y + p(t)y = g(t) for each t I, and that also satisfies the initial condition y(t 0 ) = y 0 where y 0 is an arbitrary prescribed initial value.

Transcription

1 p1 Differences Between Linear and Nonlinear Equation Theorem 1: If the function p and g are continuous on an open interval I : α < t < β containing the point t = t 0, then there exists a unique function y = φ(t) that satisfies the differential equation y + p(t)y = g(t) for each t I, and that also satisfies the initial condition y(t 0 ) = y 0 where y 0 is an arbitrary prescribed initial value.

2 p2 Differences Between Linear and Nonlinear Equation Theorem 2:(Containing nonlinear differential equation) Let the functions f and f y be continuous in some rectangle α < t < β, γ < y < δ containing the point (t 0, y 0 ). Then, in some interval t 0 h < t < t 0 + h contained in α < t < β, there is a unique solution y = φ(t) of the initial value problem y = f(t, y), y(t 0 ) = y 0

3 p3 Example. Use Theorem 1 to find an interval in which the initial value problem ty + 2y = 4t 2, y(1) = 2 has a unique solution. Sol. Rewriting ty + 2y = 4t 2 in the form y + 2 t y = 4t. Thus, p(t) = 2 t is continuous only for t < 0 or t > 0, and g(t) = 4t is continuous for all t. Since t 0 = 1 (0, ); consequently, the initial value problem ty + 2y = 4t 2, y(1) = 2

4 p4: continuation p3 has a unique solution on the interval 0 < t < since Quiz: ty + 2y = 4t 2 y + 2 y = 4t, for t > 0. t 1. Determine(without solving the problem) an interval in which the solution of the initial value problem is certain to exist. (4 t 2 )y + 2ty = 3t 2, y(1) = 3

5 p5 Example. Use Theorem 2 to find an interval in which the initial value problem dy dx = 3x2 + 4x + 2, y(0) = 1 2(y 1) has a unique solution. Sol. f(x, y) = 3x2 +4x+2 2(y 1) and f y (x, y) = 3x2 +4x+2. Thus each of 2(y 1) 2 f and f y functions are continuous everywhere except on the line y = 1.

6 p6: continuation p5 Consequently, the initial value problem dy dx = 3x2 + 4x + 2, y(0) = 1 2(y 1) has a unique solution in some interval about x = 0. Note: Though f and f y are continuous on (, ) (, 1) = {(x, y) < x <, < y < 1} containing a point (0, 1). This does not necessarily mean that the solution exists for all x. Exercise. Find the unique solution, and verify that the initial value problem has a unique solution only for x > 2.

7 p7 Example. Consider the initial value problem y = y 1 3, y(0) = 0 for t 0. Apply Theorem 2 to this initial value problem and then solve the problem. Sol. f(x, y) = y 1/3 and f y = 1 3 y 2/3 = 1. Thus f is 3y 2/3 continuous everywhere, f y does not exist when y = 0 and not continuous at y = 0. Hence there is not a rectangle R containing (0, 0) such that f and f y are continuous on R. So Theorem 2 does not apply to this problem.

8 p8: continuation p7 y = y 1/3 is rewritten as y 1/3 dy dt = 1 (separable) y 1/3 dy = 1 dt 3 2 y2/3 = t + C and y = [ ] 2 3/2 [ 2 (t + C) or y = 3 (t + C) 3 ] 3/2 t = 0 y = so y = φ 1 (t) = ( 2 3 t)3/2, the function y = φ 2 (t) = ( 2 3 t)3/2, initial value problem. [ ] 2 3/2 (0 + C) = 0 C = 0 3 t 0 is a solution. On the other hand, t 0 is also a solution of the

9 p9: continuation p8 Moreover, y = ψ(t) = 0, t 0 is yet another solution. For an arbitrary positive t 0, the function { 0, if 0 t < t0 y = X(t) = ± [ 2 3 (t t 0) ] 3/2, if t t0 are continuous, differentiable, and are solutions of the initial value problem y = y 1 3, y(0) = 0. Hence this problem has an infinite family of solutions.

10 p10: continuation p9 Remark. The continuity of f does assure the existence of solutions, but not their uniqueness.

11 p11 Quiz 2. Solve the initial value problem y = t 2 y(1 + t 3 ), y(0) = y 0 and determine how the interval in which the solution exists depends on the initial value y 0.

12 p12 Compare the first linear equation y + p(t)y = g(t) with nonlinear differential equation, such as y = y 2 : Interval of Definition Remark. The solution of a linear equation y + p(t)y = g(t) subject to the initial condition y(t 0 ) = y 0, exists throughout any interval about t = t 0 in which the function p and g are continuous. Thus, vertical asymptotes or other discontinuous in the solution can occur only at points of discontinuity of p or g.

13 p13 On the other hand, for a nonlinear initial value problem satisfying the hypotheses of Theorem 2, the interval in which a solution exists may be difficult to determine. Example. Solve the initial value problem y = y 2, y(0) = 1 and determine the interval in which the solution exists. Sol. f(t, y) = y 2 and f y (t, y) = 2y are continuous everywhere. So, by Theorem 2, there exists a h > 0 such that the initial value problem has a unique solution in interval for t in (0 h, 0 + h).

14 p14: continuation p13 and y = y 2 dy dt = y2 y 2 dy = dt y 2 dy = t = 0 y = 1 0+C (separable) dt y 1 = t + C y = 1 t + C = 1 C = 1, thus y = 1 1 t is the solution of the given initial value problem. Clearly, 1 lim t 1 1 t =, the solution exists only in the interval < t < 1. However, there is no indication from the differential equation itself.

15 p15 General Solution Example. y = 1 t+c is solutions of the nonlinear equation y = y 2 for arbitrary number C. But y(t) = 0 is also a solution, implies y = 1 t+c is not general solution since there is no a vlue of C such that y(t) = 1 t+c = 0 for all t. Implicit Solution The situation for nonlinear equations is much less satisfactory( 令人滿意的 ). Usually, the best that we can hope for is to find an equation F (t, y) = 0 involving t and y that is satisfied by the solution y = φ(t).

16 p16 Graphical or Numerical Construction of Integral Curves Because of the difficulty in obtaining exact analytical solutions of nonlinear differential equations, methods that yield approximate solutions or other qualitative information about solutions are of correspondingly greater importance. Summary. The linear equation y + p(t)y = g(t) has several nice properties: 1. Assuming that the coefficients are continuous, there is a general solution, containing an arbitrary constant, that includes all solutions of the differential equation. A particular solution that satisfies a given initial condition can be picked out by choosing the proper value for the arbitrary constant.

17 p17 u(t)g(t)dt u(t) 2. There is an expression for the solution, y = or [ ] y = 1 t u(t) t 0 u(s)g(s)ds + c where u(t) = e p(t)dt. Moreover, although it involves two integrations, the expression is an explicit one for the solution y = φ(t) rather than an equation that defines φ implicitly. 3. The possible points of discontinuity, or singularities, of the solution can be identified (without solving the problem) merely by finding the points of discontinuity of the coefficients. Thus, if the coefficients are continuous for all t, then the soloution also exists and is continuous for all t. None of these statements is true, in general, of nonlinear equations.

18 p18 Exercise. 1. It is possible to solve a nonlinear equation by making a change of the dependent variable that converts it into a linear equation. The most important such equations has the form y + p(t)y = q(t)y n and is called a Bernoulli equation after Jakob Bernoulli. (a) Solve Bernoulli s equation when n = 0, when n = 1 (b) Show that if n 0, 1, then the substitution v = y n 1 reduces Bernoulli s equation to a linear equation. (Note: This method of solution was found by Leibniz in 1696 )

19 p19: continuation p18 2. Solve the following equations by using the substitution (a) y = ry ky 2, r > 0 and k > 0 (This equation is important in population dynamics) Ans: r y = k+cre rt (b) y = ɛy σy 3, ɛ > 0 and σ > 0. (This equation occurs in the study of the stability of fluid flow) [ Ans: y = ± ] 1/2 ɛ σ+cɛe 2ɛt