scales distances by a factor of 2)
• Conformal mappings do not necessarily preserve areas of figures (e.g., a mapping that doubles areas while preserving angles)
• Conformal mappings do not necessarily preserve the global shapes of figures (e.g., a mapping that maps a large circle to an ellipse)

Effects of Conformal Mappings on Functions

Local Behavior

• Locally, conformal mappings behave like rotations and dilations, preserving angles and the shape of infinitesimal figures
• The local behavior of a conformal mapping at a point is determined by its complex derivative at that point
• The magnitude of the complex derivative at a point represents the local scaling factor, while the argument of the complex derivative represents the local rotation angle
• For example, if $f'(z_0) = 2e^{i\pi/4}$, then the mapping locally scales distances by a factor of 2 and rotates angles by $\pi/4$ radians (45 degrees) counterclockwise

Global Effects

• Globally, conformal mappings can drastically alter the appearance of a function's graph, as they do not necessarily preserve distances or global shapes
• Conformal mappings can be used to simplify the geometry of a problem by mapping a complicated domain to a simpler one (e.g., mapping the upper half-plane to the unit disk)
• Conformal mappings can be used to study the behavior of complex functions by mapping their domains to more tractable regions (e.g., mapping the exterior of the unit disk to the upper half-plane)

Conformal Mappings vs Cauchy-Riemann Equations

Cauchy-Riemann Equations as a Criterion for Conformality

• The Cauchy-Riemann equations are a necessary and sufficient condition for a complex function to be analytic and, consequently, conformal
• For a complex function $f(z) = u(x, y) + iv(x, y)$, the Cauchy-Riemann equations are:
  • $\frac{\partial u}{\partial x} = \frac{\partial v}{\partial y}$
  • $\frac{\partial u}{\partial y} = -\frac{\partial v}{\partial x}$
• If the Cauchy-Riemann equations are satisfied at every point in the domain of the function, then the mapping is conformal
• If the Cauchy-Riemann equations are not satisfied at some point, then the mapping is not conformal in any neighborhood of that point

Determining Conformality using Cauchy-Riemann Equations

• To determine if a mapping is conformal, calculate the partial derivatives of the real and imaginary parts and check if they satisfy the Cauchy-Riemann equations
• For example, consider the function $f(z) = z^2 = (x^2 - y^2) + i(2xy)$. Here, $u(x, y) = x^2 - y^2$ and $v(x, y) = 2xy$. Checking the Cauchy-Riemann equations:
  • $\frac{\partial u}{\partial x} = 2x$, $\frac{\partial v}{\partial y} = 2x$
  • $\frac{\partial u}{\partial y} = -2y$, $-\frac{\partial v}{\partial x} = -2y$
• Since the Cauchy-Riemann equations are satisfied for all $x$ and $y$, the mapping $f(z) = z^2$ is conformal everywhere in the complex plane