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OpenBLAS/lapack-netlib/SRC/claunhr_col_getrfnp.f

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*> \brief \b CLAUNHR_COL_GETRFNP
*
* =========== DOCUMENTATION ===========
*
* Online html documentation available at
* http://www.netlib.org/lapack/explore-html/
*
*> \htmlonly
*> Download CLAUNHR_COL_GETRFNP + dependencies
*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/claunhr_col_getrfnp.f">
*> [TGZ]</a>
*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/claunhr_col_getrfnp.f">
*> [ZIP]</a>
*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/claunhr_col_getrfnp.f">
*> [TXT]</a>
*> \endhtmlonly
*
* Definition:
* ===========
*
* SUBROUTINE CLAUNHR_COL_GETRFNP( M, N, A, LDA, D, INFO )
*
* .. Scalar Arguments ..
* INTEGER INFO, LDA, M, N
* ..
* .. Array Arguments ..
* COMPLEX A( LDA, * ), D( * )
* ..
*
*
*> \par Purpose:
* =============
*>
*> \verbatim
*>
*> CLAUNHR_COL_GETRFNP computes the modified LU factorization without
*> pivoting of a complex general M-by-N matrix A. The factorization has
*> the form:
*>
*> A - S = L * U,
*>
*> where:
*> S is a m-by-n diagonal sign matrix with the diagonal D, so that
*> D(i) = S(i,i), 1 <= i <= min(M,N). The diagonal D is constructed
*> as D(i)=-SIGN(A(i,i)), where A(i,i) is the value after performing
*> i-1 steps of Gaussian elimination. This means that the diagonal
*> element at each step of "modified" Gaussian elimination is
*> at least one in absolute value (so that division-by-zero not
*> not possible during the division by the diagonal element);
*>
*> L is a M-by-N lower triangular matrix with unit diagonal elements
*> (lower trapezoidal if M > N);
*>
*> and U is a M-by-N upper triangular matrix
*> (upper trapezoidal if M < N).
*>
*> This routine is an auxiliary routine used in the Householder
*> reconstruction routine CUNHR_COL. In CUNHR_COL, this routine is
*> applied to an M-by-N matrix A with orthonormal columns, where each
*> element is bounded by one in absolute value. With the choice of
*> the matrix S above, one can show that the diagonal element at each
*> step of Gaussian elimination is the largest (in absolute value) in
*> the column on or below the diagonal, so that no pivoting is required
*> for numerical stability [1].
*>
*> For more details on the Householder reconstruction algorithm,
*> including the modified LU factorization, see [1].
*>
*> This is the blocked right-looking version of the algorithm,
*> calling Level 3 BLAS to update the submatrix. To factorize a block,
*> this routine calls the recursive routine CLAUNHR_COL_GETRFNP2.
*>
*> [1] "Reconstructing Householder vectors from tall-skinny QR",
*> G. Ballard, J. Demmel, L. Grigori, M. Jacquelin, H.D. Nguyen,
*> E. Solomonik, J. Parallel Distrib. Comput.,
*> vol. 85, pp. 3-31, 2015.
*> \endverbatim
*
* Arguments:
* ==========
*
*> \param[in] M
*> \verbatim
*> M is INTEGER
*> The number of rows of the matrix A. M >= 0.
*> \endverbatim
*>
*> \param[in] N
*> \verbatim
*> N is INTEGER
*> The number of columns of the matrix A. N >= 0.
*> \endverbatim
*>
*> \param[in,out] A
*> \verbatim
*> A is COMPLEX array, dimension (LDA,N)
*> On entry, the M-by-N matrix to be factored.
*> On exit, the factors L and U from the factorization
*> A-S=L*U; the unit diagonal elements of L are not stored.
*> \endverbatim
*>
*> \param[in] LDA
*> \verbatim
*> LDA is INTEGER
*> The leading dimension of the array A. LDA >= max(1,M).
*> \endverbatim
*>
*> \param[out] D
*> \verbatim
*> D is COMPLEX array, dimension min(M,N)
*> The diagonal elements of the diagonal M-by-N sign matrix S,
*> D(i) = S(i,i), where 1 <= i <= min(M,N). The elements can be
*> only ( +1.0, 0.0 ) or (-1.0, 0.0 ).
*> \endverbatim
*>
*> \param[out] INFO
*> \verbatim
*> INFO is INTEGER
*> = 0: successful exit
*> < 0: if INFO = -i, the i-th argument had an illegal value
*> \endverbatim
*>
* Authors:
* ========
*
*> \author Univ. of Tennessee
*> \author Univ. of California Berkeley
*> \author Univ. of Colorado Denver
*> \author NAG Ltd.
*
*> \ingroup complexGEcomputational
*
*> \par Contributors:
* ==================
*>
*> \verbatim
*>
*> November 2019, Igor Kozachenko,
*> Computer Science Division,
*> University of California, Berkeley
*>
*> \endverbatim
*
* =====================================================================
SUBROUTINE CLAUNHR_COL_GETRFNP( M, N, A, LDA, D, INFO )
IMPLICIT NONE
*
* -- LAPACK computational routine --
* -- LAPACK is a software package provided by Univ. of Tennessee, --
* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*
* .. Scalar Arguments ..
INTEGER INFO, LDA, M, N
* ..
* .. Array Arguments ..
COMPLEX A( LDA, * ), D( * )
* ..
*
* =====================================================================
*
* .. Parameters ..
COMPLEX CONE
PARAMETER ( CONE = ( 1.0E+0, 0.0E+0 ) )
* ..
* .. Local Scalars ..
INTEGER IINFO, J, JB, NB
* ..
* .. External Subroutines ..
EXTERNAL CGEMM, CLAUNHR_COL_GETRFNP2, CTRSM, XERBLA
* ..
* .. External Functions ..
INTEGER ILAENV
EXTERNAL ILAENV
* ..
* .. Intrinsic Functions ..
INTRINSIC MAX, MIN
* ..
* .. Executable Statements ..
*
* Test the input parameters.
*
INFO = 0
IF( M.LT.0 ) THEN
INFO = -1
ELSE IF( N.LT.0 ) THEN
INFO = -2
ELSE IF( LDA.LT.MAX( 1, M ) ) THEN
INFO = -4
END IF
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'CLAUNHR_COL_GETRFNP', -INFO )
RETURN
END IF
*
* Quick return if possible
*
IF( MIN( M, N ).EQ.0 )
$ RETURN
*
* Determine the block size for this environment.
*
NB = ILAENV( 1, 'CLAUNHR_COL_GETRFNP', ' ', M, N, -1, -1 )
IF( NB.LE.1 .OR. NB.GE.MIN( M, N ) ) THEN
*
* Use unblocked code.
*
CALL CLAUNHR_COL_GETRFNP2( M, N, A, LDA, D, INFO )
ELSE
*
* Use blocked code.
*
DO J = 1, MIN( M, N ), NB
JB = MIN( MIN( M, N )-J+1, NB )
*
* Factor diagonal and subdiagonal blocks.
*
CALL CLAUNHR_COL_GETRFNP2( M-J+1, JB, A( J, J ), LDA,
$ D( J ), IINFO )
*
IF( J+JB.LE.N ) THEN
*
* Compute block row of U.
*
CALL CTRSM( 'Left', 'Lower', 'No transpose', 'Unit', JB,
$ N-J-JB+1, CONE, A( J, J ), LDA, A( J, J+JB ),
$ LDA )
IF( J+JB.LE.M ) THEN
*
* Update trailing submatrix.
*
CALL CGEMM( 'No transpose', 'No transpose', M-J-JB+1,
$ N-J-JB+1, JB, -CONE, A( J+JB, J ), LDA,
$ A( J, J+JB ), LDA, CONE, A( J+JB, J+JB ),
$ LDA )
END IF
END IF
END DO
END IF
RETURN
*
* End of CLAUNHR_COL_GETRFNP
*
END
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