ZLAHRD(3f)

NAME
ZLAHRD - the first NB columns of a complex general n-by-(n-k+1) matrix A so that elements below the k-th subdiagonal are zero

SYNOPSIS

SUBROUTINE ZLAHRD(
    N, K, NB, A, LDA, TAU, T, LDT, Y, LDY )

INTEGER
    K, LDA, LDT, LDY, N, NB

COMPLEX*16
    A( LDA, * ), T( LDT, NB ), TAU( NB ), Y( LDY, NB )

PURPOSE
ZLAHRD reduces the first NB columns of a complex general n-by-(n-k+1) matrix A so that elements below the k-th subdiagonal are zero. The reduction is performed by a unitary 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 OBSOLETE auxiliary routine.
This routine will be 'deprecated' in a future release.
Please use the new routine ZLAHR2 instead.

ARGUMENTS

N       (input) INTEGER
    The order of the matrix A.

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

NB      (input) INTEGER
    The number of columns to be reduced.

A       (input/output) COMPLEX*16 array, dimension (LDA,N-K+1)
    On entry, the n-by-(n-k+1) general matrix A. 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 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 are unchanged. See Further Details. LDA (input) INTEGER The leading dimension of the array A. LDA >= max(1,N).

TAU     (output) COMPLEX*16 array, dimension (NB)
    The scalar factors of the elementary reflectors. See Further Details.

T       (output) COMPLEX*16 array, dimension (LDT,NB)
    The upper triangular matrix T.

LDT     (input) INTEGER
    The leading dimension of the array T. LDT >= NB.

Y       (output) COMPLEX*16 array, dimension (LDY,NB)
    The n-by-nb matrix Y.

LDY     (input) INTEGER
    The leading dimension of the array Y. LDY >= max(1,N).

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 complex scalar, and v is a complex vector with v(1:i+k-1) = 0, v(i+k) = 1; v(i+k+1:n) is stored on exit in A(i+k+1:n,i), and tau in TAU(i).
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 := (I - V*T*V') * (A - Y*V').
The contents of A 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, h denotes a modified element of the upper Hessenberg matrix H, and vi denotes an element of the vector defining H(i).