LAPACK  3.8.0
LAPACK: Linear Algebra PACKage

◆ dlatrz()

subroutine dlatrz ( integer  M,
integer  N,
integer  L,
double precision, dimension( lda, * )  A,
integer  LDA,
double precision, dimension( * )  TAU,
double precision, dimension( * )  WORK 
)

DLATRZ factors an upper trapezoidal matrix by means of orthogonal transformations.

Download DLATRZ + dependencies [TGZ] [ZIP] [TXT]

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.
Parameters
[in]M
          M is INTEGER
          The number of rows of the matrix A.  M >= 0.
[in]N
          N is INTEGER
          The number of columns of the matrix A.  N >= 0.
[in]L
          L is INTEGER
          The number of columns of the matrix A containing the
          meaningful part of the Householder vectors. N-M >= L >= 0.
[in,out]A
          A is 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.
[in]LDA
          LDA is INTEGER
          The leading dimension of the array A.  LDA >= max(1,M).
[out]TAU
          TAU is DOUBLE PRECISION array, dimension (M)
          The scalar factors of the elementary reflectors.
[out]WORK
          WORK is DOUBLE PRECISION array, dimension (M)
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Date
December 2016
Contributors:
A. Petitet, Computer Science Dept., Univ. of Tenn., Knoxville, USA
Further Details:
  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 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 ).

Definition at line 142 of file dlatrz.f.

142 *
143 * -- LAPACK computational routine (version 3.7.0) --
144 * -- LAPACK is a software package provided by Univ. of Tennessee, --
145 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
146 * December 2016
147 *
148 * .. Scalar Arguments ..
149  INTEGER l, lda, m, n
150 * ..
151 * .. Array Arguments ..
152  DOUBLE PRECISION a( lda, * ), tau( * ), work( * )
153 * ..
154 *
155 * =====================================================================
156 *
157 * .. Parameters ..
158  DOUBLE PRECISION zero
159  parameter( zero = 0.0d+0 )
160 * ..
161 * .. Local Scalars ..
162  INTEGER i
163 * ..
164 * .. External Subroutines ..
165  EXTERNAL dlarfg, dlarz
166 * ..
167 * .. Executable Statements ..
168 *
169 * Test the input arguments
170 *
171 * Quick return if possible
172 *
173  IF( m.EQ.0 ) THEN
174  RETURN
175  ELSE IF( m.EQ.n ) THEN
176  DO 10 i = 1, n
177  tau( i ) = zero
178  10 CONTINUE
179  RETURN
180  END IF
181 *
182  DO 20 i = m, 1, -1
183 *
184 * Generate elementary reflector H(i) to annihilate
185 * [ A(i,i) A(i,n-l+1:n) ]
186 *
187  CALL dlarfg( l+1, a( i, i ), a( i, n-l+1 ), lda, tau( i ) )
188 *
189 * Apply H(i) to A(1:i-1,i:n) from the right
190 *
191  CALL dlarz( 'Right', i-1, n-i+1, l, a( i, n-l+1 ), lda,
192  $ tau( i ), a( 1, i ), lda, work )
193 *
194  20 CONTINUE
195 *
196  RETURN
197 *
198 * End of DLATRZ
199 *
subroutine dlarz(SIDE, M, N, L, V, INCV, TAU, C, LDC, WORK)
DLARZ applies an elementary reflector (as returned by stzrzf) to a general matrix.
Definition: dlarz.f:147
subroutine dlarfg(N, ALPHA, X, INCX, TAU)
DLARFG generates an elementary reflector (Householder matrix).
Definition: dlarfg.f:108
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