PDLABRD(l)

NAME
PDLABRD - reduce the first NB rows and columns of a real general M-by-N distributed matrix sub( A ) = A(IA:IA+M-1,JA:JA+N-1) to upper or lower bidiagonal form by an orthogonal transformation Q' * A * P,

SYNOPSIS

SUBROUTINE PDLABRD(
    M, N, NB, A, IA, JA, DESCA, D, E, TAUQ, TAUP, X, IX, JX, DESCX, Y, IY, JY, DESCY, WORK )

INTEGER
    IA, IX, IY, JA, JX, JY, M, N, NB

INTEGER
    DESCA( * ), DESCX( * ), DESCY( * )

DOUBLE
    PRECISION A( * ), D( * ), E( * ), TAUP( * ), TAUQ( * ), X( * ), Y( * ), WORK( * )

PURPOSE
PDLABRD reduces the first NB rows and columns of a real general M-by-N distributed matrix sub( A ) = A(IA:IA+M-1,JA:JA+N-1) to upper or lower bidiagonal form by an orthogonal transformation Q' * A * P, and returns the matrices X and Y which are needed to apply the transformation to the unreduced part of sub( A ).

If M >= N, sub( A ) is reduced to upper bidiagonal form; if M < N, to lower bidiagonal form.

This is an auxiliary routine called by PDGEBRD.

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.

NB      (global input) INTEGER
    The number of leading rows and columns of sub( A ) to be reduced.

A       (local input/local output) DOUBLE PRECISION 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 general distributed matrix sub( A ) to be reduced. On exit, the first NB rows and columns of the matrix are overwritten; the rest of the distributed matrix sub( A ) is unchanged. If m >= n, elements on and below the diagonal in the first NB columns, with the array TAUQ, represent the orthogonal matrix Q as a product of elementary reflectors; and elements above the diagonal in the first NB rows, with the array TAUP, represent the orthogonal matrix P as a product of elementary reflectors. If m < n, elements below the diagonal in the first NB columns, with the array TAUQ, represent the orthogonal matrix Q as a product of elementary reflectors, and elements on and above the diagonal in the first NB rows, with the array TAUP, represent the orthogonal matrix P 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.

D       (local output) DOUBLE PRECISION array, dimension
    LOCr(IA+MIN(M,N)-1) if M >= N; LOCc(JA+MIN(M,N)-1) otherwise. The distributed diagonal elements of the bidiagonal matrix B: D(i) = A(ia+i-1,ja+i-1). D is tied to the distributed matrix A.

E       (local output) DOUBLE PRECISION array, dimension
    LOCr(IA+MIN(M,N)-1) if M >= N; LOCc(JA+MIN(M,N)-2) otherwise. The distributed off-diagonal elements of the bidiagonal distributed matrix B: if m >= n, E(i) = A(ia+i-1,ja+i) for i = 1,2,...,n-1; if m < n, E(i) = A(ia+i,ja+i-1) for i = 1,2,...,m-1. E is tied to the distributed matrix A.

TAUQ    (local output) DOUBLE PRECISION array dimension
    LOCc(JA+MIN(M,N)-1). The scalar factors of the elementary reflectors which represent the orthogonal matrix Q. TAUQ is tied to the distributed matrix A. See Further Details. TAUP (local output) DOUBLE PRECISION array, dimension LOCr(IA+MIN(M,N)-1). The scalar factors of the elementary reflectors which represent the orthogonal matrix P. TAUP is tied to the distributed matrix A. See Further Details. X (local output) DOUBLE PRECISION pointer into the local memory to an array of dimension (LLD_X,NB). On exit, the local pieces of the distributed M-by-NB matrix X(IX:IX+M-1,JX:JX+NB-1) required to update the unreduced part of sub( A ).

IX      (global input) INTEGER
    The row index in the global array X indicating the first row of sub( X ).

JX      (global input) INTEGER
    The column index in the global array X indicating the first column of sub( X ).

DESCX   (global and local input) INTEGER array of dimension DLEN_.
    The array descriptor for the distributed matrix X.

Y       (local output) DOUBLE PRECISION pointer into the local memory
    to an array of dimension (LLD_Y,NB). On exit, the local pieces of the distributed N-by-NB matrix Y(IY:IY+N-1,JY:JY+NB-1) required to update the unreduced part of sub( A ).

IY      (global input) INTEGER
    The row index in the global array Y indicating the first row of sub( Y ).

JY      (global input) INTEGER
    The column index in the global array Y indicating the first column of sub( Y ).

DESCY   (global and local input) INTEGER array of dimension DLEN_.
    The array descriptor for the distributed matrix Y.

WORK    (local workspace) DOUBLE PRECISION array, dimension (LWORK)
    LWORK >= NB_A + NQ, with

    NQ = NUMROC( N+MOD( IA-1, NB_Y ), NB_Y, MYCOL, IACOL, NPCOL ) IACOL = INDXG2P( JA, NB_A, MYCOL, CSRC_A, NPCOL )

    INDXG2P and NUMROC are ScaLAPACK tool functions; MYROW, MYCOL, NPROW and NPCOL can be determined by calling the subroutine BLACS_GRIDINFO.

FURTHER DETAILS
The matrices Q and P are represented as products of elementary reflectors:

   Q = H(1) H(2) . . . H(nb) and P = G(1) G(2) . . . G(nb)

Each H(i) and G(i) has the form:

   H(i) = I - tauq * v * v' and G(i) = I - taup * u * u'

where tauq and taup are real scalars, and v and u are real vectors.

If m >= n, v(1:i-1) = 0, v(i) = 1, and v(i:m) is stored on exit in A(ia+i-1:ia+m-1,ja+i-1); u(1:i) = 0, u(i+1) = 1, and u(i+1:n) is stored on exit in A(ia+i-1,ja+i:ja+n-1); tauq is stored in TAUQ(ja+i-1) and taup in TAUP(ia+i-1).

If m < n, v(1:i) = 0, v(i+1) = 1, and v(i+1:m) is stored on exit in A(ia+i+1:ia+m-1,ja+i-1); u(1:i-1) = 0, u(i) = 1, and u(i:n) is stored on exit in A(ia+i-1,ja+i:ja+n-1); tauq is stored in TAUQ(ja+i-1) and taup in TAUP(ia+i-1).

The elements of the vectors v and u together form the m-by-nb matrix V and the nb-by-n matrix U' which are needed, with X and Y, to apply the transformation to the unreduced part of the matrix, using a block update of the form: sub( A ) := sub( A ) - V*Y' - X*U'.

The contents of sub( A ) on exit are illustrated by the following examples with nb = 2:

m = 6 and n = 5 (m > n): m = 5 and n = 6 (m < n):

  ( 1 1 u1 u1 u1 ) ( 1 u1 u1 u1 u1 u1 )
  ( v1 1 1 u2 u2 ) ( 1 1 u2 u2 u2 u2 )
  ( v1 v2 a a a ) ( v1 1 a a a a )
  ( v1 v2 a a a ) ( v1 v2 a a a a )
  ( v1 v2 a a a ) ( v1 v2 a a a a )
  ( v1 v2 a a a )

where a denotes an element of the original matrix which is unchanged, vi denotes an element of the vector defining H(i), and ui an element of the vector defining G(i).