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sgghrd.f
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1 *> \brief \b SGGHRD
2 *
3 * =========== DOCUMENTATION ===========
4 *
5 * Online html documentation available at
6 * http://www.netlib.org/lapack/explore-html/
7 *
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16 *> \endhtmlonly
17 *
18 * Definition:
19 * ===========
20 *
21 * SUBROUTINE SGGHRD( COMPQ, COMPZ, N, ILO, IHI, A, LDA, B, LDB, Q,
22 * LDQ, Z, LDZ, INFO )
23 *
24 * .. Scalar Arguments ..
25 * CHARACTER COMPQ, COMPZ
26 * INTEGER IHI, ILO, INFO, LDA, LDB, LDQ, LDZ, N
27 * ..
28 * .. Array Arguments ..
29 * REAL A( LDA, * ), B( LDB, * ), Q( LDQ, * ),
30 * $ Z( LDZ, * )
31 * ..
32 *
33 *
34 *> \par Purpose:
35 * =============
36 *>
37 *> \verbatim
38 *>
39 *> SGGHRD reduces a pair of real matrices (A,B) to generalized upper
40 *> Hessenberg form using orthogonal transformations, where A is a
41 *> general matrix and B is upper triangular. The form of the
42 *> generalized eigenvalue problem is
43 *> A*x = lambda*B*x,
44 *> and B is typically made upper triangular by computing its QR
45 *> factorization and moving the orthogonal matrix Q to the left side
46 *> of the equation.
47 *>
48 *> This subroutine simultaneously reduces A to a Hessenberg matrix H:
49 *> Q**T*A*Z = H
50 *> and transforms B to another upper triangular matrix T:
51 *> Q**T*B*Z = T
52 *> in order to reduce the problem to its standard form
53 *> H*y = lambda*T*y
54 *> where y = Z**T*x.
55 *>
56 *> The orthogonal matrices Q and Z are determined as products of Givens
57 *> rotations. They may either be formed explicitly, or they may be
58 *> postmultiplied into input matrices Q1 and Z1, so that
59 *>
60 *> Q1 * A * Z1**T = (Q1*Q) * H * (Z1*Z)**T
61 *>
62 *> Q1 * B * Z1**T = (Q1*Q) * T * (Z1*Z)**T
63 *>
64 *> If Q1 is the orthogonal matrix from the QR factorization of B in the
65 *> original equation A*x = lambda*B*x, then SGGHRD reduces the original
66 *> problem to generalized Hessenberg form.
67 *> \endverbatim
68 *
69 * Arguments:
70 * ==========
71 *
72 *> \param[in] COMPQ
73 *> \verbatim
74 *> COMPQ is CHARACTER*1
75 *> = 'N': do not compute Q;
76 *> = 'I': Q is initialized to the unit matrix, and the
77 *> orthogonal matrix Q is returned;
78 *> = 'V': Q must contain an orthogonal matrix Q1 on entry,
79 *> and the product Q1*Q is returned.
80 *> \endverbatim
81 *>
82 *> \param[in] COMPZ
83 *> \verbatim
84 *> COMPZ is CHARACTER*1
85 *> = 'N': do not compute Z;
86 *> = 'I': Z is initialized to the unit matrix, and the
87 *> orthogonal matrix Z is returned;
88 *> = 'V': Z must contain an orthogonal matrix Z1 on entry,
89 *> and the product Z1*Z is returned.
90 *> \endverbatim
91 *>
92 *> \param[in] N
93 *> \verbatim
94 *> N is INTEGER
95 *> The order of the matrices A and B. N >= 0.
96 *> \endverbatim
97 *>
98 *> \param[in] ILO
99 *> \verbatim
100 *> ILO is INTEGER
101 *> \endverbatim
102 *>
103 *> \param[in] IHI
104 *> \verbatim
105 *> IHI is INTEGER
106 *>
107 *> ILO and IHI mark the rows and columns of A which are to be
108 *> reduced. It is assumed that A is already upper triangular
109 *> in rows and columns 1:ILO-1 and IHI+1:N. ILO and IHI are
110 *> normally set by a previous call to SGGBAL; otherwise they
111 *> should be set to 1 and N respectively.
112 *> 1 <= ILO <= IHI <= N, if N > 0; ILO=1 and IHI=0, if N=0.
113 *> \endverbatim
114 *>
115 *> \param[in,out] A
116 *> \verbatim
117 *> A is REAL array, dimension (LDA, N)
118 *> On entry, the N-by-N general matrix to be reduced.
119 *> On exit, the upper triangle and the first subdiagonal of A
120 *> are overwritten with the upper Hessenberg matrix H, and the
121 *> rest is set to zero.
122 *> \endverbatim
123 *>
124 *> \param[in] LDA
125 *> \verbatim
126 *> LDA is INTEGER
127 *> The leading dimension of the array A. LDA >= max(1,N).
128 *> \endverbatim
129 *>
130 *> \param[in,out] B
131 *> \verbatim
132 *> B is REAL array, dimension (LDB, N)
133 *> On entry, the N-by-N upper triangular matrix B.
134 *> On exit, the upper triangular matrix T = Q**T B Z. The
135 *> elements below the diagonal are set to zero.
136 *> \endverbatim
137 *>
138 *> \param[in] LDB
139 *> \verbatim
140 *> LDB is INTEGER
141 *> The leading dimension of the array B. LDB >= max(1,N).
142 *> \endverbatim
143 *>
144 *> \param[in,out] Q
145 *> \verbatim
146 *> Q is REAL array, dimension (LDQ, N)
147 *> On entry, if COMPQ = 'V', the orthogonal matrix Q1,
148 *> typically from the QR factorization of B.
149 *> On exit, if COMPQ='I', the orthogonal matrix Q, and if
150 *> COMPQ = 'V', the product Q1*Q.
151 *> Not referenced if COMPQ='N'.
152 *> \endverbatim
153 *>
154 *> \param[in] LDQ
155 *> \verbatim
156 *> LDQ is INTEGER
157 *> The leading dimension of the array Q.
158 *> LDQ >= N if COMPQ='V' or 'I'; LDQ >= 1 otherwise.
159 *> \endverbatim
160 *>
161 *> \param[in,out] Z
162 *> \verbatim
163 *> Z is REAL array, dimension (LDZ, N)
164 *> On entry, if COMPZ = 'V', the orthogonal matrix Z1.
165 *> On exit, if COMPZ='I', the orthogonal matrix Z, and if
166 *> COMPZ = 'V', the product Z1*Z.
167 *> Not referenced if COMPZ='N'.
168 *> \endverbatim
169 *>
170 *> \param[in] LDZ
171 *> \verbatim
172 *> LDZ is INTEGER
173 *> The leading dimension of the array Z.
174 *> LDZ >= N if COMPZ='V' or 'I'; LDZ >= 1 otherwise.
175 *> \endverbatim
176 *>
177 *> \param[out] INFO
178 *> \verbatim
179 *> INFO is INTEGER
180 *> = 0: successful exit.
181 *> < 0: if INFO = -i, the i-th argument had an illegal value.
182 *> \endverbatim
183 *
184 * Authors:
185 * ========
186 *
187 *> \author Univ. of Tennessee
188 *> \author Univ. of California Berkeley
189 *> \author Univ. of Colorado Denver
190 *> \author NAG Ltd.
191 *
192 *> \date November 2011
193 *
194 *> \ingroup realOTHERcomputational
195 *
196 *> \par Further Details:
197 * =====================
198 *>
199 *> \verbatim
200 *>
201 *> This routine reduces A to Hessenberg and B to triangular form by
202 *> an unblocked reduction, as described in _Matrix_Computations_,
203 *> by Golub and Van Loan (Johns Hopkins Press.)
204 *> \endverbatim
205 *>
206 * =====================================================================
207  SUBROUTINE sgghrd( COMPQ, COMPZ, N, ILO, IHI, A, LDA, B, LDB, Q,
208  $ ldq, z, ldz, info )
209 *
210 * -- LAPACK computational routine (version 3.4.0) --
211 * -- LAPACK is a software package provided by Univ. of Tennessee, --
212 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
213 * November 2011
214 *
215 * .. Scalar Arguments ..
216  CHARACTER compq, compz
217  INTEGER ihi, ilo, info, lda, ldb, ldq, ldz, n
218 * ..
219 * .. Array Arguments ..
220  REAL a( lda, * ), b( ldb, * ), q( ldq, * ),
221  $ z( ldz, * )
222 * ..
223 *
224 * =====================================================================
225 *
226 * .. Parameters ..
227  REAL one, zero
228  parameter( one = 1.0e+0, zero = 0.0e+0 )
229 * ..
230 * .. Local Scalars ..
231  LOGICAL ilq, ilz
232  INTEGER icompq, icompz, jcol, jrow
233  REAL c, s, temp
234 * ..
235 * .. External Functions ..
236  LOGICAL lsame
237  EXTERNAL lsame
238 * ..
239 * .. External Subroutines ..
240  EXTERNAL slartg, slaset, srot, xerbla
241 * ..
242 * .. Intrinsic Functions ..
243  INTRINSIC max
244 * ..
245 * .. Executable Statements ..
246 *
247 * Decode COMPQ
248 *
249  IF( lsame( compq, 'N' ) ) THEN
250  ilq = .false.
251  icompq = 1
252  ELSE IF( lsame( compq, 'V' ) ) THEN
253  ilq = .true.
254  icompq = 2
255  ELSE IF( lsame( compq, 'I' ) ) THEN
256  ilq = .true.
257  icompq = 3
258  ELSE
259  icompq = 0
260  END IF
261 *
262 * Decode COMPZ
263 *
264  IF( lsame( compz, 'N' ) ) THEN
265  ilz = .false.
266  icompz = 1
267  ELSE IF( lsame( compz, 'V' ) ) THEN
268  ilz = .true.
269  icompz = 2
270  ELSE IF( lsame( compz, 'I' ) ) THEN
271  ilz = .true.
272  icompz = 3
273  ELSE
274  icompz = 0
275  END IF
276 *
277 * Test the input parameters.
278 *
279  info = 0
280  IF( icompq.LE.0 ) THEN
281  info = -1
282  ELSE IF( icompz.LE.0 ) THEN
283  info = -2
284  ELSE IF( n.LT.0 ) THEN
285  info = -3
286  ELSE IF( ilo.LT.1 ) THEN
287  info = -4
288  ELSE IF( ihi.GT.n .OR. ihi.LT.ilo-1 ) THEN
289  info = -5
290  ELSE IF( lda.LT.max( 1, n ) ) THEN
291  info = -7
292  ELSE IF( ldb.LT.max( 1, n ) ) THEN
293  info = -9
294  ELSE IF( ( ilq .AND. ldq.LT.n ) .OR. ldq.LT.1 ) THEN
295  info = -11
296  ELSE IF( ( ilz .AND. ldz.LT.n ) .OR. ldz.LT.1 ) THEN
297  info = -13
298  END IF
299  IF( info.NE.0 ) THEN
300  CALL xerbla( 'SGGHRD', -info )
301  return
302  END IF
303 *
304 * Initialize Q and Z if desired.
305 *
306  IF( icompq.EQ.3 )
307  $ CALL slaset( 'Full', n, n, zero, one, q, ldq )
308  IF( icompz.EQ.3 )
309  $ CALL slaset( 'Full', n, n, zero, one, z, ldz )
310 *
311 * Quick return if possible
312 *
313  IF( n.LE.1 )
314  $ return
315 *
316 * Zero out lower triangle of B
317 *
318  DO 20 jcol = 1, n - 1
319  DO 10 jrow = jcol + 1, n
320  b( jrow, jcol ) = zero
321  10 continue
322  20 continue
323 *
324 * Reduce A and B
325 *
326  DO 40 jcol = ilo, ihi - 2
327 *
328  DO 30 jrow = ihi, jcol + 2, -1
329 *
330 * Step 1: rotate rows JROW-1, JROW to kill A(JROW,JCOL)
331 *
332  temp = a( jrow-1, jcol )
333  CALL slartg( temp, a( jrow, jcol ), c, s,
334  $ a( jrow-1, jcol ) )
335  a( jrow, jcol ) = zero
336  CALL srot( n-jcol, a( jrow-1, jcol+1 ), lda,
337  $ a( jrow, jcol+1 ), lda, c, s )
338  CALL srot( n+2-jrow, b( jrow-1, jrow-1 ), ldb,
339  $ b( jrow, jrow-1 ), ldb, c, s )
340  IF( ilq )
341  $ CALL srot( n, q( 1, jrow-1 ), 1, q( 1, jrow ), 1, c, s )
342 *
343 * Step 2: rotate columns JROW, JROW-1 to kill B(JROW,JROW-1)
344 *
345  temp = b( jrow, jrow )
346  CALL slartg( temp, b( jrow, jrow-1 ), c, s,
347  $ b( jrow, jrow ) )
348  b( jrow, jrow-1 ) = zero
349  CALL srot( ihi, a( 1, jrow ), 1, a( 1, jrow-1 ), 1, c, s )
350  CALL srot( jrow-1, b( 1, jrow ), 1, b( 1, jrow-1 ), 1, c,
351  $ s )
352  IF( ilz )
353  $ CALL srot( n, z( 1, jrow ), 1, z( 1, jrow-1 ), 1, c, s )
354  30 continue
355  40 continue
356 *
357  return
358 *
359 * End of SGGHRD
360 *
361  END