# Elementary differential equations 7th edition - Boyce W.E

ISBN 0-471-31999-6

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In the preceding paragraph we have described how to draw by hand a qualitatively correct sketch of the trajectories of a system such as Eq. (5), once the eigenvalues and eigenvectors have been determined. However, to produce a detailed and accurate drawing, such as Figure 7.5.2a and other figures that appear later in this chapter, a computer is extremely helpful, if not indispensable.

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Chapter 7. Systems of First Order Linear Equations

FIGURE 7.5.2 (a) Trajectories of the system (5); the origin is a saddle point. (b) Plots of x1 versus t for the system (5).

As an alternative to Figure 7.5.2a one can also plot x1 or x2 as a function of t; some typical plots of x1 versus t are shown in Figure 7.5.2b, and those of x2 versus t are similar. For certain initial conditions it follows that c1 = 0 in Eq. (13), so that x1 = c2e-t and x1 ^ 0 as t ^ro. One such graph is shown in Figure 7.5.2b, corresponding to a trajectory that approaches the origin in Figure 7.5.2a. For most initial conditions, however, c1 = 0and x1 is given by x1 = c1e3t + c2e~t. Then the presence of the positive exponential term causes x1 to grow exponentially in magnitude as t increases. Several graphs of this type are shown in Figure 7.5.2b, corresponding to trajectories that depart from the neighborhood of the origin in Figure 7.5.2a. It is important to understand the relation between parts (a) and (b) of Figure 7.5.2 and other similar figures that appear later, since one may want to visualize solutions either in the x1 x2-plane or as functions of the independent variable t.

Consider the system EXAMPLE 7 x

2 x'=(-2 -[)x (14)

Draw a direction field for this system; then find its general solution and plot several trajectories in the phase plane.

The direction field for the system (14) in Figure 7.5.3 shows clearly that all solutions approach the origin. To find the solutions assume that x = ?ert; then we obtain the algebraic system

v/2 — 2 — r/ If?/ \°

(15)

7.5 Homogeneous Linear Systems with Constant Coefficients

377

*2 w\\\\\\\\ ' xXXXXXXXX\' 2 XXXXXXXXXX' \\\\\\\\\\' XXXXXXXXXX' 1 'S»'' | XXX | N^XX' >N\,\ » ^

-i -2 —^ / / / ( \ \ \ « 1 2 xi ^V\’sNNNNWN ^XXXXXXXXXX -VXXXXXXXxx .xxxxxxxxxx ,X XXX XXXXXX vXXXXXXXXXX -xxxxxxxxxx s XXXXXXXXXX -\ X XX XXX X XX

FIGURE 7.5.3 Direction field for the system (14).

The eigenvalues satisfy

- r)(-2 - r) - 2

= (r + 1)(r + 4) = 0, (16)

(-3 - r)(-2 - r) - 2 = r2 + 5r + 4

so r1 = -1 and r2 = —4. For r =-1, Eq. (15) becomes

-2 V2\(^i\_(0

V2 -1 / \%2/ V°

(17)

Hence ?2 = V2 ^ and the eigenvector ?(1) corresponding to the eigenvalue r1 = — 1 can be taken as

«(1) = (jy( (18)

Similarly, corresponding to the eigenvalue r2 = —4, we have f1 = —V2?2, so the eigenvector is

g (2) = (— • (19)

Thus a fundamental set of solutions of the system (14) is

iF1)^ - ( 0—t x(2)(t)_ f—V2\ e—4t

x (t) = e , x (t) = ^ e"4t, (20)

and the general solution is

x = C1x(1)(t) + C2x(2) = e-t + c2 ^-^ e-4t. (21)

Graphs of the solution (21) for several values of c1 and c2 are shown in Figure

7.5.4a. The solution x(1)(t) approaches the origin along the line x2 = V2x1, while the solution x(2)(t) approaches the origin along the line x1 = - V2x2. The directions of

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Chapter 7. Systems of First Order Linear Equations

these lines are determined by the eigenvectors ?(1) and ?(2), respectively. In general, we have a combination of these two fundamental solutions. As t ^ro, the solution x(2) (t) is negligible compared to x(1)(t). Thus, unless c1 = 0, the solution (21) approaches the origin tangent to the line x2 = \f2xr The pattern of trajectories shown in Figure

7.5.4a is typical of all second order systems x; = Ax for which the eigenvalues are real, different, and of the same sign. The origin is called a node for such a system. If the eigenvalues were positive rather than negative, then the trajectories would be similar but traversed in the outward direction. Nodes are asymptotically stable if the eigenvalues are negative and unstable if the eigenvalues are positive.

Although Figure 7.5.4a was computer-generated, a qualitatively correct sketch of the trajectories can be drawn quickly by hand, based on a knowledge of the eigenvalues and eigenvectors.

Some typical plots of x1 versus t are shown in Figure 7.5.4b. Observe that each of the graphs approaches the t-axis asymptotically as t increases, corresponding to a trajectory that approaches the origin in Figure 7.5.2a. The behavior of x2 as a function of t is similar.

FIGURE 7.5.4 (a) Trajectories of the system (14); the origin is a node. (b) Plots of x1 versus t for the system (14).

The two preceding examples illustrate the two main cases for 2 x 2 systems having eigenvalues that are real and different: Either the eigenvalues have opposite signs (Example 1) or the same sign (Example 2). The other possibility is that zero is an eigenvalue, but in this case it follows that det A = 0, which violates the assumption made at the beginning of this section.

Returning to the general system (1) we proceed as in the examples. To find solutions of the differential equation (1) we must find the eigenvalues and eigenvectors of A

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