FUNCTION PCUBIC( XI,X,N,C )                                               
C                                                                               
C        SUBROUTINE FUNCTION PCUBIC  EVALUATES THE FINAL INTERPOLATION VALUE    
C        OF THE COMPUTED PIECEWISE CUBIC FUNCTION FOR A GIVEN ARGUMENT  XI      
C                                                                               
      IMPLICIT REAL*8(A-H,O-Z)                                                  
      DIMENSION X(N),C(4,N)                                                     
      DATA I/1/                                                                 
      DX = XI  - X(I)                                                           
      IF(DX) 1,3,2                                                              
    1 IF(I.EQ.1) GO TO 3                                                        
      I = I - 1                                                                 
      DX = XI - X(I)                                                            
      IF(DX) 1,3,3                                                              
    4 I = I + 1                                                                 
      DX = DDX                                                                  
    2 IF(I.EQ.N) GO TO 3                                                        
      DDX = XI - X(I+1)                                                         
      IF(DDX) 3,4,4                                                             
    3 PCUBIC = C(1,I) + DX*( C(2,I) + DX*( C(3,I) + DX*C(4,I)))                 
      RETURN                                                                    
      END