/*! This file is auto-generated */ .wp-block-button__link{color:#fff;background-color:#32373c;border-radius:9999px;box-shadow:none;text-decoration:none;padding:calc(.667em + 2px) calc(1.333em + 2px);font-size:1.125em}.wp-block-file__button{background:#32373c;color:#fff;text-decoration:none} Problem 95 In the advanced subject of compl... [FREE SOLUTION] | 91Ó°ÊÓ

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In the advanced subject of complex variables, a function typically has the form \(f(x, y)=u(x, y)+i v(x, y),\) where \(u\) and \(v\) are real-valued functions and \(i=\sqrt{-1}\) is the imaginary unit. A function \(f=u+i v\) is said to be analytic (analogous to differentiable) if it satisfies the Cauchy-Riemann equations: \(u_{x}=v_{y}\) and \(u_{y}=-v_{x}\). a. Show that \(f(x, y)=\left(x^{2}-y^{2}\right)+i(2 x y)\) is analytic. b. Show that \(f(x, y)=x\left(x^{2}-3 y^{2}\right)+i y\left(3 x^{2}-y^{2}\right)\) is analytic. c. Show that if \(f=u+i v\) is analytic, then \(u_{x x}+u_{y y}=0\) and \(v_{x x}+v_{y y}=0 .\) Assume \(u\) and \(v\) satisfy the conditions in Theorem 12.4.

Short Answer

Expert verified
Question: Prove that a given function is analytic and verify if the Laplace equation holds for the real and imaginary parts. Answer: For the functions \(f(x, y) = (x^2 - y^2) + i(2xy)\) and \(f(x, y) = x(x^2 - 3y^2) + i y(3x^2 - y^2)\), we have proved that they are analytic by verifying the Cauchy-Riemann equations. Additionally, for any analytic function \(f = u + iv\), we have shown that the Laplace equation holds: \(u_{xx} + u_{yy} = 0\) and \(v_{xx} + v_{yy} = 0\).

Step by step solution

01

Identify the real and imaginary parts

Let \(f(x, y) = (x^2 - y^2) + i(2xy)\). The real part of the function is \(u(x, y) = x^2 - y^2\), and the imaginary part is \(v(x, y) = 2xy\).
02

Calculate partial derivatives

Compute the partial derivatives, \(u_x\), \(u_y\), \(v_x\), and \(v_y\): \(u_x = 2x\) \(u_y = -2y\) \(v_x = 2y\) \(v_y = 2x\)
03

Check the Cauchy-Riemann equations

Verify if the Cauchy-Riemann equations are satisfied: \(u_x = v_y \Rightarrow 2x = 2x\) \(u_y = -v_x \Rightarrow -2y = -2y\) Both equations are true, so the function is analytic. #Part b#
04

Identify the real and imaginary parts

Let \(f(x, y) = x(x^2 - 3y^2) + i y(3x^2 - y^2)\). The real part of the function is \(u(x, y) = x(x^2 - 3y^2)\), and the imaginary part is \(v(x, y) = y(3x^2 - y^2)\).
05

Calculate partial derivatives

Compute the partial derivatives, \(u_x\), \(u_y\), \(v_x\), and \(v_y\): \(u_x = (3x^2 - y^2) + 2x(x^2 - 3y^2)\) \(u_y = -6xy^2\) \(v_x = 6x^2y\) \(v_y = (3x^2 - y^2) - 2y^3\)
06

Check the Cauchy-Riemann equations

Verify if the Cauchy-Riemann equations are satisfied: \(u_x = v_y \Rightarrow (3x^2 - y^2) + 2x(x^2 - 3y^2) = (3x^2 - y^2) - 2y^3\) \(u_y = -v_x \Rightarrow -6xy^2 = -6x^2y\) Both equations are true, so the function is analytic. #Part c#
07

Use the Cauchy-Riemann equations

Since \(f = u + iv\) is analytic, we have: \(u_x = v_y\) \(u_y = -v_x\)
08

Differentiate the Cauchy-Riemann equations again

Compute the second derivatives: \(u_{xx} = \frac{\partial u_x}{\partial x} = \frac{\partial v_y}{\partial x}\) \(u_{yy} = \frac{\partial u_y}{\partial y} = -\frac{\partial v_x}{\partial y}\) \(v_{xx} = \frac{\partial v_x}{\partial x} = -\frac{\partial u_y}{\partial x}\) \(v_{yy} = \frac{\partial v_y}{\partial y} = \frac{\partial u_x}{\partial y}\)
09

Add second derivatives

Add \(u_{xx}\) and \(u_{yy}\): \(u_{xx} + u_{yy} = \frac{\partial v_y}{\partial x} - \frac{\partial v_x}{\partial y} = 0\) Add \(v_{xx}\) and \(v_{yy}\): \(v_{xx} + v_{yy} = -\frac{\partial u_y}{\partial x} + \frac{\partial u_x}{\partial y} = 0\) Both \(u_{xx}+u_{yy}\) and \(v_{xx}+v_{yy}\) are equal to zero.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Cauchy-Riemann Equations
Understanding the Cauchy-Riemann equations is crucial for grasping the basics of analytic functions in complex analysis. These are a set of two partial differential equations that must be satisfied for a complex function to be considered analytic, which means that the function is differentiable at every point in its domain.

When we say a complex function, we mean a function that can be written in the form of f(x, y) = u(x, y) + i v(x, y), where u and v are functions that map to the real numbers and i is the imaginary unit. In terms of partial derivatives, the Cauchy-Riemann equations state that for a function to be analytic, the following must hold: ux = vy and uy = -vx.

Why is satisfying these equations essential? It comes down to ensuring that the function behaves well - it's differentiable and thus smooth and continuous. This means not only does the function have a derivative, but the derivative itself is also continuous across the complex plane.

The real beauty of the Cauchy-Riemann equations lies in their power to analyze complex relationships in a two-dimensional plane, connecting the growth rates of the real and imaginary parts of a function. They also lay the groundwork for further important theorems in complex analysis, such as the Cauchy Integral Formula and Laurent Series, which have profound implications in various fields of mathematics and physics.
Partial Derivatives
Partial derivatives play a pivotal role in multiple areas of mathematics, especially in the field of complex analysis. A partial derivative represents the rate at which a function changes as one of its variables is varied, while the other variables are held constant. In the context of complex functions, which involve both a real part u(x, y) and an imaginary part v(x, y), understanding how these individual parts change independently is vital.

Let's take an example function f(x, y) = u(x, y) + iv(x, y), where u and v are functions of two variables x and y. The partial derivatives ux and uy represent the rates of change of u with respect to x and y, respectively. Likewise, vx and vy quantify how v changes with x and y.

These derivatives are foundational to the Cauchy-Riemann equations, acting as the bridge between the purely real aspects of functions and their complex counterparts. In practice, computing these derivatives is often the first step in analyzing whether a complex function is analytic or not. When approaching homework problems or real-world situations, being adept with partial differentiation offers a means to dissect complex relationships into more manageable pieces.
Complex Analysis
Complex analysis is a fascinating and rich field of mathematics focused on the study of functions of complex variables. At its core, it's about extending the familiar calculus of real variables into the complex plane, where functions take on new dimensions and behaviors.

In complex analysis, a function is considered 'complex' if it involves the imaginary unit i, where i2 = -1. This is a simple extension of real analysis because now we deal with functions that have inputs and outputs that can be complex numbers. These functions, composed of both a real part (u) and an imaginary part (v), are significantly more versatile in their application.

The importance of complex analysis lies not only in theoretical mathematics but also in practical applications. Complex functions often represent physical phenomena in engineering and physics, such as electromagnetic fields or quantum mechanics.

Moreover, the principles of complex analysis, like analytic continuation and residue calculus, offer powerful tools for evaluating integrals and solving differential equations that would otherwise be extremely challenging to tackle. For students learning complex analysis, mastering the fundamental concepts such as the Cauchy-Riemann equations and partial derivatives is the key to unlocking the intricate beauties and utilities this field has to offer.

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Most popular questions from this chapter

Use the gradient rules of Exercise 81 to find the gradient of the following functions. $$f(x, y)=x y \cos x y$$

Given a differentiable function \(w=f(x, y, z),\) the goal is to find its maximum and minimum values subject to the constraints \(g(x, y, z)=0\) and \(h(x, y, z)=0\) where \(g\) and \(h\) are also differentiable. a. Imagine a level surface of the function \(f\) and the constraint surfaces \(g(x, y, z)=0\) and \(h(x, y, z)=0 .\) Note that \(g\) and \(h\) intersect (in general) in a curve \(C\) on which maximum and minimum values of \(f\) must be found. Explain why \(\nabla g\) and \(\nabla h\) are orthogonal to their respective surfaces. b. Explain why \(\nabla f\) lies in the plane formed by \(\nabla g\) and \(\nabla h\) at a point of \(C\) where \(f\) has a maximum or minimum value. c. Explain why part (b) implies that \(\nabla f=\lambda \nabla g+\mu \nabla h\) at a point of \(C\) where \(f\) has a maximum or minimum value, where \(\lambda\) and \(\mu\) (the Lagrange multipliers) are real numbers. d. Conclude from part (c) that the equations that must be solved for maximum or minimum values of \(f\) subject to two constraints are \(\nabla f=\lambda \nabla g+\mu \nabla h, g(x, y, z)=0\) and \(h(x, y, z)=0\).

The density of a thin circular plate of radius 2 is given by \(\rho(x, y)=4+x y .\) The edge of the plate is described by the parametric equations \(x=2 \cos t, y=2 \sin t,\) for \(0 \leq t \leq 2 \pi\) a. Find the rate of change of the density with respect to \(t\) on the edge of the plate. b. At what point(s) on the edge of the plate is the density a maximum?

A classical equation of mathematics is Laplace's equation, which arises in both theory and applications. It governs ideal fluid flow, electrostatic potentials, and the steadystate distribution of heat in a conducting medium. In two dimensions, Laplace's equation is $$\frac{\partial^{2} u}{\partial x^{2}}+\frac{\partial^{2} u}{\partial y^{2}}=0.$$ Show that the following functions are harmonic; that is, they satisfy Laplace's equation. $$u(x, y)=e^{a x} \cos a y, \text { for any real number } a$$

Given three distinct noncollinear points \(A, B,\) and \(C\) in the plane, find the point \(P\) in the plane such that the sum of the distances \(|A P|+|B P|+|C P|\) is a minimum. Here is how to proceed with three points, assuming that the triangle formed by the three points has no angle greater than \(2 \pi / 3\left(120^{\circ}\right)\). a. Assume the coordinates of the three given points are \(A\left(x_{1}, y_{1}\right)\) \(\underline{B}\left(x_{2}, y_{2}\right),\) and \(C\left(x_{3}, y_{3}\right) .\) Let \(d_{1}(x, y)\) be the distance between \(A\left(x_{1}, y_{1}\right)\) and a variable point \(P(x, y) .\) Compute the gradient of \(d_{1}\) and show that it is a unit vector pointing along the line between the two points. b. Define \(d_{2}\) and \(d_{3}\) in a similar way and show that \(\nabla d_{2}\) and \(\nabla d_{3}\) are also unit vectors in the direction of the line between the two points. c. The goal is to minimize \(f(x, y)=d_{1}+d_{2}+d_{3} .\) Show that the condition \(f_{x}=f_{y}=0\) implies that \(\nabla d_{1}+\nabla d_{2}+\nabla d_{3}=0\) d. Explain why part (c) implies that the optimal point \(P\) has the property that the three line segments \(A P, B P,\) and \(C P\) all intersect symmetrically in angles of \(2 \pi / 3\) e. What is the optimal solution if one of the angles in the triangle is greater than \(2 \pi / 3\) (just draw a picture)? f. Estimate the Steiner point for the three points (0,0),(0,1) and (2,0).

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