SUBROUTINE DLAHRD( N, K, NB, A, LDA, TAU, T, LDT, Y, LDY )
*
* -- LAPACK auxiliary routine (version 3.1) --
* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
* November 2006
*
* .. Scalar Arguments ..
INTEGER K, LDA, LDT, LDY, N, NB
* ..
* .. Array Arguments ..
DOUBLE PRECISION A( LDA, * ), T( LDT, NB ), TAU( NB ),
$ Y( LDY, NB )
* ..
*
* Purpose
* =======
*
* DLAHRD 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' * 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 DLAHR2 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) DOUBLE PRECISION 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) DOUBLE PRECISION array, dimension (NB)
* The scalar factors of the elementary reflectors. See Further
* Details.
*
* T (output) DOUBLE PRECISION array, dimension (LDT,NB)
* The upper triangular matrix T.
*
* LDT (input) INTEGER
* The leading dimension of the array T. LDT >= NB.
*
* Y (output) DOUBLE PRECISION array, dimension (LDY,NB)
* The n-by-nb matrix Y.
*
* LDY (input) INTEGER
* The leading dimension of the array Y. LDY >= 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 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') * (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).
*
* =====================================================================
*
* .. Parameters ..
DOUBLE PRECISION ZERO, ONE
PARAMETER ( ZERO = 0.0D+0, ONE = 1.0D+0 )
* ..
* .. Local Scalars ..
INTEGER I
DOUBLE PRECISION EI
* ..
* .. External Subroutines ..
EXTERNAL DAXPY, DCOPY, DGEMV, DLARFG, DSCAL, DTRMV
* ..
* .. Intrinsic Functions ..
INTRINSIC MIN
* ..
* .. Executable Statements ..
*
* Quick return if possible
*
IF( N.LE.1 )
$ RETURN
*
DO 10 I = 1, NB
IF( I.GT.1 ) THEN
*
* Update A(1:n,i)
*
* Compute i-th column of A - Y * V'
*
CALL DGEMV( 'No transpose', N, I-1, -ONE, Y, LDY,
$ A( K+I-1, 1 ), LDA, ONE, A( 1, I ), 1 )
*
* Apply I - V * T' * V' to this column (call it b) from the
* left, using the last column of T as workspace
*
* Let V = ( V1 ) and b = ( b1 ) (first I-1 rows)
* ( V2 ) ( b2 )
*
* where V1 is unit lower triangular
*
* w := V1' * b1
*
CALL DCOPY( I-1, A( K+1, I ), 1, T( 1, NB ), 1 )
CALL DTRMV( 'Lower', 'Transpose', 'Unit', I-1, A( K+1, 1 ),
$ LDA, T( 1, NB ), 1 )
*
* w := w + V2'*b2
*
CALL DGEMV( 'Transpose', N-K-I+1, I-1, ONE, A( K+I, 1 ),
$ LDA, A( K+I, I ), 1, ONE, T( 1, NB ), 1 )
*
* w := T'*w
*
CALL DTRMV( 'Upper', 'Transpose', 'Non-unit', I-1, T, LDT,
$ T( 1, NB ), 1 )
*
* b2 := b2 - V2*w
*
CALL DGEMV( 'No transpose', N-K-I+1, I-1, -ONE, A( K+I, 1 ),
$ LDA, T( 1, NB ), 1, ONE, A( K+I, I ), 1 )
*
* b1 := b1 - V1*w
*
CALL DTRMV( 'Lower', 'No transpose', 'Unit', I-1,
$ A( K+1, 1 ), LDA, T( 1, NB ), 1 )
CALL DAXPY( I-1, -ONE, T( 1, NB ), 1, A( K+1, I ), 1 )
*
A( K+I-1, I-1 ) = EI
END IF
*
* Generate the elementary reflector H(i) to annihilate
* A(k+i+1:n,i)
*
CALL DLARFG( N-K-I+1, A( K+I, I ), A( MIN( K+I+1, N ), I ), 1,
$ TAU( I ) )
EI = A( K+I, I )
A( K+I, I ) = ONE
*
* Compute Y(1:n,i)
*
CALL DGEMV( 'No transpose', N, N-K-I+1, ONE, A( 1, I+1 ), LDA,
$ A( K+I, I ), 1, ZERO, Y( 1, I ), 1 )
CALL DGEMV( 'Transpose', N-K-I+1, I-1, ONE, A( K+I, 1 ), LDA,
$ A( K+I, I ), 1, ZERO, T( 1, I ), 1 )
CALL DGEMV( 'No transpose', N, I-1, -ONE, Y, LDY, T( 1, I ), 1,
$ ONE, Y( 1, I ), 1 )
CALL DSCAL( N, TAU( I ), Y( 1, I ), 1 )
*
* Compute T(1:i,i)
*
CALL DSCAL( I-1, -TAU( I ), T( 1, I ), 1 )
CALL DTRMV( 'Upper', 'No transpose', 'Non-unit', I-1, T, LDT,
$ T( 1, I ), 1 )
T( I, I ) = TAU( I )
*
10 CONTINUE
A( K+NB, NB ) = EI
*
RETURN
*
* End of DLAHRD
*
END