FUNCTION CHISQ(A,N)                                                       
C                                                                               
C   COMPUTES THE PERCENTILES OF THE CHI-SQUARE DISTRIBUTION X(N)                
C                                                                               
C   PARAMETERS                                                                  
C     A - PROBABILITY INTEGRAL FROM ZERO TO CHISQ                               
C     N - NUMBER OF DEGREES OF FREEDOM                                          
C     CHISQ - COMPUTED ABSCISSA VALUE OF X(N) CORRESPONDING TO PROBABILITY A    
C   ACCURACY BETTER THAN 0.04 FOR N>1                                           
      IMPLICIT REAL*8(A-H,O-Z)                                                  
      CHISQ     =N*(1-2./(9.*N)+XNORM(A,0D0,1D0)*DSQRT(2D0/(9.*N)))**3          
      RETURN                                                                    
      END