SUBROUTINE CSI1 (X,Y,Y1,N,C )                                             
C                                                                               
C        SUBROUTINE  CSI1  COMPUTES THE POLYNOMIAL COEFFICIENTS OF EACH 3RD     
C        DEGREE POLYNOMIAL PIECE OF THE CUBIC SPLINE AT EACH INTERVAL BETWEEN   
C        DATA POINT I AND (I+1) UTILIZING THE COMPUTED SLOPES AT THESE POINTS   
C                                                                               
C        DESCRIPTION OF PARAMETERS                                              
C            X - VECTOR OF ARGUMENTS                                            
C           Y1 - VECTOR OF FIRST DERIVATIVES                                    
C            N - NUMBER OF MESH POINTS                                          
C            C - MATRIX OF COMPUTED COEFFICIENTS                                
C                                                                               
      IMPLICIT REAL*8(A-H,O-Z)                                                  
      DIMENSION X(N),Y(N),Y1(N),C(4,N)                                          
C                                                                               
      NM1 = N - 1                                                               
      DO 1 I = 1,NM1                                                            
      DX = X(I+1)- X(I)                                                         
      DDIF1 = (Y(I+1) - Y(I))/DX                                                
      DDIF2 = Y1(I+1) + Y1(I) - 2.*DDIF1                                        
      C(1,I) = Y(I)                                                             
      C(2,I) = Y1(I)                                                            
      C(3,I) = (DDIF1 - Y1(I) - DDIF2)/DX                                       
    1 C(4,I) = DDIF2/(DX*DX)                                                    
      RETURN                                                                    
      END