stzrqf.f

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*> \brief \b STZRQF
*
*  =========== DOCUMENTATION ===========
*
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*
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*
*  Definition:
*  ===========
*
*       SUBROUTINE STZRQF( M, N, A, LDA, TAU, INFO )
*
*       .. Scalar Arguments ..
*       INTEGER            INFO, LDA, M, N
*       ..
*       .. Array Arguments ..
*       REAL               A( LDA, * ), TAU( * )
*       ..
*
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*> This routine is deprecated and has been replaced by routine STZRZF.
*>
*> STZRQF reduces the M-by-N ( M<=N ) real upper trapezoidal matrix A
*> to upper triangular form by means of orthogonal transformations.
*>
*> The upper trapezoidal matrix A is factored as
*>
*>    A = ( R  0 ) * Z,
*>
*> where Z is an N-by-N orthogonal matrix and R is an M-by-M upper
*> triangular matrix.
*> \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 >= M.
*> \endverbatim
*>
*> \param[in,out] A
*> \verbatim
*>          A is REAL array, dimension (LDA,N)
*>          On entry, the leading M-by-N upper trapezoidal part of the
*>          array A must contain the matrix to be factorized.
*>          On exit, the leading M-by-M upper triangular part of A
*>          contains the upper triangular matrix R, and elements M+1 to
*>          N of the first M rows of A, with the array TAU, represent the
*>          orthogonal matrix Z as a product of M elementary reflectors.
*> \endverbatim
*>
*> \param[in] LDA
*> \verbatim
*>          LDA is INTEGER
*>          The leading dimension of the array A.  LDA >= max(1,M).
*> \endverbatim
*>
*> \param[out] TAU
*> \verbatim
*>          TAU is REAL array, dimension (M)
*>          The scalar factors of the elementary reflectors.
*> \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.
*
*> \date December 2016
*
*> \ingroup realOTHERcomputational
*
*> \par Further Details:
*  =====================
*>
*> \verbatim
*>
*>  The factorization is obtained by Householder's method.  The kth
*>  transformation matrix, Z( k ), which is used to introduce zeros into
*>  the ( m - k + 1 )th row of A, is given in the form
*>
*>     Z( k ) = ( I     0   ),
*>              ( 0  T( k ) )
*>
*>  where
*>
*>     T( k ) = I - tau*u( k )*u( k )**T,   u( k ) = (   1    ),
*>                                                   (   0    )
*>                                                   ( z( k ) )
*>
*>  tau is a scalar and z( k ) is an ( n - m ) element vector.
*>  tau and z( k ) are chosen to annihilate the elements of the kth row
*>  of X.
*>
*>  The scalar tau is returned in the kth element of TAU and the vector
*>  u( k ) in the kth row of A, such that the elements of z( k ) are
*>  in  a( k, m + 1 ), ..., a( k, n ). The elements of R are returned in
*>  the upper triangular part of A.
*>
*>  Z is given by
*>
*>     Z =  Z( 1 ) * Z( 2 ) * ... * Z( m ).
*> \endverbatim
*>
*  =====================================================================
      SUBROUTINE stzrqf( M, N, A, LDA, TAU, INFO )
*
*  -- LAPACK computational 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            INFO, LDA, M, N
*     ..
*     .. Array Arguments ..
      REAL               A( lda, * ), TAU( * )
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               ONE, ZERO
      parameter( one = 1.0e+0, zero = 0.0e+0 )
*     ..
*     .. Local Scalars ..
      INTEGER            I, K, M1
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          max, min
*     ..
*     .. External Subroutines ..
      EXTERNAL           saxpy, scopy, sgemv, sger, slarfg, xerbla
*     ..
*     .. Executable Statements ..
*
*     Test the input parameters.
*
      info = 0
      IF( m.LT.0 ) THEN
         info = -1
      ELSE IF( n.LT.m ) THEN
         info = -2
      ELSE IF( lda.LT.max( 1, m ) ) THEN
         info = -4
      END IF
      IF( info.NE.0 ) THEN
         CALL xerbla( 'STZRQF', -info )
         RETURN
      END IF
*
*     Perform the factorization.
*
      IF( m.EQ.0 )
     $   RETURN
      IF( m.EQ.n ) THEN
         DO 10 i = 1, n
            tau( i ) = zero
   10    CONTINUE
      ELSE
         m1 = min( m+1, n )
         DO 20 k = m, 1, -1
*
*           Use a Householder reflection to zero the kth row of A.
*           First set up the reflection.
*
            CALL slarfg( n-m+1, a( k, k ), a( k, m1 ), lda, tau( k ) )
*
            IF( ( tau( k ).NE.zero ) .AND. ( k.GT.1 ) ) THEN
*
*              We now perform the operation  A := A*P( k ).
*
*              Use the first ( k - 1 ) elements of TAU to store  a( k ),
*              where  a( k ) consists of the first ( k - 1 ) elements of
*              the  kth column  of  A.  Also  let  B  denote  the  first
*              ( k - 1 ) rows of the last ( n - m ) columns of A.
*
               CALL scopy( k-1, a( 1, k ), 1, tau, 1 )
*
*              Form   w = a( k ) + B*z( k )  in TAU.
*
               CALL sgemv( 'No transpose', k-1, n-m, one, a( 1, m1 ),
     $                     lda, a( k, m1 ), lda, one, tau, 1 )
*
*              Now form  a( k ) := a( k ) - tau*w
*              and       B      := B      - tau*w*z( k )**T.
*
               CALL saxpy( k-1, -tau( k ), tau, 1, a( 1, k ), 1 )
               CALL sger( k-1, n-m, -tau( k ), tau, 1, a( k, m1 ), lda,
     $                    a( 1, m1 ), lda )
            END IF
   20    CONTINUE
      END IF
*
      RETURN
*
*     End of STZRQF
*
      END