slahrd (3)

Functions/Subroutines

subroutine slahrd (N, K, NB, A, LDA, TAU, T, LDT, Y, LDY)

SLAHRD reduces the first nb columns of a general rectangular matrix A so that elements below the k-th subdiagonal are zero, and returns auxiliary matrices which are needed to apply the transformation to the unreduced part of A.

subroutine slahrd (integerN, integerK, integerNB, real, dimension( lda, * )A, integerLDA, real, dimension( nb )TAU, real, dimension( ldt, nb )T, integerLDT, real, dimension( ldy, nb )Y, integerLDY)

SLAHRD reduces the first nb columns of a general rectangular matrix A so that elements below the k-th subdiagonal are zero, and returns auxiliary matrices which are needed to apply the transformation to the unreduced part of A.

Purpose:

 SLAHRD reduces the first NB columns of a real general n-by-(n-k+1)
 matrix A so that elements below the k-th subdiagonal are zero. The
 reduction is performed by an orthogonal similarity transformation
 Q**T * A * Q. The routine returns the matrices V and T which determine
 Q as a block reflector I - V*T*V**T, 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 SLAHR2 instead.

Parameters:

N

          N is INTEGER
          The order of the matrix A.

K

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

NB

          NB is INTEGER
          The number of columns to be reduced.

A

          A is REAL 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

          LDA is INTEGER
          The leading dimension of the array A.  LDA >= max(1,N).

TAU

          TAU is REAL array, dimension (NB)
          The scalar factors of the elementary reflectors. See Further
          Details.

T

          T is REAL array, dimension (LDT,NB)
          The upper triangular matrix T.

LDT

          LDT is INTEGER
          The leading dimension of the array T.  LDT >= NB.

Y

          Y is REAL array, dimension (LDY,NB)
          The n-by-nb matrix Y.

LDY

          LDY is INTEGER
          The leading dimension of the array Y. LDY >= N.

Author:

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Date:

September 2012

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**T

  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(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**T) * (A - Y*V**T).

  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).

Definition at line 170 of file slahrd.f.

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