SUBROUTINE STBT03( UPLO, TRANS, DIAG, N, KD, NRHS, AB, LDAB,
$ SCALE, CNORM, TSCAL, X, LDX, B, LDB, WORK,
$ RESID )
*
* -- LAPACK test routine (version 3.0) --
* Univ. of Tennessee, Univ. of California Berkeley, NAG Ltd.,
* Courant Institute, Argonne National Lab, and Rice University
* February 29, 1992
*
* .. Scalar Arguments ..
CHARACTER DIAG, TRANS, UPLO
INTEGER KD, LDAB, LDB, LDX, N, NRHS
REAL RESID, SCALE, TSCAL
* ..
* .. Array Arguments ..
REAL AB( LDAB, * ), B( LDB, * ), CNORM( * ),
$ WORK( * ), X( LDX, * )
* ..
*
* Purpose
* =======
*
* STBT03 computes the residual for the solution to a scaled triangular
* system of equations A*x = s*b or A'*x = s*b when A is a
* triangular band matrix. Here A' is the 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 or A' and EPS is the machine epsilon.
*
* Arguments
* =========
*
* UPLO (input) CHARACTER*1
* Specifies whether the matrix A is upper or lower triangular.
* = 'U': Upper triangular
* = 'L': Lower triangular
*
* TRANS (input) CHARACTER*1
* Specifies the operation applied to A.
* = 'N': A *x = b (No transpose)
* = 'T': A'*x = b (Transpose)
* = 'C': A'*x = b (Conjugate transpose = Transpose)
*
* DIAG (input) CHARACTER*1
* Specifies whether or not the matrix A is unit triangular.
* = 'N': Non-unit triangular
* = 'U': Unit triangular
*
* N (input) INTEGER
* The order of the matrix A. N >= 0.
*
* KD (input) INTEGER
* The number of superdiagonals or subdiagonals of the
* triangular band matrix A. KD >= 0.
*
* NRHS (input) INTEGER
* The number of right hand sides, i.e., the number of columns
* of the matrices X and B. NRHS >= 0.
*
* AB (input) REAL 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).
*
* LDAB (input) INTEGER
* The leading dimension of the array AB. LDAB >= KD+1.
*
* SCALE (input) REAL
* The scaling factor s used in solving the triangular system.
*
* CNORM (input) REAL array, dimension (N)
* The 1-norms of the columns of A, not counting the diagonal.
*
* TSCAL (input) REAL
* The scaling factor used in computing the 1-norms in CNORM.
* CNORM actually contains the column norms of TSCAL*A.
*
* X (input) REAL array, dimension (LDX,NRHS)
* The computed solution vectors for the system of linear
* equations.
*
* LDX (input) INTEGER
* The leading dimension of the array X. LDX >= max(1,N).
*
* B (input) REAL array, dimension (LDB,NRHS)
* The right hand side vectors for the system of linear
* equations.
*
* LDB (input) INTEGER
* The leading dimension of the array B. LDB >= max(1,N).
*
* WORK (workspace) REAL array, dimension (N)
*
* RESID (output) REAL
* The maximum over the number of right hand sides of
* norm(op(A)*x - s*b) / ( norm(op(A)) * norm(x) * EPS ).
*
* =====================================================================
*
* .. Parameters ..
REAL ONE, ZERO
PARAMETER ( ONE = 1.0E+0, ZERO = 0.0E+0 )
* ..
* .. Local Scalars ..
INTEGER IX, J
REAL BIGNUM, EPS, ERR, SMLNUM, TNORM, XNORM, XSCAL
* ..
* .. External Functions ..
LOGICAL LSAME
INTEGER ISAMAX
REAL SLAMCH
EXTERNAL LSAME, ISAMAX, SLAMCH
* ..
* .. External Subroutines ..
EXTERNAL SAXPY, SCOPY, SLABAD, SSCAL, STBMV
* ..
* .. Intrinsic Functions ..
INTRINSIC ABS, 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' )
BIGNUM = ONE / SMLNUM
CALL SLABAD( SMLNUM, BIGNUM )
*
* Compute the norm of the triangular matrix A using the column
* norms already computed by SLATBS.
*
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 SCOPY( N, X( 1, J ), 1, WORK, 1 )
IX = ISAMAX( N, WORK, 1 )
XNORM = MAX( ONE, ABS( X( IX, J ) ) )
XSCAL = ( ONE / XNORM ) / REAL( KD+1 )
CALL SSCAL( N, XSCAL, WORK, 1 )
CALL STBMV( UPLO, TRANS, DIAG, N, KD, AB, LDAB, WORK, 1 )
CALL SAXPY( N, -SCALE*XSCAL, B( 1, J ), 1, WORK, 1 )
IX = ISAMAX( N, WORK, 1 )
ERR = TSCAL*ABS( WORK( IX ) )
IX = ISAMAX( 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 STBT03
*
END