LAPACK  3.5.0
LAPACK: Linear Algebra PACKage
 All Classes Files Functions Variables Typedefs Macros
zgebd2.f
Go to the documentation of this file.
1 *> \brief \b ZGEBD2 reduces a general matrix to bidiagonal form using an unblocked algorithm.
2 *
3 * =========== DOCUMENTATION ===========
4 *
5 * Online html documentation available at
6 * http://www.netlib.org/lapack/explore-html/
7 *
8 *> \htmlonly
9 *> Download ZGEBD2 + dependencies
10 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/zgebd2.f">
11 *> [TGZ]</a>
12 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/zgebd2.f">
13 *> [ZIP]</a>
14 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/zgebd2.f">
15 *> [TXT]</a>
16 *> \endhtmlonly
17 *
18 * Definition:
19 * ===========
20 *
21 * SUBROUTINE ZGEBD2( M, N, A, LDA, D, E, TAUQ, TAUP, WORK, INFO )
22 *
23 * .. Scalar Arguments ..
24 * INTEGER INFO, LDA, M, N
25 * ..
26 * .. Array Arguments ..
27 * DOUBLE PRECISION D( * ), E( * )
28 * COMPLEX*16 A( LDA, * ), TAUP( * ), TAUQ( * ), WORK( * )
29 * ..
30 *
31 *
32 *> \par Purpose:
33 * =============
34 *>
35 *> \verbatim
36 *>
37 *> ZGEBD2 reduces a complex general m by n matrix A to upper or lower
38 *> real bidiagonal form B by a unitary transformation: Q**H * A * P = B.
39 *>
40 *> If m >= n, B is upper bidiagonal; if m < n, B is lower bidiagonal.
41 *> \endverbatim
42 *
43 * Arguments:
44 * ==========
45 *
46 *> \param[in] M
47 *> \verbatim
48 *> M is INTEGER
49 *> The number of rows in the matrix A. M >= 0.
50 *> \endverbatim
51 *>
52 *> \param[in] N
53 *> \verbatim
54 *> N is INTEGER
55 *> The number of columns in the matrix A. N >= 0.
56 *> \endverbatim
57 *>
58 *> \param[in,out] A
59 *> \verbatim
60 *> A is COMPLEX*16 array, dimension (LDA,N)
61 *> On entry, the m by n general matrix to be reduced.
62 *> On exit,
63 *> if m >= n, the diagonal and the first superdiagonal are
64 *> overwritten with the upper bidiagonal matrix B; the
65 *> elements below the diagonal, with the array TAUQ, represent
66 *> the unitary matrix Q as a product of elementary
67 *> reflectors, and the elements above the first superdiagonal,
68 *> with the array TAUP, represent the unitary matrix P as
69 *> a product of elementary reflectors;
70 *> if m < n, the diagonal and the first subdiagonal are
71 *> overwritten with the lower bidiagonal matrix B; the
72 *> elements below the first subdiagonal, with the array TAUQ,
73 *> represent the unitary matrix Q as a product of
74 *> elementary reflectors, and the elements above the diagonal,
75 *> with the array TAUP, represent the unitary matrix P as
76 *> a product of elementary reflectors.
77 *> See Further Details.
78 *> \endverbatim
79 *>
80 *> \param[in] LDA
81 *> \verbatim
82 *> LDA is INTEGER
83 *> The leading dimension of the array A. LDA >= max(1,M).
84 *> \endverbatim
85 *>
86 *> \param[out] D
87 *> \verbatim
88 *> D is DOUBLE PRECISION array, dimension (min(M,N))
89 *> The diagonal elements of the bidiagonal matrix B:
90 *> D(i) = A(i,i).
91 *> \endverbatim
92 *>
93 *> \param[out] E
94 *> \verbatim
95 *> E is DOUBLE PRECISION array, dimension (min(M,N)-1)
96 *> The off-diagonal elements of the bidiagonal matrix B:
97 *> if m >= n, E(i) = A(i,i+1) for i = 1,2,...,n-1;
98 *> if m < n, E(i) = A(i+1,i) for i = 1,2,...,m-1.
99 *> \endverbatim
100 *>
101 *> \param[out] TAUQ
102 *> \verbatim
103 *> TAUQ is COMPLEX*16 array dimension (min(M,N))
104 *> The scalar factors of the elementary reflectors which
105 *> represent the unitary matrix Q. See Further Details.
106 *> \endverbatim
107 *>
108 *> \param[out] TAUP
109 *> \verbatim
110 *> TAUP is COMPLEX*16 array, dimension (min(M,N))
111 *> The scalar factors of the elementary reflectors which
112 *> represent the unitary matrix P. See Further Details.
113 *> \endverbatim
114 *>
115 *> \param[out] WORK
116 *> \verbatim
117 *> WORK is COMPLEX*16 array, dimension (max(M,N))
118 *> \endverbatim
119 *>
120 *> \param[out] INFO
121 *> \verbatim
122 *> INFO is INTEGER
123 *> = 0: successful exit
124 *> < 0: if INFO = -i, the i-th argument had an illegal value.
125 *> \endverbatim
126 *
127 * Authors:
128 * ========
129 *
130 *> \author Univ. of Tennessee
131 *> \author Univ. of California Berkeley
132 *> \author Univ. of Colorado Denver
133 *> \author NAG Ltd.
134 *
135 *> \date September 2012
136 *
137 *> \ingroup complex16GEcomputational
138 *
139 *> \par Further Details:
140 * =====================
141 *>
142 *> \verbatim
143 *>
144 *> The matrices Q and P are represented as products of elementary
145 *> reflectors:
146 *>
147 *> If m >= n,
148 *>
149 *> Q = H(1) H(2) . . . H(n) and P = G(1) G(2) . . . G(n-1)
150 *>
151 *> Each H(i) and G(i) has the form:
152 *>
153 *> H(i) = I - tauq * v * v**H and G(i) = I - taup * u * u**H
154 *>
155 *> where tauq and taup are complex scalars, and v and u are complex
156 *> vectors; v(1:i-1) = 0, v(i) = 1, and v(i+1:m) is stored on exit in
157 *> A(i+1:m,i); u(1:i) = 0, u(i+1) = 1, and u(i+2:n) is stored on exit in
158 *> A(i,i+2:n); tauq is stored in TAUQ(i) and taup in TAUP(i).
159 *>
160 *> If m < n,
161 *>
162 *> Q = H(1) H(2) . . . H(m-1) and P = G(1) G(2) . . . G(m)
163 *>
164 *> Each H(i) and G(i) has the form:
165 *>
166 *> H(i) = I - tauq * v * v**H and G(i) = I - taup * u * u**H
167 *>
168 *> where tauq and taup are complex scalars, v and u are complex vectors;
169 *> v(1:i) = 0, v(i+1) = 1, and v(i+2:m) is stored on exit in A(i+2:m,i);
170 *> u(1:i-1) = 0, u(i) = 1, and u(i+1:n) is stored on exit in A(i,i+1:n);
171 *> tauq is stored in TAUQ(i) and taup in TAUP(i).
172 *>
173 *> The contents of A on exit are illustrated by the following examples:
174 *>
175 *> m = 6 and n = 5 (m > n): m = 5 and n = 6 (m < n):
176 *>
177 *> ( d e u1 u1 u1 ) ( d u1 u1 u1 u1 u1 )
178 *> ( v1 d e u2 u2 ) ( e d u2 u2 u2 u2 )
179 *> ( v1 v2 d e u3 ) ( v1 e d u3 u3 u3 )
180 *> ( v1 v2 v3 d e ) ( v1 v2 e d u4 u4 )
181 *> ( v1 v2 v3 v4 d ) ( v1 v2 v3 e d u5 )
182 *> ( v1 v2 v3 v4 v5 )
183 *>
184 *> where d and e denote diagonal and off-diagonal elements of B, vi
185 *> denotes an element of the vector defining H(i), and ui an element of
186 *> the vector defining G(i).
187 *> \endverbatim
188 *>
189 * =====================================================================
190  SUBROUTINE zgebd2( M, N, A, LDA, D, E, TAUQ, TAUP, WORK, INFO )
191 *
192 * -- LAPACK computational routine (version 3.4.2) --
193 * -- LAPACK is a software package provided by Univ. of Tennessee, --
194 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
195 * September 2012
196 *
197 * .. Scalar Arguments ..
198  INTEGER info, lda, m, n
199 * ..
200 * .. Array Arguments ..
201  DOUBLE PRECISION d( * ), e( * )
202  COMPLEX*16 a( lda, * ), taup( * ), tauq( * ), work( * )
203 * ..
204 *
205 * =====================================================================
206 *
207 * .. Parameters ..
208  COMPLEX*16 zero, one
209  parameter( zero = ( 0.0d+0, 0.0d+0 ),
210  $ one = ( 1.0d+0, 0.0d+0 ) )
211 * ..
212 * .. Local Scalars ..
213  INTEGER i
214  COMPLEX*16 alpha
215 * ..
216 * .. External Subroutines ..
217  EXTERNAL xerbla, zlacgv, zlarf, zlarfg
218 * ..
219 * .. Intrinsic Functions ..
220  INTRINSIC dconjg, max, min
221 * ..
222 * .. Executable Statements ..
223 *
224 * Test the input parameters
225 *
226  info = 0
227  IF( m.LT.0 ) THEN
228  info = -1
229  ELSE IF( n.LT.0 ) THEN
230  info = -2
231  ELSE IF( lda.LT.max( 1, m ) ) THEN
232  info = -4
233  END IF
234  IF( info.LT.0 ) THEN
235  CALL xerbla( 'ZGEBD2', -info )
236  RETURN
237  END IF
238 *
239  IF( m.GE.n ) THEN
240 *
241 * Reduce to upper bidiagonal form
242 *
243  DO 10 i = 1, n
244 *
245 * Generate elementary reflector H(i) to annihilate A(i+1:m,i)
246 *
247  alpha = a( i, i )
248  CALL zlarfg( m-i+1, alpha, a( min( i+1, m ), i ), 1,
249  $ tauq( i ) )
250  d( i ) = alpha
251  a( i, i ) = one
252 *
253 * Apply H(i)**H to A(i:m,i+1:n) from the left
254 *
255  IF( i.LT.n )
256  $ CALL zlarf( 'Left', m-i+1, n-i, a( i, i ), 1,
257  $ dconjg( tauq( i ) ), a( i, i+1 ), lda, work )
258  a( i, i ) = d( i )
259 *
260  IF( i.LT.n ) THEN
261 *
262 * Generate elementary reflector G(i) to annihilate
263 * A(i,i+2:n)
264 *
265  CALL zlacgv( n-i, a( i, i+1 ), lda )
266  alpha = a( i, i+1 )
267  CALL zlarfg( n-i, alpha, a( i, min( i+2, n ) ), lda,
268  $ taup( i ) )
269  e( i ) = alpha
270  a( i, i+1 ) = one
271 *
272 * Apply G(i) to A(i+1:m,i+1:n) from the right
273 *
274  CALL zlarf( 'Right', m-i, n-i, a( i, i+1 ), lda,
275  $ taup( i ), a( i+1, i+1 ), lda, work )
276  CALL zlacgv( n-i, a( i, i+1 ), lda )
277  a( i, i+1 ) = e( i )
278  ELSE
279  taup( i ) = zero
280  END IF
281  10 CONTINUE
282  ELSE
283 *
284 * Reduce to lower bidiagonal form
285 *
286  DO 20 i = 1, m
287 *
288 * Generate elementary reflector G(i) to annihilate A(i,i+1:n)
289 *
290  CALL zlacgv( n-i+1, a( i, i ), lda )
291  alpha = a( i, i )
292  CALL zlarfg( n-i+1, alpha, a( i, min( i+1, n ) ), lda,
293  $ taup( i ) )
294  d( i ) = alpha
295  a( i, i ) = one
296 *
297 * Apply G(i) to A(i+1:m,i:n) from the right
298 *
299  IF( i.LT.m )
300  $ CALL zlarf( 'Right', m-i, n-i+1, a( i, i ), lda,
301  $ taup( i ), a( i+1, i ), lda, work )
302  CALL zlacgv( n-i+1, a( i, i ), lda )
303  a( i, i ) = d( i )
304 *
305  IF( i.LT.m ) THEN
306 *
307 * Generate elementary reflector H(i) to annihilate
308 * A(i+2:m,i)
309 *
310  alpha = a( i+1, i )
311  CALL zlarfg( m-i, alpha, a( min( i+2, m ), i ), 1,
312  $ tauq( i ) )
313  e( i ) = alpha
314  a( i+1, i ) = one
315 *
316 * Apply H(i)**H to A(i+1:m,i+1:n) from the left
317 *
318  CALL zlarf( 'Left', m-i, n-i, a( i+1, i ), 1,
319  $ dconjg( tauq( i ) ), a( i+1, i+1 ), lda,
320  $ work )
321  a( i+1, i ) = e( i )
322  ELSE
323  tauq( i ) = zero
324  END IF
325  20 CONTINUE
326  END IF
327  RETURN
328 *
329 * End of ZGEBD2
330 *
331  END