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      SUBROUTINE DLATRZ( M, N, L, A, LDA, TAU, WORK )

*

*  -- LAPACK routine (version 3.2.2) --

*  -- LAPACK is a software package provided by Univ. of Tennessee,    --

*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--

*     June 2010

*

*     .. Scalar Arguments ..

      INTEGER            L, LDA, M, N

*     ..

*     .. Array Arguments ..

      DOUBLE PRECISION   A( LDA, * ), TAU( * ), WORK( * )

*     ..

*

*  Purpose

*  =======

*

*  DLATRZ factors the M-by-(M+L) real upper trapezoidal matrix

*  [ A1 A2 ] = [ A(1:M,1:M) A(1:M,N-L+1:N) ] as ( R  0 ) * Z, by means

*  of orthogonal transformations.  Z is an (M+L)-by-(M+L) orthogonal

*  matrix and, R and A1 are M-by-M upper triangular matrices.

*

*  Arguments

*  =========

*

*  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.

*

*  L       (input) INTEGER

*          The number of columns of the matrix A containing the

*          meaningful part of the Householder vectors. N-M >= L >= 0.

*

*  A       (input/output) DOUBLE PRECISION 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 N-L+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.

*

*  LDA     (input) INTEGER

*          The leading dimension of the array A.  LDA >= max(1,M).

*

*  TAU     (output) DOUBLE PRECISION array, dimension (M)

*          The scalar factors of the elementary reflectors.

*

*  WORK    (workspace) DOUBLE PRECISION array, dimension (M)

*

*  Further Details

*  ===============

*

*  Based on contributions by

*    A. Petitet, Computer Science Dept., Univ. of Tenn., Knoxville, USA

*

*  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 )',   u( k ) = (   1    ),

*                                                 (   0    )

*                                                 ( z( k ) )

*

*  tau is a scalar and z( k ) is an l element vector. tau and z( k )

*  are chosen to annihilate the elements of the kth row of A2.

*

*  The scalar tau is returned in the kth element of TAU and the vector

*  u( k ) in the kth row of A2, such that the elements of z( k ) are

*  in  a( k, l + 1 ), ..., a( k, n ). The elements of R are returned in

*  the upper triangular part of A1.

*

*  Z is given by

*

*     Z =  Z( 1 ) * Z( 2 ) * ... * Z( m ).

*

*  =====================================================================

*

*     .. Parameters ..

      DOUBLE PRECISION   ZERO

      PARAMETER          ( ZERO = 0.0D+0 )

*     ..

*     .. Local Scalars ..

      INTEGER            I

*     ..

*     .. External Subroutines ..

      EXTERNAL           DLARFG, DLARZ

*     ..

*     .. Executable Statements ..

*

*     Test the input arguments

*

*     Quick return if possible

*

      IF( M.EQ.0 ) THEN

         RETURN

      ELSE IF( M.EQ.N ) THEN

         DO 10 I = 1, N

            TAU( I ) = ZERO

   10    CONTINUE

         RETURN

      END IF

*

      DO 20 I = M, 1, -1

*

*        Generate elementary reflector H(i) to annihilate

*        [ A(i,i) A(i,n-l+1:n) ]

*

         CALL DLARFG( L+1, A( I, I ), A( I, N-L+1 ), LDA, TAU( I ) )

*

*        Apply H(i) to A(1:i-1,i:n) from the right

*

         CALL DLARZ( 'Right', I-1, N-I+1, L, A( I, N-L+1 ), LDA,

     $               TAU( I ), A( 1, I ), LDA, WORK )

*

   20 CONTINUE

*

      RETURN

*

*     End of DLATRZ

*

      END