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      SUBROUTINE FIBIC2 (N,M,A,X,Y,XU,YU,XO,YO,VALUE,IERR)
C
C*****************************************************************
C                                                                *
C  The subroutine determines a double integral of a spline       *
C  function over a rectangle [XU, XO] x [YU, YO].                *
C                                                                *
C----------------------------------------------------------------*
C                                                                *
C  subroutines required: XYINTV, FIBIC1                          *
C                                                                *
C*****************************************************************
C                                                                *
C  author   : Eberhard Heyne                                     *
C  date     : 02.15.1983                                         *
C  source   : FORTRAN 77                                         *
C                                                                *
C*****************************************************************
C
      IMPLICIT DOUBLE PRECISION (A-H,O-Z)
C
      PARAMETER (KDIM=3,LDIM=3)
C
      DIMENSION A(0:N,0:M,0:KDIM,0:LDIM),X(0:N),Y(0:M)
C
C*  determine intervals and relative coordinates
C
      CALL XYINTV(N,M,X,Y,IU,JU,XIU,ETAU,XU,YU,JERR)
      IERR=JERR
      IF(JERR .NE. 0) RETURN
      CALL XYINTV(N,M,X,Y,IO,JO,XIO,ETAO,XO,YO,JERR)
      IERR=JERR+1
      IF(JERR .NE. 0) RETURN
      FACTOR=1.0D0
      IF(IU .GT. IO) THEN
        I=IO
        IO=IU
        IU=I
        XI=XIO
        XIO=XIU
        XIU=XI
        FACTOR=-FACTOR
      ENDIF
      IF(JU .GT. JO) THEN
        J=JO
        JO=JU
        JU=J
        ETA=ETAO
        ETAO=ETAU
        ETAU=ETA
        FACTOR=-FACTOR
      ENDIF
      S=FIBIC1(N,M,A,IU,JU,XIU,ETAU)
      S=S-FIBIC1(N,M,A,IU,JO,XIU,ETAO)
      S=S-FIBIC1(N,M,A,IO,JU,XIO,ETAU)
      S=S+FIBIC1(N,M,A,IO,JO,XIO,ETAO)
      DO 101 I=IU,IO-1
        S=S+FIBIC1(N,M,A,I,JO,X(I+1)-X(I),ETAO)
        S=S-FIBIC1(N,M,A,I,JU,X(I+1)-X(I),ETAU)
  101 CONTINUE
      DO 103 J=JU,JO-1
        S=S+FIBIC1(N,M,A,IO,J,XIO,Y(J+1)-Y(J))
        S=S-FIBIC1(N,M,A,IU,J,XIU,Y(J+1)-Y(J))
        DO 102 I=IU,IO-1
          S=S+FIBIC1(N,M,A,I,J,X(I+1)-X(I),Y(J+1)-Y(J))
  102   CONTINUE
  103 CONTINUE
      IERR=0
      VALUE=FACTOR*S
      RETURN
      END


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