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21 * SUBROUTINE CHETRD( UPLO, N, A, LDA, D, E, TAU, WORK, LWORK, INFO )
23 * .. Scalar Arguments ..
25 * INTEGER INFO, LDA, LWORK, N
27 * .. Array Arguments ..
29 * COMPLEX A( LDA, * ), TAU( * ), WORK( * )
38 *> CHETRD reduces a complex Hermitian matrix A to real symmetric
39 *> tridiagonal form T by a unitary similarity transformation:
48 *> UPLO is CHARACTER*1
49 *> = 'U': Upper triangle of A is stored;
50 *> = 'L': Lower triangle of A is stored.
56 *> The order of the matrix A. N >= 0.
61 *> A is COMPLEX array, dimension (LDA,N)
62 *> On entry, the Hermitian matrix A. If UPLO = 'U', the leading
63 *> N-by-N upper triangular part of A contains the upper
64 *> triangular part of the matrix A, and the strictly lower
65 *> triangular part of A is not referenced. If UPLO = 'L', the
66 *> leading N-by-N lower triangular part of A contains the lower
67 *> triangular part of the matrix A, and the strictly upper
68 *> triangular part of A is not referenced.
69 *> On exit, if UPLO = 'U', the diagonal and first superdiagonal
70 *> of A are overwritten by the corresponding elements of the
71 *> tridiagonal matrix T, and the elements above the first
72 *> superdiagonal, with the array TAU, represent the unitary
73 *> matrix Q as a product of elementary reflectors; if UPLO
74 *> = 'L', the diagonal and first subdiagonal of A are over-
75 *> written by the corresponding elements of the tridiagonal
76 *> matrix T, and the elements below the first subdiagonal, with
77 *> the array TAU, represent the unitary matrix Q as a product
78 *> of elementary reflectors. See Further Details.
84 *> The leading dimension of the array A. LDA >= max(1,N).
89 *> D is REAL array, dimension (N)
90 *> The diagonal elements of the tridiagonal matrix T:
96 *> E is REAL array, dimension (N-1)
97 *> The off-diagonal elements of the tridiagonal matrix T:
98 *> E(i) = A(i,i+1) if UPLO = 'U', E(i) = A(i+1,i) if UPLO = 'L'.
103 *> TAU is COMPLEX array, dimension (N-1)
104 *> The scalar factors of the elementary reflectors (see Further
110 *> WORK is COMPLEX array, dimension (MAX(1,LWORK))
111 *> On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
117 *> The dimension of the array WORK. LWORK >= 1.
118 *> For optimum performance LWORK >= N*NB, where NB is the
119 *> optimal blocksize.
121 *> If LWORK = -1, then a workspace query is assumed; the routine
122 *> only calculates the optimal size of the WORK array, returns
123 *> this value as the first entry of the WORK array, and no error
124 *> message related to LWORK is issued by XERBLA.
130 *> = 0: successful exit
131 *> < 0: if INFO = -i, the i-th argument had an illegal value
137 *> \author Univ. of Tennessee
138 *> \author Univ. of California Berkeley
139 *> \author Univ. of Colorado Denver
142 *> \date November 2011
144 *> \ingroup complexHEcomputational
146 *> \par Further Details:
147 * =====================
151 *> If UPLO = 'U', the matrix Q is represented as a product of elementary
154 *> Q = H(n-1) . . . H(2) H(1).
156 *> Each H(i) has the form
158 *> H(i) = I - tau * v * v**H
160 *> where tau is a complex scalar, and v is a complex vector with
161 *> v(i+1:n) = 0 and v(i) = 1; v(1:i-1) is stored on exit in
162 *> A(1:i-1,i+1), and tau in TAU(i).
164 *> If UPLO = 'L', the matrix Q is represented as a product of elementary
167 *> Q = H(1) H(2) . . . H(n-1).
169 *> Each H(i) has the form
171 *> H(i) = I - tau * v * v**H
173 *> where tau is a complex scalar, and v is a complex vector with
174 *> v(1:i) = 0 and v(i+1) = 1; v(i+2:n) is stored on exit in A(i+2:n,i),
175 *> and tau in TAU(i).
177 *> The contents of A on exit are illustrated by the following examples
180 *> if UPLO = 'U': if UPLO = 'L':
182 *> ( d e v2 v3 v4 ) ( d )
183 *> ( d e v3 v4 ) ( e d )
184 *> ( d e v4 ) ( v1 e d )
185 *> ( d e ) ( v1 v2 e d )
186 *> ( d ) ( v1 v2 v3 e d )
188 *> where d and e denote diagonal and off-diagonal elements of T, and vi
189 *> denotes an element of the vector defining H(i).
192 * =====================================================================
193 SUBROUTINE CHETRD( UPLO, N, A, LDA, D, E, TAU, WORK, LWORK, INFO )
195 * -- LAPACK computational routine (version 3.4.0) --
196 * -- LAPACK is a software package provided by Univ. of Tennessee, --
197 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
200 * .. Scalar Arguments ..
202 INTEGER INFO, LDA, LWORK, N
204 * .. Array Arguments ..
206 COMPLEX A( LDA, * ), TAU( * ), WORK( * )
209 * =====================================================================
213 PARAMETER ( ONE = 1.0E+0 )
215 PARAMETER ( CONE = ( 1.0E+0, 0.0E+0 ) )
217 * .. Local Scalars ..
218 LOGICAL LQUERY, UPPER
219 INTEGER I, IINFO, IWS, J, KK, LDWORK, LWKOPT, NB,
222 * .. External Subroutines ..
223 EXTERNAL CHER2K, CHETD2, CLATRD, XERBLA
225 * .. Intrinsic Functions ..
228 * .. External Functions ..
231 EXTERNAL LSAME, ILAENV
233 * .. Executable Statements ..
235 * Test the input parameters
238 UPPER = LSAME( UPLO, 'U' )
239 LQUERY = ( LWORK.EQ.-1 )
240 IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
242 ELSE IF( N.LT.0 ) THEN
244 ELSE IF( LDA.LT.MAX( 1, N ) ) THEN
246 ELSE IF( LWORK.LT.1 .AND. .NOT.LQUERY ) THEN
252 * Determine the block size.
254 NB = ILAENV( 1, 'CHETRD', UPLO, N, -1, -1, -1 )
260 CALL XERBLA( 'CHETRD', -INFO )
262 ELSE IF( LQUERY ) THEN
266 * Quick return if possible
275 IF( NB.GT.1 .AND. NB.LT.N ) THEN
277 * Determine when to cross over from blocked to unblocked code
278 * (last block is always handled by unblocked code).
280 NX = MAX( NB, ILAENV( 3, 'CHETRD', UPLO, N, -1, -1, -1 ) )
283 * Determine if workspace is large enough for blocked code.
287 IF( LWORK.LT.IWS ) THEN
289 * Not enough workspace to use optimal NB: determine the
290 * minimum value of NB, and reduce NB or force use of
291 * unblocked code by setting NX = N.
293 NB = MAX( LWORK / LDWORK, 1 )
294 NBMIN = ILAENV( 2, 'CHETRD', UPLO, N, -1, -1, -1 )
307 * Reduce the upper triangle of A.
308 * Columns 1:kk are handled by the unblocked method.
310 KK = N - ( ( N-NX+NB-1 ) / NB )*NB
311 DO 20 I = N - NB + 1, KK + 1, -NB
313 * Reduce columns i:i+nb-1 to tridiagonal form and form the
314 * matrix W which is needed to update the unreduced part of
317 CALL CLATRD( UPLO, I+NB-1, NB, A, LDA, E, TAU, WORK,
320 * Update the unreduced submatrix A(1:i-1,1:i-1), using an
321 * update of the form: A := A - V*W**H - W*V**H
323 CALL CHER2K( UPLO, 'No transpose', I-1, NB, -CONE,
324 $ A( 1, I ), LDA, WORK, LDWORK, ONE, A, LDA )
326 * Copy superdiagonal elements back into A, and diagonal
329 DO 10 J = I, I + NB - 1
330 A( J-1, J ) = E( J-1 )
335 * Use unblocked code to reduce the last or only block
337 CALL CHETD2( UPLO, KK, A, LDA, D, E, TAU, IINFO )
340 * Reduce the lower triangle of A
342 DO 40 I = 1, N - NX, NB
344 * Reduce columns i:i+nb-1 to tridiagonal form and form the
345 * matrix W which is needed to update the unreduced part of
348 CALL CLATRD( UPLO, N-I+1, NB, A( I, I ), LDA, E( I ),
349 $ TAU( I ), WORK, LDWORK )
351 * Update the unreduced submatrix A(i+nb:n,i+nb:n), using
352 * an update of the form: A := A - V*W**H - W*V**H
354 CALL CHER2K( UPLO, 'No transpose', N-I-NB+1, NB, -CONE,
355 $ A( I+NB, I ), LDA, WORK( NB+1 ), LDWORK, ONE,
356 $ A( I+NB, I+NB ), LDA )
358 * Copy subdiagonal elements back into A, and diagonal
361 DO 30 J = I, I + NB - 1
367 * Use unblocked code to reduce the last or only block
369 CALL CHETD2( UPLO, N-I+1, A( I, I ), LDA, D( I ), E( I ),