Provided by: scalapack-doc_1.5-10_all 
NAME
PSLATRZ - reduce the M-by-N ( M<=N ) real upper trapezoidal matrix sub( A ) = [
A(IA:IA+M-1,JA:JA+M-1) A(IA:IA+M-1,JA+N-L:JA+N-1) ] to upper triangular form by means of
orthogonal transformations
SYNOPSIS
SUBROUTINE PSLATRZ( M, N, L, A, IA, JA, DESCA, TAU, WORK )
INTEGER IA, JA, L, M, N
INTEGER DESCA( * )
REAL A( * ), TAU( * ), WORK( * )
PURPOSE
PSLATRZ reduces the M-by-N ( M<=N ) real upper trapezoidal matrix sub( A ) = [
A(IA:IA+M-1,JA:JA+M-1) A(IA:IA+M-1,JA+N-L:JA+N-1) ] to upper triangular form by means of
orthogonal transformations.
The upper trapezoidal matrix sub( A ) is factored as
sub( A ) = ( R 0 ) * Z,
where Z is an N-by-N orthogonal matrix and R is an M-by-M upper triangular matrix.
Notes
=====
Each global data object is described by an associated description vector. This vector
stores the information required to establish the mapping between an object element and its
corresponding process and memory location.
Let A be a generic term for any 2D block cyclicly distributed array. Such a global array
has an associated description vector DESCA. In the following comments, the character _
should be read as "of the global array".
NOTATION STORED IN EXPLANATION
--------------- -------------- -------------------------------------- DTYPE_A(global)
DESCA( DTYPE_ )The descriptor type. In this case,
DTYPE_A = 1.
CTXT_A (global) DESCA( CTXT_ ) The BLACS context handle, indicating
the BLACS process grid A is distribu-
ted over. The context itself is glo-
bal, but the handle (the integer
value) may vary.
M_A (global) DESCA( M_ ) The number of rows in the global
array A.
N_A (global) DESCA( N_ ) The number of columns in the global
array A.
MB_A (global) DESCA( MB_ ) The blocking factor used to distribute
the rows of the array.
NB_A (global) DESCA( NB_ ) The blocking factor used to distribute
the columns of the array.
RSRC_A (global) DESCA( RSRC_ ) The process row over which the first
row of the array A is distributed. CSRC_A (global) DESCA(
CSRC_ ) The process column over which the
first column of the array A is
distributed.
LLD_A (local) DESCA( LLD_ ) The leading dimension of the local
array. LLD_A >= MAX(1,LOCr(M_A)).
Let K be the number of rows or columns of a distributed matrix, and assume that its
process grid has dimension p x q.
LOCr( K ) denotes the number of elements of K that a process would receive if K were
distributed over the p processes of its process column.
Similarly, LOCc( K ) denotes the number of elements of K that a process would receive if K
were distributed over the q processes of its process row.
The values of LOCr() and LOCc() may be determined via a call to the ScaLAPACK tool
function, NUMROC:
LOCr( M ) = NUMROC( M, MB_A, MYROW, RSRC_A, NPROW ),
LOCc( N ) = NUMROC( N, NB_A, MYCOL, CSRC_A, NPCOL ). An upper bound for these
quantities may be computed by:
LOCr( M ) <= ceil( ceil(M/MB_A)/NPROW )*MB_A
LOCc( N ) <= ceil( ceil(N/NB_A)/NPCOL )*NB_A
ARGUMENTS
M (global input) INTEGER
The number of rows to be operated on, i.e. the number of rows of the distributed
submatrix sub( A ). M >= 0.
N (global input) INTEGER
The number of columns to be operated on, i.e. the number of columns of the
distributed submatrix sub( A ). N >= 0.
L (global input) INTEGER
The columns of the distributed submatrix sub( A ) containing the meaningful part
of the Householder reflectors. L > 0.
A (local input/local output) REAL pointer into the
local memory to an array of dimension (LLD_A, LOCc(JA+N-1)). On entry, the local
pieces of the M-by-N distributed matrix sub( A ) which is to be factored. On exit,
the leading M-by-M upper triangular part of sub( A ) contains the upper trian-
gular matrix R, and elements N-L+1 to N of the first M rows of sub( A ), with the
array TAU, represent the orthogonal matrix Z as a product of M elementary
reflectors.
IA (global input) INTEGER
The row index in the global array A indicating the first row of sub( A ).
JA (global input) INTEGER
The column index in the global array A indicating the first column of sub( A ).
DESCA (global and local input) INTEGER array of dimension DLEN_.
The array descriptor for the distributed matrix A.
TAU (local output) REAL, array, dimension LOCr(IA+M-1)
This array contains the scalar factors of the elementary reflectors. TAU is tied
to the distributed matrix A.
WORK (local workspace) REAL array, dimension (LWORK)
LWORK >= Nq0 + MAX( 1, Mp0 ), where
IROFF = MOD( IA-1, MB_A ), ICOFF = MOD( JA-1, NB_A ), IAROW = INDXG2P( IA, MB_A,
MYROW, RSRC_A, NPROW ), IACOL = INDXG2P( JA, NB_A, MYCOL, CSRC_A, NPCOL ), Mp0 =
NUMROC( M+IROFF, MB_A, MYROW, IAROW, NPROW ), Nq0 = NUMROC( N+ICOFF, NB_A,
MYCOL, IACOL, NPCOL ),
and NUMROC, INDXG2P are ScaLAPACK tool functions; MYROW, MYCOL, NPROW and NPCOL
can be determined by calling the subroutine BLACS_GRIDINFO.
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 sub( 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 ( n - m ) element vector. tau and z( k ) are chosen to
annihilate the elements of the kth row of sub( A ).
The scalar tau is returned in the kth element of TAU and the vector u( k ) in the kth row
of sub( 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 sub( A ).
Z is given by
Z = Z( 1 ) * Z( 2 ) * ... * Z( m ).