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Differential Equations Brannan Copyright © 2010 by John Wiley & Sons, Inc. All rights reserved. Chapter 08: Series Solutions of Second Order Linear Equations

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Chapter 8 Chapter 8 Series Solutions of Second Order Linear Equations The general solution of a linear second order equation P(x)y'' + Q(x)y' + R(x)y = 0 is y = c 1 y 1 (x) + c 2 y 2 (x), where y 1 and y 2 are a fundamental set of solutions of the differential equation. To deal with equations that have general nonconstant coefficients, we need alternative solution techniques. For some applications we may find that approximations using an initial value problem solver are satisfactory for our needs. However there are some variable coefficient equations that frequently recur in applications where infinite series representation of solutions useful.

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Chapter 8 - Chapter 8 - Series Solutions of Second Order Linear Equations 8.1 Review of Power Series 8.2 Series Solutions Near an Ordinary Point, Part I 8.3 Series Solutions Near an Ordinary Point, Part II 8.4 Regular Singular Points 8.5 Series Solutions Near a Regular Singular Point, Part I 8.6 Series Solutions Near a Regular Singular Point, Part II 8.7 Bessel’s Equation

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8.1 Review of Power Series DEFINITION A power series is an infinite series of the form The constants a 0, a 1, a 2,... are called the coefficients of the series, the constant x 0 is called the center of the series, and x is a variable. Setting x 0 = 0 in Eq. (1) gives us a power series centered at x0 = 0:

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Convergence Concepts DEFINITION A power series is said to converge at a point x if the sequence of partial sums converges as m →∞. The sum of the series at the point x is defined to be the limit of the sequence of partial sums, and we write If the limit of the sequence of partial sums does not exist, then the series is said to diverge at x.

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Example DEFINITION 8.1.3

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Theorems THEOREM (absolute convergence implies convergence) THEOREM (The Ratio Test)

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Radius of Convergence. If is a power series, then either 1. The series converges absolutely for all x, or 2. The series converges only for x = x 0, or 3. There exists a number ρ > 0 such that converges absolutely for | x − x 0 | < ρ and diverges for | x − x 0 | > ρ. The number ρ in Case 3 is called the radius of convergence and the interval | x − x 0 | < ρ is called the interval of convergence. This case is illustrated by the shaded region in Figure We write ρ =∞ in Case 1 and ρ = 0 in Case 2. Using these conventions, we can state that each power series has a radius of convergence ρ, where 0 ≤ ρ ≤∞. If 0 < ρ < ∞, the series may either converge or diverge when |x− x 0 | = ρ.

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FIGURE The interval of convergence of a power series if 0 < ρ < ∞.

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Example Question Answer The given power series converges for −3 ≤ x < 1 and diverges otherwise. It converges absolutely for −3 < x < 1 and has a radius of convergence 2.

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Algebraic Operations on Power Series

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THEOREM 8.1.6

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Taylor Series and Analytic Functions DEFINITION Let f be a function with derivatives of all orders throughout the interval |x − x 0 | 0. Then the Taylor series of f at x 0 is the power series DEFINITION A function f that has a power series expansion of the form with a radius of convergence ρ > 0 is said to be analytic at x0.

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Shift of Index of Summation Shifting the index of summation in a power series is analogous to changing the variable of integration in an integral. This operation, in conjunction with Theorem 8.1.6, is a useful tool for computing power series solutions of differential equations. EXAMPLE

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8.2 Series Solutions Near an Ordinary Point, Part I It is sufficient to consider the homogeneous equation (1) Examples

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Ordinary and Singular Points DEFINITION A point x 0 is said to be an ordinary point of Eq. (1) if the coefficients P, Q, and R are analytic at x 0, and P(x 0 ) = 0. If x 0 is not an ordinary point, it is called a singular point of the equation. If x 0 is an ordinary point, then we can divide Eq. (1) by P(x) to obtain where p(x) = Q(x)/P(x) and q(x) = R(x)/P(x) are analytic at x 0, and therefore continuous in an interval around x 0.

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Examples We now take up the problem of solving Eq. (1) in the neighborhood of an ordinary point x 0. We look for solutions of the form Example 1. Answer y =

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Example - Airy Equation. The general solution of Airy’s equation is

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The Airy Functions Ai(x) and Bi(x) Since Airy’s equation is linear, we can form other fundamental sets of solutions by taking pairs of linear combinations of y 1 and y 2 in above eq., provided that the resultant pairs of functions are linearly independent. Using the gamma function, denoted by Γ(p) and defined by the integral

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The Airy Functions Ai(x) and Bi(x) Asymptotic analysis yields the following approximations to Ai(x) and Bi(x) for large values of |x|:

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(a) The graphs of Ai(x) and the asymptotic approximations for values of |x| ≥ 0.2. (b) The graphs of Bi(x) and the asymptotic approximations for values of |x| ≥ 0.2.

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8.3 Series Solutions Near an Ordinary Point, Part II In the preceding section, we considered the problem of finding solutions of (1) where P, Q, and R are polynomials, in the neighborhood of an ordinary point x0. Assuming that Eq. (1) does have a solution y = φ(x) and that φ has a Taylor series (2) which converges for |x − x0| 0, we found that the an can be determined by directly substituting the series (2) for y in Eq. (1). Let us now consider how we might justify the statement that if x0 is an ordinary point of Eq. (1), then there exist solutions of the form (2). We also consider the question of the radius of convergence of such a series.

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General Solutions in Neighborhoods of Ordinary Points THEOREM 8.3.1

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Examples 1. What is the radius of convergence of the Taylor series for (1 + x 2 ) −1 about x = 0? 2. Determine a lower bound for the radius of convergence of series solutions of the differential equation (1 + x 2 )y'' + 2xy' + 4x 2 y = 0 about the point x = 0 and about the point x = −1/2.

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8.4 Regular Singular Points We consider the nonconstant coefficient equation P(x)y'' + Q(x)y' + R(x)y = 0, (1) where x 0 is a singular point. This means that P(x 0 ) = 0 and at least one of Q and R is not zero at x 0. Power series method to solve in the neighborhood of a singular point x 0, fails. We need to use a more general type of series expansion to solve.

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Cauchy–Euler Equations A relatively simple differential equation that has a singular point is the Cauchy–Euler equation, L[y] = x 2 y'' + αxy' + βy = 0, (2) where α and β are real constants. If we assume that Eq. (2) has a solution of the form y = x r, (3) then we obtain L[x r ] = x 2 (x r ) + αx(x r ) + β x r = x r [r (r − 1) + αr + β]. (4) If r is a root of the quadratic equation F(r ) = r (r − 1) + αr + β = 0, (5) then L[x r ] is zero, and y = x r is a solution of Eq. (2).

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The roots of Eq. (5) The roots of Eq. (5) are and F(r) = (r − r 1 )(r − r 2 ). Three possible cases: 1. Real, Distinct Roots. 2. Equal Roots. 3. Complex Roots.

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1. Real, Distinct Roots. If F(r) = 0 has real roots r 1 and r 2, with r 1 ≠ r 2, then y 1 (x) = x r1 and y 2 (x) = x r2 are solutions of Eq. (2). Since W(x r1, x r2 ) = (r 2 − r 1 )x r1+r2−1 is nonvanishing for r 1 ≠ r 2 and x > 0, it follows that the general solution of Eq. (2) is y = c 1 x r1 + c 2 x r2, x > 0. (7) Note that if r is not a rational number, then xr is defined by x r = e r ln x. Example: Solve

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2. Equal Roots. If the roots r 1 and r 2 are equal, then we obtain one solution as y 1 (x) = x r1. A second solution of Eq. (2) is y 2 (x) = x r1 ln x, x > 0 By evaluating the Wronskian, we find that W(x r1, x r1 ln x) = x 2r1−1. Hence x r1 and x r1 ln x are a fundamental set of solutions for x > 0, and the general solution of Eq. (2) is y = (c 1 + c 2 ln x)x r1, x > 0. Example: Solve

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3. Complex Roots. Finally, suppose that the roots r 1 and r 2 of Eq. (5) are complex conjugates, say, r 1 = μ + iν and r 2 = μ − iν, with ν = 0. The general solution of Eq. (2) in this case is y = c 1 x μ cos (ν ln x) + c 2 x μ sin (ν ln x), x > 0. Example: Solve

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Solutions of Euler Equation – Real Roots

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Solutions of an Euler equation; complex roots with a negative real part.

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Solutions of an Euler equation; complex roots with a positive real part.

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Solutions of an Euler equation; repeated roots.

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Table 8.4.1

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DEFINITION Regular and Irregular Singular Points EXAMPLE

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