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

ISBN 0-471-31999-6

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What is the radius of convergence of the Taylor series for (x2 2x + 2)-1 about x = 0? About x = 1?

First notice that

x2 2x + 2 = 0

has solutions x = 1 ± i. The distance from x = 0 to either x = 1 + i or x = 1 i in the complex plane is V2; hence the radius of convergence of the Taylor series expansion

?

Y anxn about x = 0 is V2.

n=0

The distance from x = 1 to either x = 1 + i or x = 1 i is 1; hence the radius of

?

convergence of the Taylor series expansion Y bn (x 1)n about x = 1 is 1.

n=0

6Immanuel Lazarus Fuchs (1833-1902) was a student and later a professor at the University of Berlin. He proved the result of Theorem 5.3.1 in 1866. His most important research was on singular points of linear differential equations. He recognized the significance of regular singular points (Section 5.4), and equations whose only singularities, including the point at infinity, are regular singular points are known as Fuchsian equations.

252

Chapter 5. Series Solutions of Second Order Linear Equations

EXAMPLE

3

According to Theorem 5.3.1 the series solutions of the Airy equation in Examples 2 and 3 of the preceding section converge for all values of x and x 1, respectively, since in each problem P (x) = 1 and hence is never zero.

A series solution may converge for a wider range of x than indicated by Theorem

5.3.1, so the theorem actually gives only a lower bound on the radius of convergence of the series solution. This is illustrated by the Legendre polynomial solution of the Legendre equation given in the next example.

Determine a lower bound for the radius of convergence of series solutions about x = 0 for the Legendre equation

(1 x 2)y" 2xy' + a(a + 1)y = 0,

where a is a constant.

Note that P(x) = 1 x2, Q(x) = 2x, and R(x) = a(a + 1) are polynomials, and that the zeros of P, namely, x = ±1, are a distance 1 from x = 0. Hence a series

TO

solution of the form anxn converges for |x| < 1 at least, and possibly for larger

n=0 n

values of x. Indeed, it can be shown that if a is a positive integer, one of the series solutions terminates after a finite number of terms and hence converges not just for |x | < 1 but for all x. For example, if a = 1, the polynomial solution is y = x. See Problems 22 through 29 at the end of this section for a more complete discussion of the Legendre equation.

EXAMPLE

4

Determine a lower bound for the radius of convergence of series solutions of the differential equation

(1 + x 2)y" + 2xy' + 4x2 y = 0 (9)

about the point x = 0; about the point x = 1.

Again P, Q, and R are polynomials, and P has zeros at x = ±i. The distance in the

complex plane from 0 to ±i is 1, and from 2 to ±i is ^1 + | = V5/2. Hence in the

TO

first case the series anxn converges at least for |x| < 1, and in the second case the

n=0 n

TO

series bn (x + 2)n converges at least for |x + 21 < V5/2.

n=0 n 2 2

An interesting observation that we can make about Eq. (9) follows from Theorems

3.2.1 and 5.3.1. Suppose that initial conditions y(0) = y0 and y'(0) = y0 are given.

Since 1 + x2 = 0 for all x, we know from Theorem 3.2.1 that there exists a unique

solution of the initial value problem on to < x < to. On the other hand, Theorem

TO

5.3.1 only guarantees a series solution of the form anxn (with a0 = y0, a1 = y0)

n=0

for 1 < x < 1. The unique solution on the interval to < x < to may not have a power series about x = 0 that converges for all x.

I

EXAMPLE

5

Can we determine a series solution about x = 0 for the differential equation

y" + (sin x) y' + (1 + x2) y = 0, and if so, what is the radius of convergence?

5.3 Series Solutions near an Ordinary Point, Part II

253

For this differential equation, p(x) = sin x and q (x) = 1 + x2. Recall from calculus that sin x has a Taylor series expansion about x = 0 that converges for all x . Further, q also has a Taylor series expansion about x = 0, namely, q (x) = 1 + x2, that converges

for all x. Thus there is a series solution of the form y = anxn with a0 and a1

n=0

arbitrary, and the series converges for all x.

PROBLEMS In each of Problems 1 through 4 determine (x0), ft" (x0), and 0lv (x0) for the given point x0 if

I y = fy(x) is a solution of the given initial value problem.

1. y" + xy' + y = 0; y(0) = 1, y'(0) = 0

2. y" + (sinx)y' + (cos x)y = 0; y(0) = 0, y'(0) = 1

3. x2y" + (1 + x)y + 3(lnx)y = 0; y(1) = 2, y'(1) = 0

4. y" + x2y + (sinx)y = 0; y(0) = a0, y'(0) = ax

In each of Problems 5 through 8 determine a lower bound for the radius of convergence of series solutions about each given point x0 for the given differential equation.

5. y" + 4y' + 6xy = 0; x0 = 0, x0 = 4

6. (x2 2x 3)y" + xy' + 4y = 0; x0 = 4, x0 = 4, x0 = 0

7. (1 + x 3)y" + 4xy; + y = 0; x0 = 0, x0 = 2

8. xy" + y = 0; x0 = 1

9. Determine a lower bound for the radius of convergence of series solutions about the given x0 for each of the differential equations in Problems 1 through 14 of Section 5.2.

10. The Chebyshev Equation. The Chebyshev7 differential equation is

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