relibc_openlibm/slatec/snbfs.f

*DECK SNBFS
      SUBROUTINE SNBFS (ABE, LDA, N, ML, MU, V, ITASK, IND, WORK, IWORK)
C***BEGIN PROLOGUE  SNBFS
C***PURPOSE  Solve a general nonsymmetric banded system of linear
C            equations.
C***LIBRARY   SLATEC
C***CATEGORY  D2A2
C***TYPE      SINGLE PRECISION (SNBFS-S, DNBFS-D, CNBFS-C)
C***KEYWORDS  BANDED, LINEAR EQUATIONS, NONSYMMETRIC
C***AUTHOR  Voorhees, E. A., (LANL)
C***DESCRIPTION
C
C    Subroutine SNBFS solves a general nonsymmetric banded NxN
C    system of single precision real linear equations using
C    SLATEC subroutines SNBCO and SNBSL.  These are adaptations
C    of the LINPACK subroutines SBGCO and SGBSL, which require
C    a different format for storing the matrix elements.  If
C    A  is an NxN real matrix and if  X  and  B  are real
C    N-vectors, then SNBFS solves the equation
C
C                          A*X=B.
C
C    A band matrix is a matrix whose nonzero elements are all
C    fairly near the main diagonal, specifically  A(I,J) = 0
C    if  I-J is greater than  ML  or  J-I  is greater than
C    MU .  The integers ML and MU are called the lower and upper
C    band widths and  M = ML+MU+1  is the total band width.
C    SNBFS uses less time and storage than the corresponding
C    program for general matrices (SGEFS) if 2*ML+MU .LT. N  .
C
C    The matrix A is first factored into upper and lower tri-
C    angular matrices U and L using partial pivoting.  These
C    factors and the pivoting information are used to find the
C    solution vector X.  An approximate condition number is
C    calculated to provide a rough estimate of the number of
C    digits of accuracy in the computed solution.
C
C    If the equation A*X=B is to be solved for more than one vector
C    B, the factoring of A does not need to be performed again and
C    the option to only solve (ITASK .GT. 1) will be faster for
C    the succeeding solutions.  In this case, the contents of A,
C    LDA, N and IWORK must not have been altered by the user follow-
C    ing factorization (ITASK=1).  IND will not be changed by SNBFS
C    in this case.
C
C
C    Band Storage
C
C          If  A  is a band matrix, the following program segment
C          will set up the input.
C
C                  ML = (band width below the diagonal)
C                  MU = (band width above the diagonal)
C                  DO 20 I = 1, N
C                     J1 = MAX(1, I-ML)
C                     J2 = MIN(N, I+MU)
C                     DO 10 J = J1, J2
C                        K = J - I + ML + 1
C                        ABE(I,K) = A(I,J)
C               10    CONTINUE
C               20 CONTINUE
C
C          This uses columns  1  through  ML+MU+1  of ABE .
C          Furthermore,  ML  additional columns are needed in
C          ABE  starting with column  ML+MU+2  for elements
C          generated during the triangularization.  The total
C          number of columns needed in  ABE  is  2*ML+MU+1 .
C
C    Example:  If the original matrix is
C
C          11 12 13  0  0  0
C          21 22 23 24  0  0
C           0 32 33 34 35  0
C           0  0 43 44 45 46
C           0  0  0 54 55 56
C           0  0  0  0 65 66
C
C     then  N = 6, ML = 1, MU = 2, LDA .GE. 5  and ABE should contain
C
C           * 11 12 13  +     , * = not used
C          21 22 23 24  +     , + = used for pivoting
C          32 33 34 35  +
C          43 44 45 46  +
C          54 55 56  *  +
C          65 66  *  *  +
C
C
C  Argument Description ***
C
C    ABE    REAL(LDA,NC)
C             on entry, contains the matrix in band storage as
C               described above.  NC  must not be less than
C               2*ML+MU+1 .  The user is cautioned to specify  NC
C               with care since it is not an argument and cannot
C               be checked by SNBFS.  The rows of the original
C               matrix are stored in the rows of  ABE  and the
C               diagonals of the original matrix are stored in
C               columns  1  through  ML+MU+1  of  ABE .
C             on return, contains an upper triangular matrix U and
C               the multipliers necessary to construct a matrix L
C               so that A=L*U.
C    LDA    INTEGER
C             the leading dimension of array ABE.  LDA must be great-
C             er than or equal to N.  (terminal error message IND=-1)
C    N      INTEGER
C             the order of the matrix A.  N must be greater
C             than or equal to 1 .  (terminal error message IND=-2)
C    ML     INTEGER
C             the number of diagonals below the main diagonal.
C             ML  must not be less than zero nor greater than or
C             equal to  N .  (terminal error message IND=-5)
C    MU     INTEGER
C             the number of diagonals above the main diagonal.
C             MU  must not be less than zero nor greater than or
C             equal to  N .  (terminal error message IND=-6)
C    V      REAL(N)
C             on entry, the singly subscripted array(vector) of di-
C               mension N which contains the right hand side B of a
C               system of simultaneous linear equations A*X=B.
C             on return, V contains the solution vector, X .
C    ITASK  INTEGER
C             If ITASK=1, the matrix A is factored and then the
C               linear equation is solved.
C             If ITASK .GT. 1, the equation is solved using the existing
C               factored matrix A and IWORK.
C             If ITASK .LT. 1, then terminal error message IND=-3 is
C               printed.
C    IND    INTEGER
C             GT. 0  IND is a rough estimate of the number of digits
C                     of accuracy in the solution, X.
C             LT. 0  See error message corresponding to IND below.
C    WORK   REAL(N)
C             a singly subscripted array of dimension at least N.
C    IWORK  INTEGER(N)
C             a singly subscripted array of dimension at least N.
C
C  Error Messages Printed ***
C
C    IND=-1  terminal   N is greater than LDA.
C    IND=-2  terminal   N is less than 1.
C    IND=-3  terminal   ITASK is less than 1.
C    IND=-4  terminal   the matrix A is computationally singular.
C                         A solution has not been computed.
C    IND=-5  terminal   ML is less than zero or is greater than
C                         or equal to N .
C    IND=-6  terminal   MU is less than zero or is greater than
C                         or equal to N .
C    IND=-10 warning    the solution has no apparent significance.
C                         The solution may be inaccurate or the matrix
C                         A may be poorly scaled.
C
C               Note-  The above terminal(*fatal*) error messages are
C                      designed to be handled by XERMSG in which
C                      LEVEL=1 (recoverable) and IFLAG=2 .  LEVEL=0
C                      for warning error messages from XERMSG.  Unless
C                      the user provides otherwise, an error message
C                      will be printed followed by an abort.
C
C***REFERENCES  J. J. Dongarra, J. R. Bunch, C. B. Moler, and G. W.
C                 Stewart, LINPACK Users' Guide, SIAM, 1979.
C***ROUTINES CALLED  R1MACH, SNBCO, SNBSL, XERMSG
C***REVISION HISTORY  (YYMMDD)
C   800808  DATE WRITTEN
C   890531  Changed all specific intrinsics to generic.  (WRB)
C   890831  Modified array declarations.  (WRB)
C   890831  REVISION DATE from Version 3.2
C   891214  Prologue converted to Version 4.0 format.  (BAB)
C   900315  CALLs to XERROR changed to CALLs to XERMSG.  (THJ)
C   900510  Convert XERRWV calls to XERMSG calls.  (RWC)
C   920501  Reformatted the REFERENCES section.  (WRB)
C***END PROLOGUE  SNBFS
C
      INTEGER LDA,N,ITASK,IND,IWORK(*),ML,MU
      REAL ABE(LDA,*),V(*),WORK(*),R1MACH
      REAL RCOND
      CHARACTER*8 XERN1, XERN2
C***FIRST EXECUTABLE STATEMENT  SNBFS
      IF (LDA.LT.N) THEN
         IND = -1
         WRITE (XERN1, '(I8)') LDA
         WRITE (XERN2, '(I8)') N
         CALL XERMSG ('SLATEC', 'SNBFS', 'LDA = ' // XERN1 //
     *      ' IS LESS THAN N = ' // XERN2, -1, 1)
         RETURN
      ENDIF
C
      IF (N.LE.0) THEN
         IND = -2
         WRITE (XERN1, '(I8)') N
         CALL XERMSG ('SLATEC', 'SNBFS', 'N = ' // XERN1 //
     *      ' IS LESS THAN 1', -2, 1)
         RETURN
      ENDIF
C
      IF (ITASK.LT.1) THEN
         IND = -3
         WRITE (XERN1, '(I8)') ITASK
         CALL XERMSG ('SLATEC', 'SNBFS', 'ITASK = ' // XERN1 //
     *      ' IS LESS THAN 1', -3, 1)
         RETURN
      ENDIF
C
      IF (ML.LT.0 .OR. ML.GE.N) THEN
         IND = -5
         WRITE (XERN1, '(I8)') ML
         CALL XERMSG ('SLATEC', 'SNBFS',
     *      'ML = ' // XERN1 // ' IS OUT OF RANGE', -5, 1)
         RETURN
      ENDIF
C
      IF (MU.LT.0 .OR. MU.GE.N) THEN
         IND = -6
         WRITE (XERN1, '(I8)') MU
         CALL XERMSG ('SLATEC', 'SNBFS',
     *      'MU = ' // XERN1 // ' IS OUT OF RANGE', -6, 1)
         RETURN
      ENDIF
C
      IF (ITASK.EQ.1) THEN
C
C        FACTOR MATRIX A INTO LU
C
         CALL SNBCO(ABE,LDA,N,ML,MU,IWORK,RCOND,WORK)
C
C        CHECK FOR COMPUTATIONALLY SINGULAR MATRIX
C
         IF (RCOND.EQ.0.0) THEN
            IND = -4
            CALL XERMSG ('SLATEC', 'SNBFS',
     *         'SINGULAR MATRIX A - NO SOLUTION', -4, 1)
            RETURN
         ENDIF
C
C        COMPUTE IND (ESTIMATE OF NO. OF SIGNIFICANT DIGITS)
C        AND CHECK FOR IND GREATER THAN ZERO
C
         IND = -LOG10(R1MACH(4)/RCOND)
         IF (IND.LE.0) THEN
            IND = -10
            CALL XERMSG ('SLATEC', 'SNBFS',
     *         'SOLUTION MAY HAVE NO SIGNIFICANCE', -10, 0)
         ENDIF
      ENDIF
C
C     SOLVE AFTER FACTORING
C
      CALL SNBSL(ABE,LDA,N,ML,MU,IWORK,V,0)
      RETURN
      END