C SUBROUTINE GRADLS.F 
C
C SOURCE
C   BEVINGTON, PAGES 220-222.
C
C PURPOSE
C   MAKE A GRADIENT-SEARCH LEAST-SQUARES FIT TO DATA WITH A
C      SPECIFIED FUNCTION WHICH IS NOT LINEAR IN COEFFICIENTS
C
C USAGE 
C   CALL GRADLS (X, Y, SIGMAY, NPTS, NTERMS, MODE, A, DELTAA,
C      YFIT, CHISQR)
C
C DESCRIPTION OF PARAMETERS
C   X	   - ARRAY OF DATA POINTS FOR INDEPENDENT VARIABLE
C   Y	   - ARRAY OF DATA POINTS FOR DEPENDENT VARIABLE
C   SIGMAY - ARRAY OF STANDARD DEVIATIONS FOR Y DATA POINTS
C   NPTS   - NUMBER OF PAIRS OF DATA POINTS
C   NTERMS - NUMBER OF PARAMETERS
C   MODE   - DETERMINES METHOD OF WEIGHTING LEAST-SQUARES FIT
C	     +1 (INSTRUMENTAL) WEIGHT(I) = 1./SIGMAY(I)**2
C	      0 (NO WEIGHTING) WEIGHT(I) = 1.
C	     -1 (STATISTICAL)  WEIGHT(I) = 1./Y(I)
C   A	   - ARRAY OF PARAMETERS
C   DELTAA - ARRAY OF INCREMENTS FOR PARAMETERS A
C   YFIT   - ARRAY OF CALCULATED VALUES OF Y
C   CHISQR - REDUCED CHI SQUARE FOR FIT 
C
C SUBROUTINES AND FUNCTION SUBPROGRAMS REQUIRED 
C   FUNCTN (X, I, A)
C      EVALUATES THE FITTING FUNCTION FOR THE ITH TERM
C   FCHISQ (Y, SIGMAY, NPTS, NFREE, MODE, YFIT) 
C      EVALUATES REDUCED CHI SQUARED FOR FIT TO DATA
C
C COMMENTS
C   DIMENSION STATEMENT VALID FOR NTERMS UP TO 10
C
      SUBROUTINE GRADLS (X,Y,SIGMAY,NPTS,NTERMS,MODE,A,DELTAA,
     *YFIT,CHISQR)
      DIMENSION X(1),Y(1),SIGMAY(1),A(1),DELTAA(1),YFIT(1)
      DIMENSION GRAD(10)
      REAL FUNCTN
      EXTERNAL FUNCTN

C
C EVALUATE CHI SQUARE AT BEGINNING
C
11    NFREE=NPTS-NTERMS
      IF (NFREE) 13,13,21
13    CHISQR=0.
      GOTO 110
21    DO 22 I=1,NPTS
22    YFIT(I)=FUNCTN (X,I,A)
      CHISQ1=FCHISQ (Y,SIGMAY,NPTS,NFREE,MODE,YFIT)
C
C EVALUATE GRADIENT OF CHI SQUARE
C
31    SUM=0.
32    DO 39 J=1,NTERMS
      DELTA=0.1*DELTAA(J)
      A(J)=A(J)+DELTA 
      DO 36 I=1,NPTS
36    YFIT(I)=FUNCTN (X,I,A)
      A(J)=A(J)-DELTA 
      GRAD(J)=CHISQ1-FCHISQ (Y,SIGMAY,NPTS,NFREE,MODE,YFIT)
39    SUM=SUM+GRAD(J)**2
41    DO 42 J=1,NTERMS
42    GRAD(J)=DELTAA(J)*GRAD(J)/SQRT(SUM)
C
C EVALUATE CHI SQUARE AT NEW POINT
C
51    DO 52 J=1,NTERMS
52    A(J)=A(J)+GRAD(J)
53    DO 54 I=1,NPTS
54    YFIT(I)=FUNCTN (X,I,A)
      CHISQ2=FCHISQ (Y,SIGMAY,NPTS,NFREE,MODE,YFIT)
C
C MAKE SURE CHI SQUARE DECREASES
C
61    IF (CHISQ1-CHISQ2) 62,62,71
62    DO 64 J=1,NTERMS
      A(J)=A(J)-GRAD(J)
64    GRAD(J)=GRAD(J)/2.
      GOTO 51 
C
C INCREMENT PARAMETERS UNTIL CHI SQUARE STARTS TO INCREASE
C
71    DO 72 J=1,NTERMS
72    A(J)=A(J)+GRAD(J)
      DO 74 I=1,NPTS
74    YFIT(I)=FUNCTN (X,I,A)
75    CHISQ3=FCHISQ (Y,SIGMAY,NPTS,NFREE,MODE,YFIT)
76    IF (CHISQ3-CHISQ2) 81,91,91
81    CHISQ1=CHISQ2
82    CHISQ2=CHISQ3
      GOTO 71 
C
C FIND MINIMUM OF PARABOLA DEFINED BY LAST THREE POINTS 
C
91    DELTA=1./(1.+(CHISQ1-CHISQ2)/(CHISQ3-CHISQ2))+0.5
      DO 93 J=1,NTERMS
93    A(J)=A(J)-DELTA*GRAD(J) 
      DO 95 I=1,NPTS
95    YFIT(I)=FUNCTN (X,I,A)
      CHISQR=FCHISQ (Y,SIGMAY,NPTS,NFREE,MODE,YFIT)
101   IF (CHISQ2-CHISQR) 102,110,110
102   DO 103 J=1,NTERMS
103   A(J)=A(J)+(DELTA-1.)*GRAD(J)
104   DO 105 I=1,NPTS 
105   YFIT(I)=FUNCTN (X,I,A)
106   CHISQR=CHISQ2
110   RETURN
      END