# Elementary Differential Equations and Boundary Value Problems - Boyce W.E.

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The nonlinearity of system (2) comes from the —x3 term in the rate function g(x, y) = —x — x3 — 0.1 y. In calculus you may have seen the following formula for the linear approximation of the function g(x, y) near the point

(*0, y0):

dg dg

g(*> Ó) ™ g(*o, Óî) + òãÎî, 7o)(x- x0) + — (x0, y0)(y-yo) (3) dx dy

However, g( x0, y0) will always be zero at an equilibrium point (do you see why?), so formula (3) simplifies in this case to

dg dg

g(x,y) ™ 7“(Xo, Óî)(õ— Xo) + — (xo,/o)(y-yo) (4)

dx dy

Since we’re interested in long-term behavior and the trajectories of system (2) seem to be heading toward the origin, we want to use the equilibrium point

Linear System

x = Ó, y = —x — 0-1 y

iff? * ³ I

t f /Î---\ ³ ³

t t t ³ ³ ³

t t ³ ³ ³

t t t ³ ³ ³

t \ ³ ³ ³

t * V4^-'³ ³ ³

t * /Ï²

0 2 4

x

Nonlinear System

x = Ó y = —x — x? — 0Ëy

Figure 7.1: Trajectories of both systems have the same long-term, spiral-sink behavior, but behavior differs in the short-term.

4

2

0

x

118

Chapter 7

Linear and nonlinear look-alikes.

Look back at Chapter 6 for more on complex eigenvalues and spiral sinks.

Trajectories t-x curves

Figure 7.2: Near the equilibrium at the origin trajectories and tx-component curves of nonlinear system (2) and its linearization (1) are nearly look-alikes.

x

(xo, y0) = (0, 0) in formula (4). Near the origin, the rate function for our nonlinear spring can be approximated by

g(x, y) Ú -x — °.ly

since dg/dx = —1 and dg/dy = —0.1 at x° = 0, y° = 0. Therefore the nonlinear system (2) reduces to the linearized system (1). You can see the approximation when the phase portraits are overlaid. The trajectories and tx-component curves of both systems, issuing from a common initial point close to the origin, are shown in Figure 7.2. The linear approximation is pretty good because the nonlinearity —x3 is small near x = 0. Take another look at Figure 7.1; the linear approximation is not very good when |x| > 1.

? How good an approximation to system (2) is the linearized system (1) if the initial point of a trajectory is far away from the origin? Explain what you mean by “good” and “far away.”

In matrix notation, linear system (1) takes the form

(5)

x / 0 1 x

y ---1 ---0.1 y

so the characteristic equation of the system matrix is A2 + 0.1 A + 1 = 0. The matrix has eigenvalues A = (—0.1 ± ³ ë/3.99 ) / 2. making (0, 0) a spiral sink (due to the negative real part of both eigenvalues). This supports our earlier conclusion that was based on the computer-generated pictures in Figure 7.2. The addition of a nonlinear term to a linear system (in this example, a cubic nonlinearity) does not change the stability of the equilibrium point (a sink in this case) or the spiraling nature of the trajectories (suggested by the complex eigenvalues).

Linearization

119

The linear and nonlinear trajectories and the tx-components shown in Figure 7.2 look pretty much alike. This is often the case for a system

ibe p°mt x0is an and its linearization

equilibrium point of X = F(x) if F(xo) = 0.

X7 = F(x) x' = A(x - xo)

(6)

(7)

at an equilibrium point x0. Let’s assume that the dependent vector variable

x has n components x1,

. Xn

that F1 (x),... , Fn (x) are the components of

3Fi 3Fi

dx\ dxn

3Fn 3Fn

_ dxi dxn_

Here’s why linearization is so widely used.

F(x), and that these components are at least twice continuously differentiable functions. Then the n x n constant matrix A in system (7) is the matrix of the first partial derivatives of the components of F (x) with respect to the components of x, all evaluated at x0:

A

A is called the Jacobian matrix of F at x0, and is often denoted by J or J(x0). As an example, look back at system (1) and its linearization, system (2) or system (5).

It is known that if none of the eigenvalues of the Jacobian matrix at an equilibrium point is zero or pure imaginary, then close to the equilibrium point the trajectories and component curves of systems (6) and (7) look alike. We can use ODE Architect to find equilibrium points, calculate Jacobian matrices and their eigenvalues, and so, check out whether the eigenvalues meet the conditions just stated. If n = 2, we can apply the vocabulary of planar linear systems from Chapter 6 to nonlinear systems. We can talk about a spiral sink, a nodal source, a saddle point, etc. ODE Architect uses a solid dot for a sink, an open dot for a source, a plus sign for a center, and an open square for a saddle.

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