C$TEST PRSZ
C TO RUN AS A MAIN PROGRAM REMOVE NEXT LINE
      SUBROUTINE PRSZ
C***********************************************************************
C
C  EXAMPLE OF USE OF THE PORT PROGRAM SPFML
C
C***********************************************************************
      INTEGER I, IWRITE, I1MACH, NEQ
      REAL X(100), B(100), ERR, SASUM
      EXTERNAL AROW
      REAL RSTACK(2500)
      COMMON/CSTAK/ RSTACK
      COMMON /NN/ NX
      CALL ISTKIN(2500,3)
      NX = 10
      NEQ = 100
C
C CONSTRUCT A RANDOM VECTOR FOR X
C
      DO 10 I=1,NEQ
         X(I)=UNI(0)
  10  CONTINUE
C
C FIND THE VECTOR B=AX
C
      CALL SPFML(NEQ,AROW,X,B)
C
C SOLVE THE SYSTEM AX=B
C
      CALL SPFLE(NEQ,.TRUE.,AROW,ISIZE,B,NEQ,1)
C
C FIND THE NORM OF THE ERROR OF THE SOLUTION
C
      ERR=0.0
      IWRITE = I1MACH(2)
      DO 20 I=1,NEQ
         ERR=ERR + ABS(B(I)-X(I))
  20  CONTINUE
      ERR=ERR/SASUM(NEQ,X,1)
      WRITE(IWRITE,21)ERR
  21  FORMAT(19H RELATIVE ERROR IS ,1PE15.7)
      STOP
      END
      SUBROUTINE AROW(I, ROW, JCOL, NUM)
      REAL ROW(5)
      INTEGER JCOL(5)
      COMMON /NN/ N
C
C IN THE BLOCK TRIDIAGONAL MATRIX THERE ARE AT MOST 5
C NONZERO ELEMENTS PER ROW AND EACH ROW HAS A DIAGONAL
C OF -4.
C THE VARIABLE IN INDICATES WHICH BLOCK ONE IS IN AND
C THE VARIABLE JN INDICATES WHERE IN THE BLOCK ONE IS AT
C
      IN = (I-1)/N+1
      JN = I - (IN-1) * N
      JCOL(1)=I
      ROW(1)=-4.0
      NUM=2
C
C DO THE OFF DIAGONAL ELEMENTS IN THE CURRENT BLOCK
C
      JCOL(2)=I-1
      ROW(2)=1.0
      IF (JN.GT.1) NUM=NUM+1
      JCOL(NUM)=I+1
      ROW(NUM)=1.0
      IF (JN.LT.N)NUM=NUM+1
C
C DO THE BLOCK TO THE LEFT
C
      JCOL(NUM)=I-N
      ROW(NUM)=1.0
      IF (IN.GT.1)NUM=NUM+1
C
C DO THE BLOCK TO THE RIGHT
C
      JCOL(NUM)=I+N
      ROW(NUM)=1.0
      IF(IN.EQ.N) NUM=NUM-1
      RETURN
      END

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