ctbt03.f

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*> \brief \b CTBT03
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at 
*            http://www.netlib.org/lapack/explore-html/ 
*
*  Definition:
*  ===========
*
*       SUBROUTINE CTBT03( UPLO, TRANS, DIAG, N, KD, NRHS, AB, LDAB,
*                          SCALE, CNORM, TSCAL, X, LDX, B, LDB, WORK,
*                          RESID )
* 
*       .. Scalar Arguments ..
*       CHARACTER          DIAG, TRANS, UPLO
*       INTEGER            KD, LDAB, LDB, LDX, N, NRHS
*       REAL               RESID, SCALE, TSCAL
*       ..
*       .. Array Arguments ..
*       REAL               CNORM( * )
*       COMPLEX            AB( LDAB, * ), B( LDB, * ), WORK( * ),
*      $                   X( LDX, * )
*       ..
*  
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*> CTBT03 computes the residual for the solution to a scaled triangular
*> system of equations  A*x = s*b,  A**T *x = s*b,  or  A**H *x = s*b
*> when A is a triangular band matrix.  Here A**T  denotes the transpose
*> of A, A**H denotes the conjugate transpose of A, s is a scalar, and
*> x and b are N by NRHS matrices.  The test ratio is the maximum over
*> the number of right hand sides of
*>    norm(s*b - op(A)*x) / ( norm(op(A)) * norm(x) * EPS ),
*> where op(A) denotes A, A**T, or A**H, and EPS is the machine epsilon.
*> \endverbatim
*
*  Arguments:
*  ==========
*
*> \param[in] UPLO
*> \verbatim
*>          UPLO is CHARACTER*1
*>          Specifies whether the matrix A is upper or lower triangular.
*>          = 'U':  Upper triangular
*>          = 'L':  Lower triangular
*> \endverbatim
*>
*> \param[in] TRANS
*> \verbatim
*>          TRANS is CHARACTER*1
*>          Specifies the operation applied to A.
*>          = 'N':  A *x = s*b     (No transpose)
*>          = 'T':  A**T *x = s*b  (Transpose)
*>          = 'C':  A**H *x = s*b  (Conjugate transpose)
*> \endverbatim
*>
*> \param[in] DIAG
*> \verbatim
*>          DIAG is CHARACTER*1
*>          Specifies whether or not the matrix A is unit triangular.
*>          = 'N':  Non-unit triangular
*>          = 'U':  Unit triangular
*> \endverbatim
*>
*> \param[in] N
*> \verbatim
*>          N is INTEGER
*>          The order of the matrix A.  N >= 0.
*> \endverbatim
*>
*> \param[in] KD
*> \verbatim
*>          KD is INTEGER
*>          The number of superdiagonals or subdiagonals of the
*>          triangular band matrix A.  KD >= 0.
*> \endverbatim
*>
*> \param[in] NRHS
*> \verbatim
*>          NRHS is INTEGER
*>          The number of right hand sides, i.e., the number of columns
*>          of the matrices X and B.  NRHS >= 0.
*> \endverbatim
*>
*> \param[in] AB
*> \verbatim
*>          AB is COMPLEX array, dimension (LDAB,N)
*>          The upper or lower triangular band matrix A, stored in the
*>          first kd+1 rows of the array. The j-th column of A is stored
*>          in the j-th column of the array AB as follows:
*>          if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j;
*>          if UPLO = 'L', AB(1+i-j,j)    = A(i,j) for j<=i<=min(n,j+kd).
*> \endverbatim
*>
*> \param[in] LDAB
*> \verbatim
*>          LDAB is INTEGER
*>          The leading dimension of the array AB.  LDAB >= KD+1.
*> \endverbatim
*>
*> \param[in] SCALE
*> \verbatim
*>          SCALE is REAL
*>          The scaling factor s used in solving the triangular system.
*> \endverbatim
*>
*> \param[in] CNORM
*> \verbatim
*>          CNORM is REAL array, dimension (N)
*>          The 1-norms of the columns of A, not counting the diagonal.
*> \endverbatim
*>
*> \param[in] TSCAL
*> \verbatim
*>          TSCAL is REAL
*>          The scaling factor used in computing the 1-norms in CNORM.
*>          CNORM actually contains the column norms of TSCAL*A.
*> \endverbatim
*>
*> \param[in] X
*> \verbatim
*>          X is COMPLEX array, dimension (LDX,NRHS)
*>          The computed solution vectors for the system of linear
*>          equations.
*> \endverbatim
*>
*> \param[in] LDX
*> \verbatim
*>          LDX is INTEGER
*>          The leading dimension of the array X.  LDX >= max(1,N).
*> \endverbatim
*>
*> \param[in] B
*> \verbatim
*>          B is COMPLEX array, dimension (LDB,NRHS)
*>          The right hand side vectors for the system of linear
*>          equations.
*> \endverbatim
*>
*> \param[in] LDB
*> \verbatim
*>          LDB is INTEGER
*>          The leading dimension of the array B.  LDB >= max(1,N).
*> \endverbatim
*>
*> \param[out] WORK
*> \verbatim
*>          WORK is COMPLEX array, dimension (N)
*> \endverbatim
*>
*> \param[out] RESID
*> \verbatim
*>          RESID is REAL
*>          The maximum over the number of right hand sides of
*>          norm(op(A)*x - s*b) / ( norm(op(A)) * norm(x) * EPS ).
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee 
*> \author Univ. of California Berkeley 
*> \author Univ. of Colorado Denver 
*> \author NAG Ltd. 
*
*> \date November 2011
*
*> \ingroup complex_lin
*
*  =====================================================================
      SUBROUTINE ctbt03( UPLO, TRANS, DIAG, N, KD, NRHS, AB, LDAB,
     $                   scale, cnorm, tscal, x, ldx, b, ldb, work,
     $                   resid )
*
*  -- LAPACK test routine (version 3.4.0) --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*     November 2011
*
*     .. Scalar Arguments ..
      CHARACTER          DIAG, TRANS, UPLO
      INTEGER            KD, LDAB, LDB, LDX, N, NRHS
      REAL               RESID, SCALE, TSCAL
*     ..
*     .. Array Arguments ..
      REAL               CNORM( * )
      COMPLEX            AB( ldab, * ), B( ldb, * ), WORK( * ),
     $                   x( ldx, * )
*     ..
*
*  =====================================================================
*
*
*     .. Parameters ..
      REAL               ONE, ZERO
      parameter                ( one = 1.0e+0, zero = 0.0e+0 )
*     ..
*     .. Local Scalars ..
      INTEGER            IX, J
      REAL               EPS, ERR, SMLNUM, TNORM, XNORM, XSCAL
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      INTEGER            ICAMAX
      REAL               SLAMCH
      EXTERNAL           lsame, icamax, slamch
*     ..
*     .. External Subroutines ..
      EXTERNAL           caxpy, ccopy, csscal, ctbmv
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          abs, cmplx, max, real
*     ..
*     .. Executable Statements ..
*
*     Quick exit if N = 0
*
      IF( n.LE.0 .OR. nrhs.LE.0 ) THEN
         resid = zero
         RETURN
      END IF
      eps = slamch( 'Epsilon' )
      smlnum = slamch( 'Safe minimum' )
*
*     Compute the norm of the triangular matrix A using the column
*     norms already computed by CLATBS.
*
      tnorm = zero
      IF( lsame( diag, 'N' ) ) THEN
         IF( lsame( uplo, 'U' ) ) THEN
            DO 10 j = 1, n
               tnorm = max( tnorm, tscal*abs( ab( kd+1, j ) )+
     $                 cnorm( j ) )
   10       CONTINUE
         ELSE
            DO 20 j = 1, n
               tnorm = max( tnorm, tscal*abs( ab( 1, j ) )+cnorm( j ) )
   20       CONTINUE
         END IF
      ELSE
         DO 30 j = 1, n
            tnorm = max( tnorm, tscal+cnorm( j ) )
   30    CONTINUE
      END IF
*
*     Compute the maximum over the number of right hand sides of
*        norm(op(A)*x - s*b) / ( norm(op(A)) * norm(x) * EPS ).
*
      resid = zero
      DO 40 j = 1, nrhs
         CALL ccopy( n, x( 1, j ), 1, work, 1 )
         ix = icamax( n, work, 1 )
         xnorm = max( one, abs( x( ix, j ) ) )
         xscal = ( one / xnorm ) / REAL( kd+1 )
         CALL csscal( n, xscal, work, 1 )
         CALL ctbmv( uplo, trans, diag, n, kd, ab, ldab, work, 1 )
         CALL caxpy( n, cmplx( -scale*xscal ), b( 1, j ), 1, work, 1 )
         ix = icamax( n, work, 1 )
         err = tscal*abs( work( ix ) )
         ix = icamax( n, x( 1, j ), 1 )
         xnorm = abs( x( ix, j ) )
         IF( err*smlnum.LE.xnorm ) THEN
            IF( xnorm.GT.zero )
     $         err = err / xnorm
         ELSE
            IF( err.GT.zero )
     $         err = one / eps
         END IF
         IF( err*smlnum.LE.tnorm ) THEN
            IF( tnorm.GT.zero )
     $         err = err / tnorm
         ELSE
            IF( err.GT.zero )
     $         err = one / eps
         END IF
         resid = max( resid, err )
   40 CONTINUE
*
      RETURN
*
*     End of CTBT03
*
      END