Key Concepts of Cauchy-Riemann Equations to Know for Intro to Complex Analysis

Cauchy-Riemann equations are key to understanding complex functions. They connect the real and imaginary parts, ensuring differentiability and analyticity. These equations also play a crucial role in conformal mappings, which preserve angles and shapes in complex analysis.

1. Definition of Cauchy-Riemann equations

   • The Cauchy-Riemann equations are a set of two partial differential equations.
   • They relate the partial derivatives of the real and imaginary parts of a complex function.
   • For a function ( f(z) = u(x, y) + iv(x, y) ), the equations are ( \frac{\partial u}{\partial x} = \frac{\partial v}{\partial y} ) and ( \frac{\partial u}{\partial y} = -\frac{\partial v}{\partial x} ).

2. Relationship between real and imaginary parts of a complex function

   • The real part ( u(x, y) ) and the imaginary part ( v(x, y) ) must satisfy the Cauchy-Riemann equations for ( f(z) ) to be differentiable.
   • This relationship indicates that changes in ( u ) and ( v ) are interconnected.
   • The equations ensure that the function behaves consistently in the complex plane.

3. Necessary conditions for complex differentiability

   • A function must satisfy the Cauchy-Riemann equations at a point to be complex differentiable at that point.
   • The function must also be continuous in a neighborhood around that point.
   • If the equations are not satisfied, the function cannot be complex differentiable.

4. Sufficient conditions for complex differentiability

   • If a function satisfies the Cauchy-Riemann equations and is continuous in a neighborhood, it is complex differentiable.
   • This implies that the function is also analytic in that region.
   • Analyticity means the function can be represented by a power series.

5. Polar form of Cauchy-Riemann equations

   • In polar coordinates, where ( z = re^{i\theta} ), the equations can be expressed in terms of ( r ) and ( \theta ).
   • The equations take the form ( \frac{\partial u}{\partial r} = \frac{1}{r} \frac{\partial v}{\partial \theta} ) and ( \frac{\partial u}{\partial \theta} = -r \frac{\partial v}{\partial r} ).
   • This form is useful for analyzing functions in polar coordinates.

6. Connection to harmonic functions

   • The real and imaginary parts of an analytic function are harmonic functions.
   • A harmonic function satisfies Laplace's equation, which is a second-order partial differential equation.
   • The Cauchy-Riemann equations ensure that both ( u ) and ( v ) are harmonic if ( f(z) ) is analytic.

7. Applications in determining analyticity of functions

   • The Cauchy-Riemann equations are used to test whether a function is analytic in a given region.
   • They provide a method to identify complex functions that are differentiable.
   • This is crucial for understanding the behavior of complex functions in analysis.

8. Examples of functions satisfying and not satisfying the equations

   • Functions like ( f(z) = z^2 ) satisfy the Cauchy-Riemann equations and are analytic.
   • The function ( f(z) = \overline{z} ) does not satisfy the equations and is not analytic.
   • Identifying such functions helps in understanding the conditions for analyticity.

9. Interpretation of Cauchy-Riemann equations as orthogonality conditions

   • The equations imply that the gradients of ( u ) and ( v ) are orthogonal at points where they hold.
   • This orthogonality indicates that the level curves of ( u ) and ( v ) intersect at right angles.
   • This geometric interpretation aids in visualizing the behavior of complex functions.

10. Role in conformal mappings

    • The Cauchy-Riemann equations are essential for establishing conformal mappings, which preserve angles.
    • A function that satisfies these equations is locally one-to-one and preserves the shape of infinitesimally small figures.
    • Conformal mappings are widely used in complex analysis and applications such as fluid dynamics and electrical engineering.