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

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If each of the functions F1,..., Fn in Eqs. (12) is a linear function of the dependent variables x1,..., xn, then the system of equations is said to be linear; otherwise, it is nonlinear. Thus the most general system of n first order linear equations has the form

x1 = Pn(t)x1 + ¦¦¦ + P1n (t)xn + gl(t),

x2 = P21(t)x1 + ¦¦¦ + P2n (t)xn + g2(t), (15)

^n = Pn1(t)x1 + ¦¦¦ + Pnn(t)xn + gn(t).

If each of the functions g1(t),..., gn (t) is zero for all t in the interval I, then the system (15) is said to be homogeneous; otherwise, it is nonhomogeneous. Observe that the systems (1) and (2) are both linear, but the system (3) is nonlinear. The system (1) is nonhomogeneous unless F1(t) = F2(t) = 0, while the system (2) is homogeneous. For the linear system (15) the existence and uniqueness theorem is simpler and also has a stronger conclusion. It is analogous to Theorems 2.4.1 and 3.2.1.

Theorem 7.1.2 If the functions P11, P12,..., Pnn, g1,..., gn are continuous on an open interval

------- I: a < t < â, then there exists a unique solution x1 = ô1^),..., xn = ôï (t) ofthe

system (15) that also satisfies the initial conditions (14), where t0 is any point in I and x°,..., x° are any prescribed numbers. Moreover, the solution exists throughout the interval I.

Note that in contrast to the situation for a nonlinear system the existence and uniqueness of the solution of a linear system are guaranteed throughout the interval in which the hypotheses are satisfied. Furthermore, for a linear system the initial values x°,..., x° at t = t0 are completely arbitrary, whereas in the nonlinear case the initial point must lie in the region R defined in Theorem 7.1.1.

The rest of this chapter is devoted to systems of linear first order equations (nonlinear systems are included in the discussion in Chapters 8 and 9). Our presentation makes use of matrix notation and assumes that you have some familiarity with the properties of matrices. The basic facts about matrices are summarized in Sections 7.2 and 7.3 and some more advanced material is reviewed as needed in later sections.

344

Chapter 7. Systems ofFirst Order Linear Equations

PROBLEMS

In each of Problems 1 through 4 transform the given equation into a system of first order equations.

1.

3.

5.

6.

7.

u + 0.5u + 2u = 0

2. u + 0.5u + 2u = 3 sin t t2u" + tu'+ (t2 - 0.25)u = 0 4. u"" - u = 0

Consider the initial value problem u" + p(t)u' + q(t)u = g(t), u(0) = u0, u'(0) = u'0.

Transform this problem into an initial value problem for two first order equations.

Reduce the system (1) to a system of first order equations of the form (12).

Systems of first order equations can sometimes be transformed into a single equation of higher order. Consider the system

x1 = -2x1 + ,

x2 = x1 - 2x2.

(a) Solve the first equation for x2 and substitute into the second equation, thereby obtaining a second order equation for x1. Solve this equation for x1 and then determine x2 also.

(b) Find the solution of the given system that also satisfies the initial conditions x1 (0) = 2,

x2(0) = 3.

(c) Sketch the curve, for t > 0, given parametrically by the expressions for x1 and x2 obtained in part (b).

In each of Problems 8 through 12 proceed as in Problem 7 to transform the given system into a single equation of second order. Then find x1 and x2 that also satisfy the given initial conditions. Finally, sketch the graph of the solution in the x1 x2-plane for t > 0.

10.

x1 = 3x1 - 2x2, x2 = 2x1 - 2x2, x1 = x1 - 2x2, x2 = 3x1 - 4x2,

x1(0) = 3 x2(0) = 2 x1(0) = -1 x2(0) = 2

9. x1 = 1.25x1 + 0.75x2, x1(0) = -2

x2 = 0.75x1 + 1.25x2, x2(0) = 1

11. x1 = 2x2, x1(0) = 3

x2 = — 2x1, x2(0) = 4

12. / = —0.5x1 + 2x2, x1(0) = —2

x2 = —2x1 - 0.5x2, x2(0) = 2

13.

14.

Transform Eqs. (2) for the parallel circuit into a single second order equation. Show that if a11, a12, a21, and a22 are constants with a12 and a21 not both zer< functions g1 and g2 are differentiable, then the initial value problem

x1 = a11 x1 + a12 x2 + g1 (t), x1 (0) = x0

x2 = a21 x1 + a22 x2 + g2(t), x2(0) =

15.

16.

can be transformed into an initial value problem for a single second order equation. Can the same procedure be carried out if a11,..., a22 are functions of t?

Consider the linear homogeneous system

x = Pu(t)x + P12(t)Ó,

Ó = P21(t)x + P22 (t)y.

Show that if x = x1 (t), y = y1 (t) and x = x2(t), y = y2(t) are two solutions of the given system, then x = c1 x1 (t) + c2x2(t), y = c1 y1 (t) + c2y2(t) is also a solution for any constants c1 and c2. This is the principle of superposition.

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