1 *> \brief \b ZLAHR2 reduces the specified number of first columns of a general rectangular matrix A so that elements below the specified subdiagonal are zero, and returns auxiliary matrices which are needed to apply the transformation to the unreduced part of A.
3 * =========== DOCUMENTATION ===========
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21 * SUBROUTINE ZLAHR2( N, K, NB, A, LDA, TAU, T, LDT, Y, LDY )
23 * .. Scalar Arguments ..
24 * INTEGER K, LDA, LDT, LDY, N, NB
26 * .. Array Arguments ..
27 * COMPLEX*16 A( LDA, * ), T( LDT, NB ), TAU( NB ),
37 *> ZLAHR2 reduces the first NB columns of A complex general n-BY-(n-k+1)
38 *> matrix A so that elements below the k-th subdiagonal are zero. The
39 *> reduction is performed by an unitary similarity transformation
40 *> Q**H * A * Q. The routine returns the matrices V and T which determine
41 *> Q as a block reflector I - V*T*V**H, and also the matrix Y = A * V * T.
43 *> This is an auxiliary routine called by ZGEHRD.
52 *> The order of the matrix A.
58 *> The offset for the reduction. Elements below the k-th
59 *> subdiagonal in the first NB columns are reduced to zero.
66 *> The number of columns to be reduced.
71 *> A is COMPLEX*16 array, dimension (LDA,N-K+1)
72 *> On entry, the n-by-(n-k+1) general matrix A.
73 *> On exit, the elements on and above the k-th subdiagonal in
74 *> the first NB columns are overwritten with the corresponding
75 *> elements of the reduced matrix; the elements below the k-th
76 *> subdiagonal, with the array TAU, represent the matrix Q as a
77 *> product of elementary reflectors. The other columns of A are
78 *> unchanged. See Further Details.
84 *> The leading dimension of the array A. LDA >= max(1,N).
89 *> TAU is COMPLEX*16 array, dimension (NB)
90 *> The scalar factors of the elementary reflectors. See Further
96 *> T is COMPLEX*16 array, dimension (LDT,NB)
97 *> The upper triangular matrix T.
103 *> The leading dimension of the array T. LDT >= NB.
108 *> Y is COMPLEX*16 array, dimension (LDY,NB)
109 *> The n-by-nb matrix Y.
115 *> The leading dimension of the array Y. LDY >= N.
121 *> \author Univ. of Tennessee
122 *> \author Univ. of California Berkeley
123 *> \author Univ. of Colorado Denver
126 *> \date September 2012
128 *> \ingroup complex16OTHERauxiliary
130 *> \par Further Details:
131 * =====================
135 *> The matrix Q is represented as a product of nb elementary reflectors
137 *> Q = H(1) H(2) . . . H(nb).
139 *> Each H(i) has the form
141 *> H(i) = I - tau * v * v**H
143 *> where tau is a complex scalar, and v is a complex vector with
144 *> v(1:i+k-1) = 0, v(i+k) = 1; v(i+k+1:n) is stored on exit in
145 *> A(i+k+1:n,i), and tau in TAU(i).
147 *> The elements of the vectors v together form the (n-k+1)-by-nb matrix
148 *> V which is needed, with T and Y, to apply the transformation to the
149 *> unreduced part of the matrix, using an update of the form:
150 *> A := (I - V*T*V**H) * (A - Y*V**H).
152 *> The contents of A on exit are illustrated by the following example
153 *> with n = 7, k = 3 and nb = 2:
163 *> where a denotes an element of the original matrix A, h denotes a
164 *> modified element of the upper Hessenberg matrix H, and vi denotes an
165 *> element of the vector defining H(i).
167 *> This subroutine is a slight modification of LAPACK-3.0's DLAHRD
168 *> incorporating improvements proposed by Quintana-Orti and Van de
169 *> Gejin. Note that the entries of A(1:K,2:NB) differ from those
170 *> returned by the original LAPACK-3.0's DLAHRD routine. (This
171 *> subroutine is not backward compatible with LAPACK-3.0's DLAHRD.)
177 *> Gregorio Quintana-Orti and Robert van de Geijn, "Improving the
178 *> performance of reduction to Hessenberg form," ACM Transactions on
179 *> Mathematical Software, 32(2):180-194, June 2006.
181 * =====================================================================
182 SUBROUTINE ZLAHR2( N, K, NB, A, LDA, TAU, T, LDT, Y, LDY )
184 * -- LAPACK auxiliary routine (version 3.4.2) --
185 * -- LAPACK is a software package provided by Univ. of Tennessee, --
186 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
189 * .. Scalar Arguments ..
190 INTEGER K, LDA, LDT, LDY, N, NB
192 * .. Array Arguments ..
193 COMPLEX*16 A( LDA, * ), T( LDT, NB ), TAU( NB ),
197 * =====================================================================
201 PARAMETER ( ZERO = ( 0.0D+0, 0.0D+0 ),
202 $ ONE = ( 1.0D+0, 0.0D+0 ) )
204 * .. Local Scalars ..
208 * .. External Subroutines ..
209 EXTERNAL ZAXPY, ZCOPY, ZGEMM, ZGEMV, ZLACPY,
210 $ ZLARFG, ZSCAL, ZTRMM, ZTRMV, ZLACGV
212 * .. Intrinsic Functions ..
215 * .. Executable Statements ..
217 * Quick return if possible
227 * Update I-th column of A - Y * V**H
229 CALL ZLACGV( I-1, A( K+I-1, 1 ), LDA )
230 CALL ZGEMV( 'NO TRANSPOSE', N-K, I-1, -ONE, Y(K+1,1), LDY,
231 $ A( K+I-1, 1 ), LDA, ONE, A( K+1, I ), 1 )
232 CALL ZLACGV( I-1, A( K+I-1, 1 ), LDA )
234 * Apply I - V * T**H * V**H to this column (call it b) from the
235 * left, using the last column of T as workspace
237 * Let V = ( V1 ) and b = ( b1 ) (first I-1 rows)
240 * where V1 is unit lower triangular
244 CALL ZCOPY( I-1, A( K+1, I ), 1, T( 1, NB ), 1 )
245 CALL ZTRMV( 'Lower', 'Conjugate transpose', 'UNIT',
247 $ LDA, T( 1, NB ), 1 )
249 * w := w + V2**H * b2
251 CALL ZGEMV( 'Conjugate transpose', N-K-I+1, I-1,
253 $ LDA, A( K+I, I ), 1, ONE, T( 1, NB ), 1 )
257 CALL ZTRMV( 'Upper', 'Conjugate transpose', 'NON-UNIT',
263 CALL ZGEMV( 'NO TRANSPOSE', N-K-I+1, I-1, -ONE,
265 $ LDA, T( 1, NB ), 1, ONE, A( K+I, I ), 1 )
269 CALL ZTRMV( 'Lower', 'NO TRANSPOSE',
271 $ A( K+1, 1 ), LDA, T( 1, NB ), 1 )
272 CALL ZAXPY( I-1, -ONE, T( 1, NB ), 1, A( K+1, I ), 1 )
277 * Generate the elementary reflector H(I) to annihilate
280 CALL ZLARFG( N-K-I+1, A( K+I, I ), A( MIN( K+I+1, N ), I ), 1,
287 CALL ZGEMV( 'NO TRANSPOSE', N-K, N-K-I+1,
288 $ ONE, A( K+1, I+1 ),
289 $ LDA, A( K+I, I ), 1, ZERO, Y( K+1, I ), 1 )
290 CALL ZGEMV( 'Conjugate transpose', N-K-I+1, I-1,
291 $ ONE, A( K+I, 1 ), LDA,
292 $ A( K+I, I ), 1, ZERO, T( 1, I ), 1 )
293 CALL ZGEMV( 'NO TRANSPOSE', N-K, I-1, -ONE,
295 $ T( 1, I ), 1, ONE, Y( K+1, I ), 1 )
296 CALL ZSCAL( N-K, TAU( I ), Y( K+1, I ), 1 )
300 CALL ZSCAL( I-1, -TAU( I ), T( 1, I ), 1 )
301 CALL ZTRMV( 'Upper', 'No Transpose', 'NON-UNIT',
309 * Compute Y(1:K,1:NB)
311 CALL ZLACPY( 'ALL', K, NB, A( 1, 2 ), LDA, Y, LDY )
312 CALL ZTRMM( 'RIGHT', 'Lower', 'NO TRANSPOSE',
314 $ ONE, A( K+1, 1 ), LDA, Y, LDY )
316 $ CALL ZGEMM( 'NO TRANSPOSE', 'NO TRANSPOSE', K,
318 $ A( 1, 2+NB ), LDA, A( K+1+NB, 1 ), LDA, ONE, Y,
320 CALL ZTRMM( 'RIGHT', 'Upper', 'NO TRANSPOSE',
322 $ ONE, T, LDT, Y, LDY )