chapman.f

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c     subroutine chapmn                                                 
      subroutine chapmn(x,y,dchap)                                      
c
c**********************************************************************
      implicit real*8 (a-h,o-z)                                         
      real*8 dp,dz,dy,dsum,dchap,dt,dsq,x,pi,theta,ax(5)                
      data ax /1.061405429d+00,-1.453152027d+00,                        
     1         1.421413741d+00,-0.284496736d+00,                        
     2         0.254829592d+00/, dp /0.3275911d+00/                     
      pi = 3.141592654d0                                                
      theta = pi*theta/180.0d0                                          
      dy = y                                                            
      dz = dy*dsqrt(0.5d0*x)                                            
      dsq = dsqrt(0.5*pi*x)                                             
      dt = 1.0e+00/ (1.00e+00 + dp * dz)                                
      dsum = dt * ax(1)                                                 
      if(dy.ge.0.2d0)go to 220                                          
      do 210 i =2,5                                                     
 210  dsum = dt*(ax(i) + dsum)                                          
      dchap = dsum*dsq                                                  
                                                                        
      go to 150                                                         
 220  dt = 0.5d+00/(dz**2)                                              
      dchap = (1.0d+00-dt*(1.0d+00 - 3.0d+00*dt*                        
     1        (1.0d+00 - 5.0d+00*dt)))/dy                               
 150     return                                                         
      end                                                               
c***********************************************************************