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* =========== DOCUMENTATION ===========
*
* Online html documentation available at
* http://www.netlib.org/lapack/explore-html/
*
* Definition:
* ===========
*
* SUBROUTINE SBDT05( M, N, A, LDA, S, NS, U, LDU,
* VT, LDVT, WORK, RESID )
*
* .. Scalar Arguments ..
* INTEGER LDA, LDU, LDVT, N, NS
* REAL RESID
* ..
* .. Array Arguments ..
* REAL D( * ), E( * ), S( * ), U( LDU, * ),
* $ VT( LDVT, * ), WORK( * )
* ..
*
*
*> \par Purpose:
* =============
*>
*> \verbatim
*>
*> SBDT05 reconstructs a bidiagonal matrix B from its (partial) SVD:
*> S = U' * B * V
*> where U and V are orthogonal matrices and S is diagonal.
*>
*> The test ratio to test the singular value decomposition is
*> RESID = norm( S - U' * B * V ) / ( n * norm(B) * EPS )
*> where VT = V' and EPS is the machine precision.
*> \endverbatim
*
* Arguments:
* ==========
*
*> \param[in] M
*> \verbatim
*> M is INTEGER
*> The number of rows of the matrices A and U.
*> \endverbatim
*>
*> \param[in] N
*> \verbatim
*> N is INTEGER
*> The number of columns of the matrices A and VT.
*> \endverbatim
*>
*> \param[in] A
*> \verbatim
*> A is REAL array, dimension (LDA,N)
*> The m by n matrix A.
*>
*> \param[in] LDA
*> \verbatim
*> LDA is INTEGER
*> The leading dimension of the array A. LDA >= max(1,M).
*> \endverbatim
*>
*> \param[in] S
*> \verbatim
*> S is REAL array, dimension (NS)
*> The singular values from the (partial) SVD of B, sorted in
*> decreasing order.
*> \endverbatim
*>
*> \param[in] NS
*> \verbatim
*> NS is INTEGER
*> The number of singular values/vectors from the (partial)
*> SVD of B.
*> \endverbatim
*>
*> \param[in] U
*> \verbatim
*> U is REAL array, dimension (LDU,NS)
*> The n by ns orthogonal matrix U in S = U' * B * V.
*> \endverbatim
*>
*> \param[in] LDU
*> \verbatim
*> LDU is INTEGER
*> The leading dimension of the array U. LDU >= max(1,N)
*> \endverbatim
*>
*> \param[in] VT
*> \verbatim
*> VT is REAL array, dimension (LDVT,N)
*> The n by ns orthogonal matrix V in S = U' * B * V.
*> \endverbatim
*>
*> \param[in] LDVT
*> \verbatim
*> LDVT is INTEGER
*> The leading dimension of the array VT.
*> \endverbatim
*>
*> \param[out] WORK
*> \verbatim
*> WORK is REAL array, dimension (M,N)
*> \endverbatim
*>
*> \param[out] RESID
*> \verbatim
*> RESID is REAL
*> The test ratio: norm(S - U' * A * V) / ( n * norm(A) * EPS )
*> \endverbatim
*
* Authors:
* ========
*
*> \author Univ. of Tennessee
*> \author Univ. of California Berkeley
*> \author Univ. of Colorado Denver
*> \author NAG Ltd.
*
*> \date December 2016
*
*> \ingroup double_eig
*
* =====================================================================
SUBROUTINE SBDT05( M, N, A, LDA, S, NS, U, LDU,
$ VT, LDVT, WORK, RESID )
*
* -- LAPACK test routine (version 3.7.0) --
* -- LAPACK is a software package provided by Univ. of Tennessee, --
* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
* December 2016
*
* .. Scalar Arguments ..
INTEGER LDA, LDU, LDVT, M, N, NS
REAL RESID
* ..
* .. Array Arguments ..
REAL A( LDA, * ), S( * ), U( LDU, * ),
$ VT( LDVT, * ), WORK( * )
* ..
*
* ======================================================================
*
* .. Parameters ..
REAL ZERO, ONE
PARAMETER ( ZERO = 0.0E+0, ONE = 1.0E+0 )
* ..
* .. Local Scalars ..
INTEGER I, J
REAL ANORM, EPS
* ..
* .. External Functions ..
LOGICAL LSAME
INTEGER ISAMAX
REAL SASUM, SLAMCH, SLANGE
EXTERNAL LSAME, ISAMAX, SASUM, SLAMCH, SLANGE
* ..
* .. External Subroutines ..
EXTERNAL SGEMM
* ..
* .. Intrinsic Functions ..
INTRINSIC ABS, REAL, MAX, MIN
* ..
* .. Executable Statements ..
*
* Quick return if possible.
*
RESID = ZERO
IF( MIN( M, N ).LE.0 .OR. NS.LE.0 )
$ RETURN
*
EPS = SLAMCH( 'Precision' )
ANORM = SLANGE( 'M', M, N, A, LDA, WORK )
*
* Compute U' * A * V.
*
CALL SGEMM( 'N', 'T', M, NS, N, ONE, A, LDA, VT,
$ LDVT, ZERO, WORK( 1+NS*NS ), M )
CALL SGEMM( 'T', 'N', NS, NS, M, -ONE, U, LDU, WORK( 1+NS*NS ),
$ M, ZERO, WORK, NS )
*
* norm(S - U' * B * V)
*
J = 0
DO 10 I = 1, NS
WORK( J+I ) = WORK( J+I ) + S( I )
RESID = MAX( RESID, SASUM( NS, WORK( J+1 ), 1 ) )
J = J + NS
10 CONTINUE
*
IF( ANORM.LE.ZERO ) THEN
IF( RESID.NE.ZERO )
$ RESID = ONE / EPS
ELSE
IF( ANORM.GE.RESID ) THEN
RESID = ( RESID / ANORM ) / ( REAL( N )*EPS )
ELSE
IF( ANORM.LT.ONE ) THEN
RESID = ( MIN( RESID, REAL( N )*ANORM ) / ANORM ) /
$ ( REAL( N )*EPS )
ELSE
RESID = MIN( RESID / ANORM, REAL( N ) ) /
$ ( REAL( N )*EPS )
END IF
END IF
END IF
*
RETURN
*
* End of SBDT05
*
END
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