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

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2.6 Exact Equations and Integrating Factors

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suddenly emerge in an originally quiescent fluid. In many such cases the mathematical analysis ultimately leads to an equation10 of the form

dx/dt = (R — Rc )x — ax3. (i)

Here a and Rc are positive constants, and R is a parameter that may take on various values. For example, R may measure the amount of a certain chemical and x may measure a chemical reaction.

(a) If R < Rc, show that there is only one equilibrium solution x = 0 and that it is asymptotically stable.

(b) If R > Rc, show that there are three equilibrium solutions, x = 0 and x = ±Ó(R — Rc)/a, and that the first solution is unstable while the other two are asymptotically stable.

(c) Draw a graph in the Rx-plane showing all equilibrium solutions and label each one as asymptotically stable or unstable.

The point R = Rc is called a bifurcation point. For R < Rc one observes the asymptotically stable equilibrium solution x = 0. However, this solution loses its stability as R passes through the value Rc, and for R > Rc the asymptotically stable (and hence the observable) solutions are x = ^ (R — Rc)/a and x = ——/(R — Rc)/a. Because of the way in which the solutions branch at Rc, this type of bifurcation is called a pitchfork bifurcation; your sketch should suggest that this name is appropriate.

26. Chemical Reactions. A second order chemical reaction involves the interaction (collision) of one molecule of a substance P with one molecule of a substance Q to produce one molecule of a new substance X; this is denoted by P + Q ^ X. Suppose that p and q, where p = q, are the initial concentrations of P and Q, respectively, and let x(t) be the concentration of Xat time t. Then p — x(t) and q — x(t) are the concentrations of P and Q at time t, and the rate at which the reaction occurs is given by the equation

dx/dt = a( p — x)(q — x), (i)

where a is a positive constant.

(a) If x(0) = 0, determine the limiting value of x(t) as t ^æ without solving the differential equation. Then solve the initial value problem and find x(t) for any t.

(b) If the substances P and Q are the same, then p = q and Eq. (i) is replaced by

dx/dt = a( p — x)2. (ii)

If x(0) = 0, determine the limiting value of x(t) as t ^æ without solving the differential equation. Then solve the initial value problem and determine x(t) for any t.

2.6 Exact Equations and Integrating Factors

For first order equations there are a number of integration methods that are applicable to various classes of problems. The most important of these are linear equations and separable equations, which we have discussed previously. Here, we consider a class of

10 In fluid mechanics Eq. (i) arises in the study of the transition from laminar to turbulent flow; there it is often called the Landau equation. L. D. Landau (1908-1968) was a Russian physicist who received the Nobel prize in 1962 for his contributions to the understanding of condensed states, particularly liquid helium. He was also the coauthor, with E. M. Lifschitz, of a well-known series of physics textbooks.

90

Chapter 2. First Order Differential Equations

EXAMPLE

1

equations known as exact equations for which there is also a well-defined method of solution. Keep in mind, however, that those first order equations that can be solved by elementary integration methods are rather special; most first order equations cannot be solved in this way.

Solve the differential equation

2x + y2 + 2xyj = 0. (1)

The equation is neither linear nor separable, so the methods suitable for those types

of equations are not applicable here. However, observe that the function —(x, y) =

x2 + xy2 has the property that

2 d — d —

2x + Ó2 = 2xy = —. (2)

d x dy

Therefore the differential equation can be written as

+ d-dy = 0. (3)

d x d y dx

Assuming that y is a function of x and calling upon the chain rule, we can write Eq. (3) in the equivalent form

dx = dL (x2+xy2) = 0. (4)

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