General Solution. Linear Dependence

For a general solution of Bessel’s equation (1) in addition to J we need a second linearly independent solution. For not an integer this is easy. Replacing by

in (20), we have

Since Bessel’s equation involves 2 , the functions J and J are solutions of the equation for the same . If is not an integer, they are linearly independent, because the first terms in (20) and in (24) are finite nonzero multiples of x and x . Thus, if is not an integer,

a general solution of Bessel’s equation for all

2 J (x)

This cannot be the general solution for an integer n because, in that case, we have linear dependence. It can be seen that the first terms in (20) and (24) are finite nonzero multiples of x and x , respectively. This means that, for any integer

n , we have linear dependence because

J n (x)

SEC. 5.4 Bessel’s Equation. Bessel Functions J (x)

PROOF To prove (25), we use (24) and let approach a positive integer n. Then the gamma function in the coefficients of the first n terms becomes infinite (see Fig. 553 in App. A3.1), the coefficients become zero, and the summation starts with

. Since in this case

by (18), we obtain

The last series represents

n (x) , as you can see from (11) with m replaced by s. This completes the proof.

䊏 The difficulty caused by (25) will be overcome in the next section by introducing further

Bessel functions, called of the second kind and denoted by . Y

PROBLEM SET 5.4

1. Convergence. Show that the series (11) converges for (b) Experiment with (14) for integer n. Using graphs, all x. Why is the convergence very rapid?

find out from which

n on the curves of (11) and (14) practically coincide. How does x n change

2–10 ODE S REDUCIBLE TO BESSEL’S ODE

with n?

This is just a sample of such ODEs; some more follow in

2 ? (Our usual notation the next problem set. Find a general solution in terms of J

in this case would be .)

and J or indicate when this is not possible. Use the indicated substitutions. Show the details of your work.

(d) How does the error of (14) behave as a func- 2 4 tion of x for fixed n? [Error

2. x 2 y s xy r

exact value minus

(e) Show from the graphs that J 0 (x) has extrema where s

J 1 . Which formula proves this? Find further

5. Two-parameter ODE

relations between zeros and extrema. x 2 y 2 2 s 2 xy r x

of Bessel functions play a key role in 7. 2 1

6. 2 1 x 3 y s 4 4 13–15

ZEROS

x y s xy r 4 (x 2 modeling (e.g. of vibrations; see Sec. 12.9).

8. 2 y s

13. Interlacing of zeros. Using (21) and Rolle’s theorem, show that between any two consecutive positive zeros

9. xy s r

of J n (x) there is precisely one zero of J (x) . x y s

14. Zeros. Compute the first four positive zeros of J 0 (x) u, x

z) and J 1 (x) from (14). Determine the error and comment. 11. CAS EXPERIMENT. Change of Coefficient. Find

15. Interlacing of zeros. Using (21) and Rolle’s theorem, and graph (on common axes) the solutions of show that between any two consecutive zeros of J 0 (x) ⴚ y 1 kx y s r

there is precisely one zero of J 1 (x) . for

(or as far as you get useful

graphs). For what k do you get elementary functions?

HALF-INTEGER PARAMETER: APPROACH

Why? Try for noninteger k, particularly between 0 and 2,

BY THE ODE

to see the continuous change of the curve. Describe the 16. Elimination of first derivative. Show that change of the location of the zeros and of the extrema as

2 兰 p(x) dx) gives from the ODE k increases from 0. Can you interpret the ODE as a model

with

the ODE in mechanics, thereby explaining your observations?

s p(x)y r

2 p r (x) (a) Graph J n (x) for

1 2 s 1 4 p(x)

12. CAS EXPERIMENT. Bessel Functions for Large x.

on common axes.

not containing the first derivative of u.

CHAP. 5 Series Solutions of ODEs. Special Functions 17. Bessel’s equation. Show that for (1) the substitution ⴚ

21. Basic integral formula. Show that

in Prob. 16 is y ⫽ ux 1 >2 and gives

x ␯ J ␯ⴚ 1 (x) dx ⫽ x J ␯ (x) ⫹ c.

4 22. Basic integral formulas. Show that 18. Elementary Bessel functions. Derive (22) in Example 3

␯⫹ 1 (x) dx ⫽ ⫺x J ␯ (x) ⫹ c, from (27).

J ␯⫹ 1 (x) dx ⫽ J ␯ⴚ (x) dx ⫺ 2J ␯ (x).

19–25 APPLICATION OF (21): DERIVATIVES,

2 INTEGRALS 2 23. Integration. Show that 兰x J 0 (x) dx ⫽ x J 1 (x) ⫹ Use the powerful formulas (21) to do Probs. 19–25. Show

xJ 0 (x) ⫺ 兰J 0 (x) dx. (The last integral is nonelemen- the details of your work.

tary; tables exist, e.g., in Ref. [A13] in App. 1.)

24. Integration. Evaluate 兰x J 4 (x) dx . J 1

19. Derivatives. Show that J r 0 (x) ⫽ ⫺J 1 (x), J 1 r

25. Integration. Evaluate 兰J 5 (x) dx . 20. Bessel’s equation. Derive (1) from (21).

0 1 (x) >x, J r 2 2 [J 1 3 (x)].

General Solution. Linear Dependence