/* From: "COMPUTATIONAL PHYSICS, 2nd Ed" 
   by RH Landau, MJ Paez, and CC Bordeianu 
   Copyright Wiley-VCH, 2007.
   Electronic Materials copyright: R Landau, Oregon State Univ, 2007;
   MJ Paez, Univ Antioquia, 2007; and CC Bordeianu, Univ Bucharest, 2007.
   Support by National Science Foundation                              
*/
// Bessel.java: Spherical Bessels via up and down recursion   

import java.io.*;   
  
public class bessel {
	                                          // Global class variables
	public static double xmax = 40., xmin = 0.25, step = 0.1;   
  public static int order = 10, start = 50;      
    
  public static void main(String[] argv)
                         throws IOException, FileNotFoundException {
													 
   double x; 
	 
   PrintWriter w = 
        new PrintWriter(new FileOutputStream("Bessel.dat"), true); 
				                                        // Step thru x values
   for ( x = xmin;  x <= xmax;  x += step )
        w.println(" " +x+"  "+down(x, order, start) );    
    System.out.println("data stored in Bessel.dat"); 
   }                                                      // End main
    
  public static double down (double x, int n, int m) {  // Recur down
   double scale; 
   double j[] = new double[start + 2];        
   int k; 
	                                             // Start with anything
   j[m + 1] = j[m] = 1.;                      
   for ( k = m;  k>0 ;  k--)   j[k-1] = ((2.*k+1.)/x)*j[k] - j[k+1];
	                                    // Scale solution to known j[0]
   scale = (Math.sin(x)/x)/j[0];                           
   return j[n] * scale; 
 }
}