Bessel in AP?

Calculus Level 5

If

x ( 2 J 2 ( x ) 4 J 4 ( x ) + 6 J 6 ( x ) ) d x = x A B J k ( x ) + c \large \int x(2J_{2}(x)-4J_{4}(x)+6J_{6}(x)-\ldots) \ dx=\frac{x^{A}}{B}J_{k}(x)+c

where A , B , k A,B,k are integers and c c is a constant of integration, find A B k A^{B^{k}} .


J n J_{n} are the Bessel functions of the first kind .


The answer is 16.

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1 solution

Tanishq Varshney
Nov 17, 2015

2 n J n = x ( J n 1 + J n + 1 ) 2n J_{n}=x(J_{n-1}+J_{n+1})

x 2 J n 1 = n J n x 2 J n + 1 ( 1 ) \large{\frac{x}{2} J_{n-1}=n J_{n}-\frac{x}{2} J_{n+1}}\quad \quad \quad (1)

n n + 2 n \rightarrow n+2 in ( 1 ) (1)

x 2 J n + 1 = ( n + 2 ) J n + 2 x 2 J n + 3 \large{\frac{x}{2} J_{n+1}=(n+2) J_{n+2}-\frac{x}{2} J_{n+3}}

Put the value of x 2 J n + 1 \large{\frac{x}{2} J_{n+1}} in ( 1 ) (1)

We get x 2 J n 1 = n J n ( n + 2 ) J n + 2 + x 2 J n + 3 ( 2 ) \large{\frac{x}{2} J_{n-1}=n J_{n}-(n+2)J_{n+2}+\frac{x}{2} J_{n+3}}\quad \quad \quad (2)

Now n n + 4 n \rightarrow n+4 in ( 1 ) (1)

Find value of x 2 J n + 3 \large{\frac{x}{2} J_{n+3}} and place it in ( 2 ) (2)

On repeating these steps we finally get

x 2 J n 1 = n J n ( n + 2 ) J n + 2 + ( n + 4 ) J n + 4 . . . . . . . . \large{\frac{x}{2} J_{n-1}=n J_{n}-(n+2) J_{n+2}+(n+4) J_{n+4}-........}

In this case n = 2 n=2

Thus we are left with

1 2 x 2 J 1 d x = x 2 2 J 2 + c \large{\frac{1}{2} \int x^2 J_{1} dx=\frac{x^2}{2}J_{2}+c}

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