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/* jn.c
*
* Bessel function of integer order
*
*
*
* SYNOPSIS:
*
* int n;
* double x, y, jn();
*
* y = jn( n, x );
*
*
*
* DESCRIPTION:
*
* Returns Bessel function of order n, where n is a
* (possibly negative) integer.
*
* The ratio of jn(x) to j0(x) is computed by backward
* recurrence. First the ratio jn/jn-1 is found by a
* continued fraction expansion. Then the recurrence
* relating successive orders is applied until j0 or j1 is
* reached.
*
* If n = 0 or 1 the routine for j0 or j1 is called
* directly.
*
*
*
* ACCURACY:
*
* Absolute error:
* arithmetic range # trials peak rms
* DEC 0, 30 5500 6.9e-17 9.3e-18
* IEEE 0, 30 5000 4.4e-16 7.9e-17
*
*
* Not suitable for large n or x. Use jv() instead.
*
*/
/* jn.c
Cephes Math Library Release 2.8: June, 2000
Copyright 1984, 1987, 2000 by Stephen L. Moshier
*/
#include <math.h>
#ifdef ANSIPROT
extern double fabs ( double );
extern double j0 ( double );
extern double j1 ( double );
#else
double fabs(), j0(), j1();
#endif
extern double MACHEP;
double jn( n, x )
int n;
double x;
{
double pkm2, pkm1, pk, xk, r, ans;
int k, sign;
if( n < 0 )
{
n = -n;
if( (n & 1) == 0 ) /* -1**n */
sign = 1;
else
sign = -1;
}
else
sign = 1;
if( x < 0.0 )
{
if( n & 1 )
sign = -sign;
x = -x;
}
if( n == 0 )
return( sign * j0(x) );
if( n == 1 )
return( sign * j1(x) );
if( n == 2 )
return( sign * (2.0 * j1(x) / x - j0(x)) );
if( x < MACHEP )
return( 0.0 );
/* continued fraction */
#ifdef DEC
k = 56;
#else
k = 53;
#endif
pk = 2 * (n + k);
ans = pk;
xk = x * x;
do
{
pk -= 2.0;
ans = pk - (xk/ans);
}
while( --k > 0 );
ans = x/ans;
/* backward recurrence */
pk = 1.0;
pkm1 = 1.0/ans;
k = n-1;
r = 2 * k;
do
{
pkm2 = (pkm1 * r - pk * x) / x;
pk = pkm1;
pkm1 = pkm2;
r -= 2.0;
}
while( --k > 0 );
if( fabs(pk) > fabs(pkm1) )
ans = j1(x)/pk;
else
ans = j0(x)/pkm1;
return( sign * ans );
}
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