ctbt02()

subroutine ctbt02	(	character	uplo,
		character	trans,
		character	diag,
		integer	n,
		integer	kd,
		integer	nrhs,
		complex, dimension( ldab, * )	ab,
		integer	ldab,
		complex, dimension( ldx, * )	x,
		integer	ldx,
		complex, dimension( ldb, * )	b,
		integer	ldb,
		complex, dimension( * )	work,
		real, dimension( * )	rwork,
		real	resid )

CTBT02

Purpose:

!>
!> CTBT02 computes the residual for the computed solution to a
!> triangular system of linear equations op(A)*X = B, when A is a
!> triangular band matrix. The test ratio is the maximum over
!>    norm(b - op(A)*x) / ( ||op(A)||_1 * norm(x) * EPS ),
!> where op(A) = A, A**T, or A**H, b is the column of B, x is the
!> solution vector, and EPS is the machine epsilon.
!> 

Parameters

[in]	UPLO	!> UPLO is CHARACTER*1 !> Specifies whether the matrix A is upper or lower triangular. !> = 'U': Upper triangular !> = 'L': Lower triangular !>
[in]	TRANS	!> TRANS is CHARACTER*1 !> Specifies the operation applied to A. !> = 'N': A * X = B (No transpose) !> = 'T': A**T * X = B (Transpose) !> = 'C': A**H * X = B (Conjugate transpose) !>
[in]	DIAG	!> DIAG is CHARACTER*1 !> Specifies whether or not the matrix A is unit triangular. !> = 'N': Non-unit triangular !> = 'U': Unit triangular !>
[in]	N	!> N is INTEGER !> The order of the matrix A. N >= 0. !>
[in]	KD	!> KD is INTEGER !> The number of superdiagonals or subdiagonals of the !> triangular band matrix A. KD >= 0. !>
[in]	NRHS	!> NRHS is INTEGER !> The number of right hand sides, i.e., the number of columns !> of the matrices X and B. NRHS >= 0. !>
[in]	AB	!> AB is COMPLEX array, dimension (LDA,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). !>
[in]	LDAB	!> LDAB is INTEGER !> The leading dimension of the array AB. LDAB >= max(1,KD+1). !>
[in]	X	!> X is COMPLEX array, dimension (LDX,NRHS) !> The computed solution vectors for the system of linear !> equations. !>
[in]	LDX	!> LDX is INTEGER !> The leading dimension of the array X. LDX >= max(1,N). !>
[in]	B	!> B is COMPLEX array, dimension (LDB,NRHS) !> The right hand side vectors for the system of linear !> equations. !>
[in]	LDB	!> LDB is INTEGER !> The leading dimension of the array B. LDB >= max(1,N). !>
[out]	WORK	!> WORK is COMPLEX array, dimension (N) !>
[out]	RWORK	!> RWORK is REAL array, dimension (N) !>
[out]	RESID	!> RESID is REAL !> The maximum over the number of right hand sides of !> norm(op(A)*x - b) / ( norm(op(A)) * norm(x) * EPS ). !>

Author

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Definition at line 157 of file ctbt02.f.

*
*  -- LAPACK test routine --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*
*     .. Scalar Arguments ..
      CHARACTER          DIAG, TRANS, UPLO
      INTEGER            KD, LDAB, LDB, LDX, N, NRHS
      REAL               RESID
*     ..
*     .. Array Arguments ..
      REAL               RWORK( * )
      COMPLEX            AB( LDAB, * ), B( LDB, * ), WORK( * ),
     $                   X( LDX, * )
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               ZERO, ONE
      parameter( zero = 0.0e+0, one = 1.0e+0 )
*     ..
*     .. Local Scalars ..
      INTEGER            J
      REAL               ANORM, BNORM, EPS, XNORM
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      REAL               CLANTB, SCASUM, SLAMCH
      EXTERNAL           lsame, clantb, scasum, slamch
*     ..
*     .. External Subroutines ..
      EXTERNAL           caxpy, ccopy, ctbmv
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          cmplx, max
*     ..
*     .. Executable Statements ..
*
*     Quick exit if N = 0 or NRHS = 0
*
      IF( n.LE.0 .OR. nrhs.LE.0 ) THEN
         resid = zero
         RETURN
      END IF
*
*     Compute the 1-norm of op(A).
*
      IF( lsame( trans, 'N' ) ) THEN
         anorm = clantb( '1', uplo, diag, n, kd, ab, ldab, rwork )
      ELSE
         anorm = clantb( 'I', uplo, diag, n, kd, ab, ldab, rwork )
      END IF
*
*     Exit with RESID = 1/EPS if ANORM = 0.
*
      eps = slamch( 'Epsilon' )
      IF( anorm.LE.zero ) THEN
         resid = one / eps
         RETURN
      END IF
*
*     Compute the maximum over the number of right hand sides of
*        norm(op(A)*x - b) / ( norm(op(A)) * norm(x) * EPS ).
*
      resid = zero
      DO 10 j = 1, nrhs
         CALL ccopy( n, x( 1, j ), 1, work, 1 )
         CALL ctbmv( uplo, trans, diag, n, kd, ab, ldab, work, 1 )
         CALL caxpy( n, cmplx( -one ), b( 1, j ), 1, work, 1 )
         bnorm = scasum( n, work, 1 )
         xnorm = scasum( n, x( 1, j ), 1 )
         IF( xnorm.LE.zero ) THEN
            resid = one / eps
         ELSE
            resid = max( resid, ( ( bnorm / anorm ) / xnorm ) / eps )
         END IF
   10 CONTINUE
*
      RETURN
*
*     End of CTBT02
*

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