This chapter covers linear systems (mostly two-dimensional systems) as well as homogeneous, constant-coefficient, second-order linear equations. Linear algebra is not a prerequisite for our course, and we introduce the required concepts and notation as needed. We find that even those students who have had some linear algebra generally do not have a good geometric understanding of eigenvalues and eigenvectors, so most students learn some linear algebra while studying this chapter.
There are two examples that appear frequently throughout the chapter. They are the harmonic oscillator and Paul's and Bob's Cafes. The purpose of the Cafe model is to provide an example for understanding the meaning of the coefficients of a linear system and for practicing interpretation of solutions in everyday language. Hopefully the readers will not be annoyed by our poking fun at each other.
Section 3.1 covers matrix notation and the basic properties of linear
systems. In Sections 3.2
DETools
We use HPGSystemSolver constantly throughout the chapter along with the special purpose tools mentioned below.
In this section we introduce matrix notation for linear systems. Most
of our students have seen 2x2 matrices previously, often
in high school. The determinant of a 2x2 matrix
arises naturally when checking for equilibrium points of a linear
system.
We also mention that that a linear system of algebraic
equations has nontrivial solutions if and only if the determinant of
the matrix vanishes. This property is used in
The main goal of the section is to present the Linearity Principle (or
Principle of Superposition) and the idea that the general solution of
a two-dimensional, linear system can be obtained from two independent
solutions. Linearly independent solutions are defined as those with
linearly independent initial conditions. The Wronskian appears in
Comments on selected exercises
Exercises 1-4 consider the meaning of the parameters in the CD store model. Students are asked to describe the underlying assumptions built into these choices of coefficients.
Exercises 5-9 are straightforward practice with matrix notation for systems.
Exercises 10-13 use the geometric methods of
Exercise 14 deals with the equilibrium points of a matrix whose
determinant is nonzero (completing the discussion in the text).
There are a number of exercises throughout the chapter on second-order
equations, and it is important that the students have a lot of
practice with the relationship between second-order equations and
first-order systems.
Exercises 20-23 consider a simple model of a housing market. These
questions are similar to those of
Exercises 24-30, 33,
Exercises 31, 32,
Sections 3.2
In
Although these solutions are exponentials, we call them "straight-line solutions" because they manifest themselves as lines in the phase plane. It is important to emphasize that these lines really correspond to functions x(t) and y(t) that are exponentials.
DETools
The MatrixFields
tool helps with the geometric interpretation of
eigenvectors. The vector AY is attached to the
end of the position vector Y. Start the tool with both the
direction field and eigenvalue plane turned off. After playing with
the tool for a few minutes, turn on the
direction field. Ultimately, you should turn on the eigenvalue plane to
illustrate the eigendirections. The discussion of why the eigenvalues
are plotted in the complex plane is delayed until
Comments on selected exercises
Exercises 1-10 provide routine practice with the computation of
eigenvalues, eigenvectors, straight-line solutions, and general
solutions. The results of Exercises 1-3
Exercises 11-14 use eigenvalues and eigenvectors to solve
initial-value problems. These initial-value problems reappear in the
Exercises 15-18 are more theoretical. The students are asked to make general observations regarding eigenvalues of matrices of certain special forms.
Exercises 19-25 apply the techniques developed in this section to the
important special case of second-order, homogeneous linear equations.
The qualitative analysis of linear systems with real, distinct, nonzero eigenvalues is completed in this section. We emphasize that a good qualitative picture of the system can be obtained from the eigenvalues and eigenvectors alone and that one can often get a good idea of the behavior of the solutions without having to compute them in complete detail.
DETools
In addition to HPGSystemSolver, one can use
LinearPhasePortraits to illustrate the various types of
phase portraits. We recommend keeping the trace-determinant plane
hidden until
Comments on selected exercises
Exercises 1-8 involve phase portraits corresponding to some of the
systems that arose in the exercise set for
The model of Paul's and Bob's cafe returns in Exercises 17
Exercises 19
Exercises 21
Exercises 23-26 consider simple linear models of a pond inhabited by
two species of fish. We ask the students to make predictions based on
the phase portraits. These are good examples to revisit during the
discussion of linearization in
The careful student will have already asked if all linear systems have straight-line solutions. In this section we deal with the case of complex eigenvalues. The idea is to use the algebra developed in the previous two sections to obtain a formula for the general solution and then to use the formula to obtain the qualitative analysis. Of particular importance is the idea of a natural period for a system. This important quantity is qualitative information that cannot be seen in the phase plane.
Most students have seen complex numbers before but need a little
review in the basics, i.e., multiplication and division of complex
numbers. Also, a review of (or an introduction to) Euler's formula is
necessary. We have provided a very brief summary of some of this
material in
Again, it is stressed that a great deal of qualitative (and quantitative) information can be obtained from the eigenvalues alone. When sketching phase portraits, the easiest way to determine whether solutions spiral clockwise or counter-clockwise is by looking at any nonzero vector in the vector field.
DETools
Both MatrixFields and LinearPhasePortraits
illustrate the
geometric significance of complex eigenvalues. When you use these
tools, you should turn on the eigenvalue plane, but we still recommend
that you keep the trace-determinant plane turned off in
LinearPhasePortraits until
Comments on selected exercises
Exercises 1
Exercises 3-8
Exercises 15
Exercises 16-20 consider certain theoretical issues that arise in the case of complex eigenvalues.
Exercises 21
Exercise 23 is a theoretical exercise that relates the material of this section to the behavior of harmonic oscillators.
Exercise 25 is an essay on why there are no spiral saddles in two dimensions.
Exercise 26 involves an elliptical center.
In this section we study the atypical cases. The amount of time
spent on this section varies widely depending on how much detail you
want the students to master. If you are so inclined,
the details of how to find the
general solution can be safely skipped.
We derive the general solution for a
DETools
LinearPhasePortraits can be used to illustrate the fact that the repeated eigenvalue case separates the case of real and distinct eigenvalues from complex eigenvalues. This is a good point to illustrate here as well as when the trace-determinant plane is discussed.
Comments on selected exercises
Exercises 1-4
Exercises 17-19
Exercises 9-16, 20,
When we mentioned the harmonic oscillator as an
example in
DETools
This is a good place to bring back the MassSpring tool. RLCCircuits with A=0 is another tool that is appropriate for this section.
Comments on selected exercises
Exercises 13-20
Exercises 30
Exercises 32
Exercises 34
In this section we present a classification of the behavior of
linear systems using the trace-determinant plane.
This section is essentially a compilation of everything
that has been discussed earlier in the chapter, and therefore it
can be omitted. However, the resulting diagram is quite
appealing, and students seem to enjoy this method of classification
(as opposed to the construction of a table as is suggested in
DETools
Now turn on the trace-determinant plane in the
LinearPhasePortraits tool. Also, note the comment about
Comments on selected exercises
A word of warning regarding the exercises for this section: A detailed analysis of the qualitative behavior of any of the one- and two-parameter families that are given in the exercises is not a task for the faint of heart. Do not assign many problems from this exercise set.
Exercise 14 is an "animated" exercise involving one-parameter families of phase portraits and paths in the trace-determinant plane.
The algebra and geometry of three-dimensional linear systems is presented quickly and in a very matter-of-fact fashion in this section, and students without a background in linear algebra may find this section a bit terse. Our goals are to show that the ideas involved in the classification of two-dimensional linear systems generalize to higher dimensions and that, while the fundamental ideas remain the same in higher dimensions, the calculations become more complicated. This section is used only in the analysis of the Lorenz system in Section 5.5.
Comments on selected exercises
Exercises 1, 4-7,
Exercises 2
Exercises 8
Exercises 19
Lab 3.1. Bifurcations in Linear Systems
Students find this lab difficult but very useful in
understanding linear systems. Some students attempt to discover
the diagram necessary for
Lab 3.2. RLC Circuits
The equations modeling RLC circuits,
which are frequently given as a main motivation why second-order
equations must be studied prior to first-order systems, are actually
first expressed as a first-order system in most circuit theory
courses. The dependent variables are voltage across the capacitor and
the current. In most circuit theory texts, this system
is immediately converted
into a second-order equation. This lab is meant to emphasize this
point and to help our engineering students make the connection between
our approach and what they are learning in their circuit theory class.
This lab can be paired with
Lab 3.3. Measuring Mass in Space
A "harmonic-oscillator-like" device is actually used to measure the mass of astronauts during long space flights. ("How would you measure mass in space?" is a good question for discussion.)
Lab 3.4. Exploring a Parameter Space
This investigation of a three-parameter family of linear systems has resulted in some of our most spectacular lab "reports" ever. A number of our students who are artistically gifted have come up with especially creative ways of presenting their results.
Lab 3.5. Find Your Own Harmonic Oscillator
Lab 3.6. A Baby Bottle Harmonic Oscillator
Baby bottles are fairly foreign to the everyday life of most students in this course. The point of including the lab is that systems that can be modeled using the harmonic oscillator really do occur in unlikely places. (It also gave us something to think about when we were feeding Gib.) Students who have tried this lab have had difficulty obtaining useful data. If you assign this lab, it may be the only time that you ever hear "my baby ate my homework."