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SUBROUTINE SGBT02( TRANS, M, N, KL, KU, NRHS, A, LDA, X, LDX, B,
$ LDB, 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 TRANS
INTEGER KL, KU, LDA, LDB, LDX, M, N, NRHS
REAL RESID
* ..
* .. Array Arguments ..
REAL A( LDA, * ), B( LDB, * ), X( LDX, * )
* ..
*
* Purpose
* =======
*
* SGBT02 computes the residual for a solution of a banded system of
* equations A*x = b or A'*x = b:
* RESID = norm( B - A*X ) / ( norm(A) * norm(X) * EPS).
* where EPS is the machine precision.
*
* Arguments
* =========
*
* TRANS (input) CHARACTER*1
* Specifies the form of the system of equations:
* = 'N': A *x = b
* = 'T': A'*x = b, where A' is the transpose of A
* = 'C': A'*x = b, where A' is the transpose of A
*
* 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.
*
* NRHS (input) INTEGER
* The number of columns of B. NRHS >= 0.
*
* A (input) REAL array, dimension (LDA,N)
* The original matrix A in band storage, stored in rows 1 to
* KL+KU+1.
*
* LDA (input) INTEGER
* The leading dimension of the array A. LDA >= max(1,KL+KU+1).
*
* 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. If TRANS = 'N',
* LDX >= max(1,N); if TRANS = 'T' or 'C', LDX >= max(1,M).
*
* B (input/output) REAL array, dimension (LDB,NRHS)
* On entry, the right hand side vectors for the system of
* linear equations.
* On exit, B is overwritten with the difference B - A*X.
*
* LDB (input) INTEGER
* The leading dimension of the array B. IF TRANS = 'N',
* LDB >= max(1,M); if TRANS = 'T' or 'C', LDB >= max(1,N).
*
* RESID (output) REAL
* The maximum over the number of right hand sides of
* norm(B - A*X) / ( norm(A) * norm(X) * EPS ).
*
* =====================================================================
*
* .. Parameters ..
REAL ZERO, ONE
PARAMETER ( ZERO = 0.0E+0, ONE = 1.0E+0 )
* ..
* .. Local Scalars ..
INTEGER I1, I2, J, KD, N1
REAL ANORM, BNORM, EPS, XNORM
* ..
* .. External Functions ..
LOGICAL LSAME
REAL SASUM, SLAMCH
EXTERNAL LSAME, SASUM, SLAMCH
* ..
* .. External Subroutines ..
EXTERNAL SGBMV
* ..
* .. Intrinsic Functions ..
INTRINSIC MAX, MIN
* ..
* .. Executable Statements ..
*
* Quick return if N = 0 pr NRHS = 0
*
IF( M.LE.0 .OR. N.LE.0 .OR. NRHS.LE.0 ) THEN
RESID = ZERO
RETURN
END IF
*
* Exit with RESID = 1/EPS if ANORM = 0.
*
EPS = SLAMCH( 'Epsilon' )
KD = KU + 1
ANORM = ZERO
DO 10 J = 1, N
I1 = MAX( KD+1-J, 1 )
I2 = MIN( KD+M-J, KL+KD )
ANORM = MAX( ANORM, SASUM( I2-I1+1, A( I1, J ), 1 ) )
10 CONTINUE
IF( ANORM.LE.ZERO ) THEN
RESID = ONE / EPS
RETURN
END IF
*
IF( LSAME( TRANS, 'T' ) .OR. LSAME( TRANS, 'C' ) ) THEN
N1 = N
ELSE
N1 = M
END IF
*
* Compute B - A*X (or B - A'*X )
*
DO 20 J = 1, NRHS
CALL SGBMV( TRANS, M, N, KL, KU, -ONE, A, LDA, X( 1, J ), 1,
$ ONE, B( 1, J ), 1 )
20 CONTINUE
*
* Compute the maximum over the number of right hand sides of
* norm(B - A*X) / ( norm(A) * norm(X) * EPS ).
*
RESID = ZERO
DO 30 J = 1, NRHS
BNORM = SASUM( N1, B( 1, J ), 1 )
XNORM = SASUM( N1, X( 1, J ), 1 )
IF( XNORM.LE.ZERO ) THEN
RESID = ONE / EPS
ELSE
RESID = MAX( RESID, ( ( BNORM / ANORM ) / XNORM ) / EPS )
END IF
30 CONTINUE
*
RETURN
*
* End of SGBT02
*
END
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