slaqgb.f

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      SUBROUTINE SLAQGB( M, N, KL, KU, AB, LDAB, R, C, ROWCND, COLCND,
     $                   AMAX, EQUED )
*
*  -- LAPACK auxiliary routine (version 3.2) --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*     November 2006
*
*     .. Scalar Arguments ..
      CHARACTER          EQUED
      INTEGER            KL, KU, LDAB, M, N
      REAL               AMAX, COLCND, ROWCND
*     ..
*     .. Array Arguments ..
      REAL               AB( LDAB, * ), C( * ), R( * )
*     ..
*
*  Purpose
*  =======
*
*  SLAQGB equilibrates a general M by N band matrix A with KL
*  subdiagonals and KU superdiagonals using the row and scaling factors
*  in the vectors R and C.
*
*  Arguments
*  =========
*
*  M       (input) INTEGER
*          The number of rows of the matrix A.  M >= 0.
*
*  N       (input) INTEGER
*          The number of columns of the matrix A.  N >= 0.
*
*  KL      (input) INTEGER
*          The number of subdiagonals within the band of A.  KL >= 0.
*
*  KU      (input) INTEGER
*          The number of superdiagonals within the band of A.  KU >= 0.
*
*  AB      (input/output) REAL array, dimension (LDAB,N)
*          On entry, the matrix A in band storage, in rows 1 to KL+KU+1.
*          The j-th column of A is stored in the j-th column of the
*          array AB as follows:
*          AB(ku+1+i-j,j) = A(i,j) for max(1,j-ku)<=i<=min(m,j+kl)
*
*          On exit, the equilibrated matrix, in the same storage format
*          as A.  See EQUED for the form of the equilibrated matrix.
*
*  LDAB    (input) INTEGER
*          The leading dimension of the array AB.  LDA >= KL+KU+1.
*
*  R       (input) REAL array, dimension (M)
*          The row scale factors for A.
*
*  C       (input) REAL array, dimension (N)
*          The column scale factors for A.
*
*  ROWCND  (input) REAL
*          Ratio of the smallest R(i) to the largest R(i).
*
*  COLCND  (input) REAL
*          Ratio of the smallest C(i) to the largest C(i).
*
*  AMAX    (input) REAL
*          Absolute value of largest matrix entry.
*
*  EQUED   (output) CHARACTER*1
*          Specifies the form of equilibration that was done.
*          = 'N':  No equilibration
*          = 'R':  Row equilibration, i.e., A has been premultiplied by
*                  diag(R).
*          = 'C':  Column equilibration, i.e., A has been postmultiplied
*                  by diag(C).
*          = 'B':  Both row and column equilibration, i.e., A has been
*                  replaced by diag(R) * A * diag(C).
*
*  Internal Parameters
*  ===================
*
*  THRESH is a threshold value used to decide if row or column scaling
*  should be done based on the ratio of the row or column scaling
*  factors.  If ROWCND < THRESH, row scaling is done, and if
*  COLCND < THRESH, column scaling is done.
*
*  LARGE and SMALL are threshold values used to decide if row scaling
*  should be done based on the absolute size of the largest matrix
*  element.  If AMAX > LARGE or AMAX < SMALL, row scaling is done.
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               ONE, THRESH
      PARAMETER          ( ONE = 1.0E+0, THRESH = 0.1E+0 )
*     ..
*     .. Local Scalars ..
      INTEGER            I, J
      REAL               CJ, LARGE, SMALL
*     ..
*     .. External Functions ..
      REAL               SLAMCH
      EXTERNAL           SLAMCH
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          MAX, MIN
*     ..
*     .. Executable Statements ..
*
*     Quick return if possible
*
      IF( M.LE.0 .OR. N.LE.0 ) THEN
         EQUED = 'N'
         RETURN
      END IF
*
*     Initialize LARGE and SMALL.
*
      SMALL = SLAMCH( 'Safe minimum' ) / SLAMCH( 'Precision' )
      LARGE = ONE / SMALL
*
      IF( ROWCND.GE.THRESH .AND. AMAX.GE.SMALL .AND. AMAX.LE.LARGE )
     $     THEN
*
*        No row scaling
*
         IF( COLCND.GE.THRESH ) THEN
*
*           No column scaling
*
            EQUED = 'N'
         ELSE
*
*           Column scaling
*
            DO 20 J = 1, N
               CJ = C( J )
               DO 10 I = MAX( 1, J-KU ), MIN( M, J+KL )
                  AB( KU+1+I-J, J ) = CJ*AB( KU+1+I-J, J )
   10          CONTINUE
   20       CONTINUE
            EQUED = 'C'
         END IF
      ELSE IF( COLCND.GE.THRESH ) THEN
*
*        Row scaling, no column scaling
*
         DO 40 J = 1, N
            DO 30 I = MAX( 1, J-KU ), MIN( M, J+KL )
               AB( KU+1+I-J, J ) = R( I )*AB( KU+1+I-J, J )
   30       CONTINUE
   40    CONTINUE
         EQUED = 'R'
      ELSE
*
*        Row and column scaling
*
         DO 60 J = 1, N
            CJ = C( J )
            DO 50 I = MAX( 1, J-KU ), MIN( M, J+KL )
               AB( KU+1+I-J, J ) = CJ*R( I )*AB( KU+1+I-J, J )
   50       CONTINUE
   60    CONTINUE
         EQUED = 'B'
      END IF
*
      RETURN
*
*     End of SLAQGB
*
      END