zlatrz(3)

NAME

ZLATRZ - factor the M-by-(M+L) complex upper trapezoidal

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

by means of unitary transformations, where Z is an (M+L)-by-(M+L)

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

SYNOPSIS

SUBROUTINE ZLATRZ( M, N, L, A, LDA, TAU, WORK )
    INTEGER        L, LDA, M, N
    COMPLEX*16     A( LDA, * ), TAU( * ), WORK( * )

PURPOSE

ZLATRZ factors the M-by-(M+L) complex upper trapezoidal

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

by means of unitary transformations, where Z is an (M+L)-by-(M+L)

unitary matrix and, R and A1 are M-by-M upper triangular matri

ces.

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) COMPLEX*16 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 unitary 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) COMPLEX*16 array, dimension (M)

The scalar factors of the elementary reflectors.

WORK (workspace) COMPLEX*16 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 ).

LAPACK version 3.0 15 June 2000

ZLATRZ(3)