pslahrd3

Usage

Purpose

PSLAHRD reduces the first NB columns of a real general N-by-(N-K+1) distributed matrix A(IA:IA+N-1,JA:JA+N-K) so that elements below the k-th subdiagonal are zero. The reduction is performed by an orthogo- nal similarity transformation Q' * A * Q. The routine returns the matrices V and T which determine Q as a block reflector I - V*T*V', and also the matrix Y = A * V * T.

This is an auxiliary routine called by PSGEHRD. In the following comments sub( A ) denotes A(IA:IA+N-1,JA:JA+N-1).

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.

K (global input) INTEGER

The offset for the reduction. Elements below the k-th subdiagonal in the first NB columns are reduced to zero.

NB (global input) INTEGER

The number of columns to be reduced.

A (local input/local output) REAL pointer into

the local memory to an array of dimension (LLD_A, LOCc(JA+N-K)). On entry, this array contains the the local pieces of the N-by-(N-K+1) general distributed matrix A(IA:IA+N-1,JA:JA+N-K). On exit, the elements on and above the k-th subdiagonal in the first NB columns are overwritten with the corresponding elements of the reduced distributed matrix; the elements below the k-th subdiagonal, with the array TAU, represent the matrix Q as a product of elementary reflectors. The other columns of A(IA:IA+N-1,JA:JA+N-K) are unchanged. 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). TAU is tied to the distributed matrix A.

T (local output) REAL array, dimension (NB_A,NB_A)

The upper triangular matrix T.

Y (local output) REAL pointer into the local memory

to an array of dimension (LLD_Y,NB_A). On exit, this array contains the local pieces of the N-by-NB distributed matrix Y. LLD_Y >= LOCr(IA+N-1).

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) REAL array, dimension (NB)

Further details

The matrix Q is represented as a product of nb elementary reflectors

Q = H(1) H(2) . . . H(nb).

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+k-1) = 0, v(i+k) = 1; v(i+k+1:n) is stored on exit in A(ia+i+k:ia+n-1,ja+i-1), and tau in TAU(ja+i-1).

The elements of the vectors v together form the (n-k+1)-by-nb matrix V which is needed, with T and Y, to apply the transformation to the unreduced part of the matrix, using an update of the form: A(ia:ia+n-1,ja:ja+n-k) := (I-V*T*V')*(A(ia:ia+n-1,ja:ja+n-k)-Y*V').

The contents of A(ia:ia+n-1,ja:ja+n-k) on exit are illustrated by the following example with n = 7, k = 3 and nb = 2:

( a h a a a )

( a h a a a )

( a h a a a )

( h h a a a )

( v1 h a a a )

( v1 v2 a a a )

( v1 v2 a a a )

where a denotes an element of the original matrix
A(ia:ia+n-1,ja:ja+n-k), h denotes a modified element of the upper Hessenberg matrix H, and vi denotes an element of the vector defining H(i).