Provided by: scalapack-doc_1.5-10_all 
NAME
PSGEHD2 - reduce a real general distributed matrix sub( A ) to upper Hessenberg form H by
an orthogonal similarity transforma- tion
SYNOPSIS
SUBROUTINE PSGEHD2( N, ILO, IHI, A, IA, JA, DESCA, TAU, WORK, LWORK, INFO )
INTEGER IA, IHI, ILO, INFO, JA, LWORK, N
INTEGER DESCA( * )
REAL A( * ), TAU( * ), WORK( * )
PURPOSE
PSGEHD2 reduces a real general distributed matrix sub( A ) to upper Hessenberg form H by
an orthogonal similarity transforma- tion: Q' * sub( A ) * Q = H, where sub( A ) =
A(IA+N-1:IA+N-1,JA+N-1:JA+N-1).
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
N (global input) INTEGER
The number of rows and columns to be operated on, i.e. the order of the
distributed submatrix sub( A ). N >= 0.
ILO (global input) INTEGER
IHI (global input) INTEGER It is assumed that sub( A ) is already upper
triangular in rows IA:IA+ILO-2 and IA+IHI:IA+N-1 and columns JA:JA+JLO-2 and
JA+JHI:JA+N-1. See Further Details. If N > 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, this array
contains the local pieces of the N-by-N general distributed matrix sub( A ) to be
reduced. On exit, the upper triangle and the first subdiagonal of sub( A ) are
overwritten with the upper Hessenberg matrix H, and the ele- ments below the first
subdiagonal, with the array TAU, repre- sent the orthogonal matrix Q as a product
of elementary reflectors. See Further Details. 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 LOCc(JA+N-2)
The scalar factors of the elementary reflectors (see Further Details). Elements
JA:JA+ILO-2 and JA+IHI:JA+N-2 of TAU are set to zero. TAU is tied to the
distributed matrix A.
WORK (local workspace/local output) REAL array,
dimension (LWORK) On exit, WORK( 1 ) returns the minimal and optimal LWORK.
LWORK (local or global input) INTEGER
The dimension of the array WORK. LWORK is local input and must be at least LWORK
>= NB + MAX( NpA0, NB )
where NB = MB_A = NB_A, IROFFA = MOD( IA-1, NB ), IAROW = INDXG2P( IA, NB, MYROW,
RSRC_A, NPROW ), NpA0 = NUMROC( IHI+IROFFA, NB, MYROW, IAROW, NPROW ),
INDXG2P and NUMROC are ScaLAPACK tool functions; MYROW, MYCOL, NPROW and NPCOL can
be determined by calling the subroutine BLACS_GRIDINFO.
If LWORK = -1, then LWORK is global input and a workspace query is assumed; the
routine only calculates the minimum and optimal size for all work arrays. Each of
these values is returned in the first entry of the corresponding work array, and
no error message is issued by PXERBLA.
INFO (local output) INTEGER
= 0: successful exit
< 0: If the i-th argument is an array and the j-entry had an illegal value, then
INFO = -(i*100+j), if the i-th argument is a scalar and had an illegal value, then
INFO = -i.
FURTHER DETAILS
The matrix Q is represented as a product of (ihi-ilo) elementary reflectors
Q = H(ilo) H(ilo+1) . . . H(ihi-1).
Each H(i) has the form
H(i) = I - tau * v * v'
where tau is a real scalar, and v is a real vector with
v(1:i) = 0, v(i+1) = 1 and v(ihi+1:n) = 0; v(i+2:ihi) is stored on exit in
A(ia+ilo+i:ia+ihi-1,ja+ilo+i-2), and tau in TAU(ja+ilo+i-2).
The contents of A(IA:IA+N-1,JA:JA+N-1) are illustrated by the follo- wing example, with n
= 7, ilo = 2 and ihi = 6:
on entry on exit
( a a a a a a a ) ( a a h h h h a ) ( a a a a a
a ) ( a h h h h a ) ( a a a a a a ) ( h h h
h h h ) ( a a a a a a ) ( v2 h h h h h ) ( a a
a a a a ) ( v2 v3 h h h h ) ( a a a a a a ) (
v2 v3 v4 h h h ) ( a ) ( a )
where a denotes an element of the original matrix sub( A ), h denotes a modified element
of the upper Hessenberg matrix H, and vi denotes an element of the vector defining
H(ja+ilo+i-2).
Alignment requirements
======================
The distributed submatrix sub( A ) must verify some alignment proper- ties, namely the
following expression should be true:
( MB_A.EQ.NB_A .AND. IROFFA.EQ.ICOFFA )