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C
C     ..................................................................
C
C        SUBROUTINE APLL
C
C        PURPOSE
C           SET UP NORMAL EQUATIONS FOR A LINEAR LEAST SQUARES FIT
C           TO A GIVEN DISCRETE FUNCTION
C
C        USAGE
C           CALL APLL(FFCT,N,IP,P,WORK,DATI,IER)
C           SUBROUTINE FFCT REQUIRES AN EXTERNAL STATEMENT
C
C        DESCRIPTION OF PARAMETERS
C           FFCT  - USER CODED SUBROUTINE WHICH MUST BE DECLARED
C                   EXTERNAL IN THE MAIN PROGRAM. IT IS CALLED
C                   CALL FFCT(I,N,IP,P,DATI,WGT,IER) AND RETURNS
C                   THE VALUES OF THE FUNDAMENTAL FUNCTIONS FOR
C                   THE I-TH ARGUMENT IN P(1) UP TO P(IP)
C                   FOLLOWED BY THE I-TH FUNCTION VALUE IN P(IP+1)
C                   N IS THE NUMBER OF ALL POINTS
C                   DATI IS A DUMMY PARAMETER WHICH IS USED AS ARRAY
C                   NAME. THE GIVEN DATA SET MAY BE ALLOCATED IN DATI
C                   WGT IS THE WEIGHT FACTOR FOR THE I-TH POINT
C                   IER IS USED AS RESULTANT ERROR PARAMETER IN FFCT
C           N     - NUMBER OF GIVEN POINTS
C           IP    - NUMBER OF FUNDAMENTAL FUNCTIONS USED FOR LEAST
C                   SQUARES FIT
C                   IP SHOULD NOT EXCEED N
C           P     - WORKING STORAGE OF DIMENSION IP+1, WHICH
C                   IS USED AS INTERFACE BETWEEN APLL AND THE USER
C                   CODED SUBROUTINE FFCT
C           WORK  - WORKING STORAGE OF DIMENSION (IP+1)*(IP+2)/2.
C                   ON RETURN WORK CONTAINS THE SYMMETRIC COEFFICIENT
C                   MATRIX OF THE NORMAL EQUATIONS IN COMPRESSED FORM,
C                   I.E. UPPER TRINGULAR PART ONLY STORED COLUMNWISE.
C                   THE FOLLOWING IP POSITIONS CONTAIN THE RIGHT
C                   HAND SIDE AND WORK((IP+1)*(IP+2)/2) CONTAINS
C                   THE WEIGHTED SQUARE SUM OF THE FUNCTION VALUES
C           DATI  - DUMMY ENTRY TO COMMUNICATE AN ARRAY NAME BETWEEN
C                   MAIN LINE AND SUBROUTINE FFCT.
C           IER   - RESULTING ERROR PARAMETER
C                   IER =-1 MEANS FORMAL ERRORS IN SPECIFIED DIMENSIONS
C                   IER = 0 MEANS NO ERRORS
C                   IER = 1 MEANS ERROR IN EXTERNAL SUBROUTINE FFCT
C
C        REMARKS
C           TO ALLOW FOR EASY COMMUNICATION OF INTEGER VALUES
C           BETWEEN MAINLINE AND EXTERNAL SUBROUTINE FFCT, THE ERROR
C           PARAMETER IER IS TREATED AS A VECTOR OF DIMENSION 1 WITHIN
C           SUBROUTINE APLL. ADDITIONAL COMPONENTS OF IER MAY BE
C           INTRODUCED BY THE USER FOR COMMUNICATION BACK AND FORTH.
C           IN THIS CASE, HOWEVER, THE USER MUST SPECIFY IER AS A
C           VECTOR IN HIS MAINLINE.
C           EXECUTION OF SUBROUTINE APLL IS A PREPARATORY STEP FOR
C           CALCULATION OF THE LINEAR LEAST SQUARES FIT.
C           NORMALLY IT IS FOLLOWED BY EXECUTION OF SUBROUTINE APFS
C
C       SUBROUTINES AND FUNCTION SUBPROGRAMS REQUIRED
C           THE EXTERNAL SUBROUTINE FFCT MUST BE FURNISHED BY THE USER
C
C        METHOD
C           HANDLING OF THE GIVEN DATA SET (ARGUMENTS,FUNCTION VALUES
C           AND WEIGHTS) IS COMPLETELY LEFT TO THE USER
C           ESSENTIALLY HE HAS THREE CHOICES
C           (1) THE I-TH VALUES OF ARGUMENT, FUNCTION VALUE AND WEIGHT
C               ARE CALCULATED WITHIN SUBROUTINE FFCT FOR GIVEN I.
C           (2) THE I-TH VALUES OF ARGUMENT, FUNCTION VALUE AND WEIGHT
C               ARE DETERMINED BY TABLE LOOK UP. THE STORAGE LOCATIONS
C               REQUIRED ARE ALLOCATED WITHIN THE DUMMY ARRAY DATI
C               (POSSIBLY IN P TOO, IN EXCESS OF THE SPECIFIED IP + 1
C               LOCATIONS).
C               ANOTHER POSSIBILITY WOULD BE TO USE COMMON AS INTERFACE
C               BETWEEN MAIN LINE AND SUBROUTINE FFCT AND TO ALLOCATE
C               STORAGE FOR THE DATA SET IN COMMON.
C           (3) THE I-TH VALUES OF ARGUMENT, FUNCTION VALUE AND WEIGHT
C               ARE READ IN FROM AN EXTERNAL DEVICE. THIS MAY BE EASILY
C               ACCOMPLISHED SINCE I IS USED STRICTLY INCREASING FROM
C               ONE UP TO N WITHIN APLL
C
C     ..................................................................
C
      SUBROUTINE APLL(FFCT,N,IP,P,WORK,DATI,IER)
C
C
C        DIMENSIONED DUMMY VARIABLES
      DIMENSION P(1),WORK(1),DATI(1),IER(1)
C
C        CHECK FOR FORMAL ERRORS IN SPECIFIED DIMENSIONS
      IF(N)10,10,1
    1 IF(IP)10,10,2
    2 IF(N-IP)10,3,3
C
C        SET WORKING STORAGE AND RIGHT HAND SIDE TO ZERO
    3 IPP1=IP+1
      M=IPP1*(IP+2)/2
      IER(1)=0
      DO 4 I=1,M
    4 WORK(I)=0.
C
C        START GREAT LOOP OVER ALL GIVEN POINTS
      DO 8 I=1,N
      CALL FFCT(I,N,IP,P,DATI,WGT,IER)
      IF(IER(1))9,5,9
    5 J=0
      DO 7 K=1,IPP1
      AUX=P(K)*WGT
      DO 6 L=1,K
      J=J+1
    6 WORK(J)=WORK(J)+P(L)*AUX
    7 CONTINUE
    8 CONTINUE
C
C        NORMAL RETURN
    9 RETURN
C
C        ERROR RETURN IN CASE OF FORMAL ERRORS
   10 IER(1)=-1
      RETURN
      END