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REAL FUNCTION SQRT17( TRANS, IRESID, M, N, NRHS, A,
$ LDA, X, LDX, B, LDB, C, WORK, LWORK )
*
* -- LAPACK test routine (version 3.0) --
* Univ. of Tennessee, Univ. of California Berkeley, NAG Ltd.,
* Courant Institute, Argonne National Lab, and Rice University
* June 30, 1999
*
* .. Scalar Arguments ..
CHARACTER TRANS
INTEGER IRESID, LDA, LDB, LDX, LWORK, M, N, NRHS
* ..
* .. Array Arguments ..
REAL A( LDA, * ), B( LDB, * ), C( LDB, * ),
$ WORK( LWORK ), X( LDX, * )
* ..
*
* Purpose
* =======
*
* SQRT17 computes the ratio
*
* || R'*op(A) ||/(||A||*alpha*max(M,N,NRHS)*eps)
*
* where R = op(A)*X - B, op(A) is A or A', and
*
* alpha = ||B|| if IRESID = 1 (zero-residual problem)
* alpha = ||R|| if IRESID = 2 (otherwise).
*
* Arguments
* =========
*
* TRANS (input) CHARACTER*1
* Specifies whether or not the transpose of A is used.
* = 'N': No transpose, op(A) = A.
* = 'T': Transpose, op(A) = A'.
*
* IRESID (input) INTEGER
* IRESID = 1 indicates zero-residual problem.
* IRESID = 2 indicates non-zero residual.
*
* M (input) INTEGER
* The number of rows of the matrix A.
* If TRANS = 'N', the number of rows of the matrix B.
* If TRANS = 'T', the number of rows of the matrix X.
*
* N (input) INTEGER
* The number of columns of the matrix A.
* If TRANS = 'N', the number of rows of the matrix X.
* If TRANS = 'T', the number of rows of the matrix B.
*
* NRHS (input) INTEGER
* The number of columns of the matrices X and B.
*
* A (input) REAL array, dimension (LDA,N)
* The m-by-n matrix A.
*
* LDA (input) INTEGER
* The leading dimension of the array A. LDA >= M.
*
* X (input) REAL array, dimension (LDX,NRHS)
* If TRANS = 'N', the n-by-nrhs matrix X.
* If TRANS = 'T', the m-by-nrhs matrix X.
*
* LDX (input) INTEGER
* The leading dimension of the array X.
* If TRANS = 'N', LDX >= N.
* If TRANS = 'T', LDX >= M.
*
* B (input) REAL array, dimension (LDB,NRHS)
* If TRANS = 'N', the m-by-nrhs matrix B.
* If TRANS = 'T', the n-by-nrhs matrix B.
*
* LDB (input) INTEGER
* The leading dimension of the array B.
* If TRANS = 'N', LDB >= M.
* If TRANS = 'T', LDB >= N.
*
* C (workspace) REAL array, dimension (LDB,NRHS)
*
* WORK (workspace) REAL array, dimension (LWORK)
*
* LWORK (input) INTEGER
* The length of the array WORK. LWORK >= NRHS*(M+N).
*
* =====================================================================
*
* .. Parameters ..
REAL ZERO, ONE
PARAMETER ( ZERO = 0.0E0, ONE = 1.0E0 )
* ..
* .. Local Scalars ..
INTEGER INFO, ISCL, NCOLS, NROWS
REAL BIGNUM, ERR, NORMA, NORMB, NORMRS, NORMX,
$ SMLNUM
* ..
* .. Local Arrays ..
REAL RWORK( 1 )
* ..
* .. External Functions ..
LOGICAL LSAME
REAL SLAMCH, SLANGE
EXTERNAL LSAME, SLAMCH, SLANGE
* ..
* .. External Subroutines ..
EXTERNAL SGEMM, SLACPY, SLASCL, XERBLA
* ..
* .. Intrinsic Functions ..
INTRINSIC MAX, REAL
* ..
* .. Executable Statements ..
*
SQRT17 = ZERO
*
IF( LSAME( TRANS, 'N' ) ) THEN
NROWS = M
NCOLS = N
ELSE IF( LSAME( TRANS, 'T' ) ) THEN
NROWS = N
NCOLS = M
ELSE
CALL XERBLA( 'SQRT17', 1 )
RETURN
END IF
*
IF( LWORK.LT.NCOLS*NRHS ) THEN
CALL XERBLA( 'SQRT17', 13 )
RETURN
END IF
*
IF( M.LE.0 .OR. N.LE.0 .OR. NRHS.LE.0 ) THEN
RETURN
END IF
*
NORMA = SLANGE( 'One-norm', M, N, A, LDA, RWORK )
SMLNUM = SLAMCH( 'Safe minimum' ) / SLAMCH( 'Precision' )
BIGNUM = ONE / SMLNUM
ISCL = 0
*
* compute residual and scale it
*
CALL SLACPY( 'All', NROWS, NRHS, B, LDB, C, LDB )
CALL SGEMM( TRANS, 'No transpose', NROWS, NRHS, NCOLS, -ONE, A,
$ LDA, X, LDX, ONE, C, LDB )
NORMRS = SLANGE( 'Max', NROWS, NRHS, C, LDB, RWORK )
IF( NORMRS.GT.SMLNUM ) THEN
ISCL = 1
CALL SLASCL( 'General', 0, 0, NORMRS, ONE, NROWS, NRHS, C, LDB,
$ INFO )
END IF
*
* compute R'*A
*
CALL SGEMM( 'Transpose', TRANS, NRHS, NCOLS, NROWS, ONE, C, LDB,
$ A, LDA, ZERO, WORK, NRHS )
*
* compute and properly scale error
*
ERR = SLANGE( 'One-norm', NRHS, NCOLS, WORK, NRHS, RWORK )
IF( NORMA.NE.ZERO )
$ ERR = ERR / NORMA
*
IF( ISCL.EQ.1 )
$ ERR = ERR*NORMRS
*
IF( IRESID.EQ.1 ) THEN
NORMB = SLANGE( 'One-norm', NROWS, NRHS, B, LDB, RWORK )
IF( NORMB.NE.ZERO )
$ ERR = ERR / NORMB
ELSE
NORMX = SLANGE( 'One-norm', NCOLS, NRHS, X, LDX, RWORK )
IF( NORMX.NE.ZERO )
$ ERR = ERR / NORMX
END IF
*
SQRT17 = ERR / ( SLAMCH( 'Epsilon' )*REAL( MAX( M, N, NRHS ) ) )
RETURN
*
* End of SQRT17
*
END
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