C @(#)gauss3d.for	17.1.1.1 (ES0-DMD) 01/25/02 17:10:48
C===========================================================================
C Copyright (C) 1995 European Southern Observatory (ESO)
C
C This program is free software; you can redistribute it and/or 
C modify it under the terms of the GNU General Public License as 
C published by the Free Software Foundation; either version 2 of 
C the License, or (at your option) any later version.
C
C This program is distributed in the hope that it will be useful,
C but WITHOUT ANY WARRANTY; without even the implied warranty of
C MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
C GNU General Public License for more details.
C
C You should have received a copy of the GNU General Public 
C License along with this program; if not, write to the Free 
C Software Foundation, Inc., 675 Massachusetss Ave, Cambridge, 
C MA 02139, USA.
C
C Corresponding concerning ESO-MIDAS should be addressed as follows:
C	Internet e-mail: midas@eso.org
C	Postal address: European Southern Observatory
C			Data Management Division 
C			Karl-Schwarzschild-Strasse 2
C			D 85748 Garching bei Muenchen 
C			GERMANY
C===========================================================================
C
      SUBROUTINE GAUSS3(X,NP,PARAM,Y1,DERIV)                                   
C+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++                  
C.COPYRIGHT: Copyright (c) 1987 European Southern Observatory,                  
C                                         all rights reserved                   
C                                                                               
C.VERSION: 1.0  ESO-FORTRAN Conversion, AA  18:01 - 21 DEC 1987                 
C                                                                               
C.LANGUAGE: F77+ESOext                                                          
C                                                                               
C.AUTHOR: J.D.PONZ                                                              
C                                                                               
C--------------------------------------------------------------                 
C**********************************************                                 
C                                                                               
C      IMPLICIT NONE                                                            
C   Three-dimensional Gaussian Distribution                                     
C                                                                               
C**********************************************                                 
C                                                                               
C   Author: O.-G. Richter, ESO Garching                                         
C                                                                               
      INTEGER NP
      DOUBLE PRECISION Y1,PARAM(NP),DERIV(NP)                                   
      REAL X(3)                                                            
      DOUBLE PRECISION R11,R22,R33,R12,R13,R23,R,R1                             
      DOUBLE PRECISION A,B,C,D,E,F,O,P,Q                                        
      DOUBLE PRECISION B2,C2,D2,B22,C22,D22,E1,E2,E3                            
C  !   \                                                                        
      R11    = 1.0D0 - PARAM(10)*PARAM(10)  !    > Diagonals                    
      R22    = 1.0D0 - PARAM(9)*PARAM(9)  !   /                                 
      R33    = 1.0D0 - PARAM(8)*PARAM(8)                                        
C      ! of correlation determ.  !   \                                          
      R12    = PARAM(9)*PARAM(10) - PARAM(8)  !    > Non-diagonals              
      R13    = PARAM(8)*PARAM(10) - PARAM(9)  !   /                             
      R23    = PARAM(8)*PARAM(9) - PARAM(10)                                    
C  !   Total residual dispersion                                                
      R      = 2.0D0*R12*PARAM(8) + R11 + R22 - 1.0  !   and its inverse        
      R1     = 1.0D0/R                                                          
C                                                                               
      A      = 0.6931471806D0*R1                                                
C                                                                               
      B      = (X(1)-PARAM(2))/PARAM(5)                                         
      C      = (X(2)-PARAM(3))/PARAM(6)                                         
      D      = (X(3)-PARAM(4))/PARAM(7)                                         
      B2     = B*B                                                              
      C2     = C*C                                                              
      D2     = D*D                                                              
      B22    = B2*R11                                                           
      C22    = C2*R22                                                           
      D22    = D2*R33                                                           
C                                                                               
      DERIV(1) = DEXP(-A* (B22+C22+D22+2.0* (B*C*R12+B*D*R13+C*D*R23)))         
C                                                                               
      Y1     = DERIV(1)*PARAM(1)                                                
      A      = 2.0*A*Y1                                                         
C                                                                               
      DERIV(2) = A* (B*R11+C*R12+D*R13)/PARAM(5)                                
      DERIV(3) = A* (B*R12+C*R22+D*R23)/PARAM(6)                                
      DERIV(4) = A* (B*R13+C*R23+D*R33)/PARAM(7)                                
      DERIV(5) = DERIV(2)*B                                                     
      DERIV(6) = DERIV(3)*C                                                     
      DERIV(7) = DERIV(4)*D                                                     
      A      = A*R1                                                             
      E      = 2.0*R12*R13                                                      
      E1     = E - PARAM(8)*R                                                   
      E2     = E - PARAM(9)*R                                                   
      E3     = E - PARAM(10)*R                                                  
      F      = B22 + C22 + D22                                                  
      O      = B*C                                                              
      P      = B*D                                                              
      Q      = C*D                                                              
C                                                                               
      DERIV(8) = A* (R12*F+D2*PARAM(8)*R+O* (2.0*R12*R12+R)+P*E3+Q*E2)          
      DERIV(9) = A* (R13*F+C2*PARAM(9)*R+O*E3+P* (2.0*R13*R13+R)+Q*E1)          
      DERIV(10) = A* (R23*F+B2*PARAM(10)*R+O*E2+P*E1+Q* (2.0*R23*R23+R))        
C                                                                               
      RETURN                                                                    
                                                                                
      END