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slarft.f
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1 *> \brief \b SLARFT forms the triangular factor T of a block reflector H = I - vtvH
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 SLARFT( DIRECT, STOREV, N, K, V, LDV, TAU, T, LDT )
22 *
23 * .. Scalar Arguments ..
24 * CHARACTER DIRECT, STOREV
25 * INTEGER K, LDT, LDV, N
26 * ..
27 * .. Array Arguments ..
28 * REAL T( LDT, * ), TAU( * ), V( LDV, * )
29 * ..
30 *
31 *
32 *> \par Purpose:
33 * =============
34 *>
35 *> \verbatim
36 *>
37 *> SLARFT forms the triangular factor T of a real block reflector H
38 *> of order n, which is defined as a product of k elementary reflectors.
39 *>
40 *> If DIRECT = 'F', H = H(1) H(2) . . . H(k) and T is upper triangular;
41 *>
42 *> If DIRECT = 'B', H = H(k) . . . H(2) H(1) and T is lower triangular.
43 *>
44 *> If STOREV = 'C', the vector which defines the elementary reflector
45 *> H(i) is stored in the i-th column of the array V, and
46 *>
47 *> H = I - V * T * V**T
48 *>
49 *> If STOREV = 'R', the vector which defines the elementary reflector
50 *> H(i) is stored in the i-th row of the array V, and
51 *>
52 *> H = I - V**T * T * V
53 *> \endverbatim
54 *
55 * Arguments:
56 * ==========
57 *
58 *> \param[in] DIRECT
59 *> \verbatim
60 *> DIRECT is CHARACTER*1
61 *> Specifies the order in which the elementary reflectors are
62 *> multiplied to form the block reflector:
63 *> = 'F': H = H(1) H(2) . . . H(k) (Forward)
64 *> = 'B': H = H(k) . . . H(2) H(1) (Backward)
65 *> \endverbatim
66 *>
67 *> \param[in] STOREV
68 *> \verbatim
69 *> STOREV is CHARACTER*1
70 *> Specifies how the vectors which define the elementary
71 *> reflectors are stored (see also Further Details):
72 *> = 'C': columnwise
73 *> = 'R': rowwise
74 *> \endverbatim
75 *>
76 *> \param[in] N
77 *> \verbatim
78 *> N is INTEGER
79 *> The order of the block reflector H. N >= 0.
80 *> \endverbatim
81 *>
82 *> \param[in] K
83 *> \verbatim
84 *> K is INTEGER
85 *> The order of the triangular factor T (= the number of
86 *> elementary reflectors). K >= 1.
87 *> \endverbatim
88 *>
89 *> \param[in] V
90 *> \verbatim
91 *> V is REAL array, dimension
92 *> (LDV,K) if STOREV = 'C'
93 *> (LDV,N) if STOREV = 'R'
94 *> The matrix V. See further details.
95 *> \endverbatim
96 *>
97 *> \param[in] LDV
98 *> \verbatim
99 *> LDV is INTEGER
100 *> The leading dimension of the array V.
101 *> If STOREV = 'C', LDV >= max(1,N); if STOREV = 'R', LDV >= K.
102 *> \endverbatim
103 *>
104 *> \param[in] TAU
105 *> \verbatim
106 *> TAU is REAL array, dimension (K)
107 *> TAU(i) must contain the scalar factor of the elementary
108 *> reflector H(i).
109 *> \endverbatim
110 *>
111 *> \param[out] T
112 *> \verbatim
113 *> T is REAL array, dimension (LDT,K)
114 *> The k by k triangular factor T of the block reflector.
115 *> If DIRECT = 'F', T is upper triangular; if DIRECT = 'B', T is
116 *> lower triangular. The rest of the array is not used.
117 *> \endverbatim
118 *>
119 *> \param[in] LDT
120 *> \verbatim
121 *> LDT is INTEGER
122 *> The leading dimension of the array T. LDT >= K.
123 *> \endverbatim
124 *
125 * Authors:
126 * ========
127 *
128 *> \author Univ. of Tennessee
129 *> \author Univ. of California Berkeley
130 *> \author Univ. of Colorado Denver
131 *> \author NAG Ltd.
132 *
133 *> \date September 2012
134 *
135 *> \ingroup realOTHERauxiliary
136 *
137 *> \par Further Details:
138 * =====================
139 *>
140 *> \verbatim
141 *>
142 *> The shape of the matrix V and the storage of the vectors which define
143 *> the H(i) is best illustrated by the following example with n = 5 and
144 *> k = 3. The elements equal to 1 are not stored.
145 *>
146 *> DIRECT = 'F' and STOREV = 'C': DIRECT = 'F' and STOREV = 'R':
147 *>
148 *> V = ( 1 ) V = ( 1 v1 v1 v1 v1 )
149 *> ( v1 1 ) ( 1 v2 v2 v2 )
150 *> ( v1 v2 1 ) ( 1 v3 v3 )
151 *> ( v1 v2 v3 )
152 *> ( v1 v2 v3 )
153 *>
154 *> DIRECT = 'B' and STOREV = 'C': DIRECT = 'B' and STOREV = 'R':
155 *>
156 *> V = ( v1 v2 v3 ) V = ( v1 v1 1 )
157 *> ( v1 v2 v3 ) ( v2 v2 v2 1 )
158 *> ( 1 v2 v3 ) ( v3 v3 v3 v3 1 )
159 *> ( 1 v3 )
160 *> ( 1 )
161 *> \endverbatim
162 *>
163 * =====================================================================
164  SUBROUTINE slarft( DIRECT, STOREV, N, K, V, LDV, TAU, T, LDT )
165 *
166 * -- LAPACK auxiliary routine (version 3.4.2) --
167 * -- LAPACK is a software package provided by Univ. of Tennessee, --
168 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
169 * September 2012
170 *
171 * .. Scalar Arguments ..
172  CHARACTER direct, storev
173  INTEGER k, ldt, ldv, n
174 * ..
175 * .. Array Arguments ..
176  REAL t( ldt, * ), tau( * ), v( ldv, * )
177 * ..
178 *
179 * =====================================================================
180 *
181 * .. Parameters ..
182  REAL one, zero
183  parameter( one = 1.0e+0, zero = 0.0e+0 )
184 * ..
185 * .. Local Scalars ..
186  INTEGER i, j, prevlastv, lastv
187 * ..
188 * .. External Subroutines ..
189  EXTERNAL sgemv, strmv
190 * ..
191 * .. External Functions ..
192  LOGICAL lsame
193  EXTERNAL lsame
194 * ..
195 * .. Executable Statements ..
196 *
197 * Quick return if possible
198 *
199  IF( n.EQ.0 )
200  $ return
201 *
202  IF( lsame( direct, 'F' ) ) THEN
203  prevlastv = n
204  DO i = 1, k
205  prevlastv = max( i, prevlastv )
206  IF( tau( i ).EQ.zero ) THEN
207 *
208 * H(i) = I
209 *
210  DO j = 1, i
211  t( j, i ) = zero
212  END DO
213  ELSE
214 *
215 * general case
216 *
217  IF( lsame( storev, 'C' ) ) THEN
218 * Skip any trailing zeros.
219  DO lastv = n, i+1, -1
220  IF( v( lastv, i ).NE.zero ) exit
221  END DO
222  DO j = 1, i-1
223  t( j, i ) = -tau( i ) * v( i , j )
224  END DO
225  j = min( lastv, prevlastv )
226 *
227 * T(1:i-1,i) := - tau(i) * V(i:j,1:i-1)**T * V(i:j,i)
228 *
229  CALL sgemv( 'Transpose', j-i, i-1, -tau( i ),
230  $ v( i+1, 1 ), ldv, v( i+1, i ), 1, one,
231  $ t( 1, i ), 1 )
232  ELSE
233 * Skip any trailing zeros.
234  DO lastv = n, i+1, -1
235  IF( v( i, lastv ).NE.zero ) exit
236  END DO
237  DO j = 1, i-1
238  t( j, i ) = -tau( i ) * v( j , i )
239  END DO
240  j = min( lastv, prevlastv )
241 *
242 * T(1:i-1,i) := - tau(i) * V(1:i-1,i:j) * V(i,i:j)**T
243 *
244  CALL sgemv( 'No transpose', i-1, j-i, -tau( i ),
245  $ v( 1, i+1 ), ldv, v( i, i+1 ), ldv,
246  $ one, t( 1, i ), 1 )
247  END IF
248 *
249 * T(1:i-1,i) := T(1:i-1,1:i-1) * T(1:i-1,i)
250 *
251  CALL strmv( 'Upper', 'No transpose', 'Non-unit', i-1, t,
252  $ ldt, t( 1, i ), 1 )
253  t( i, i ) = tau( i )
254  IF( i.GT.1 ) THEN
255  prevlastv = max( prevlastv, lastv )
256  ELSE
257  prevlastv = lastv
258  END IF
259  END IF
260  END DO
261  ELSE
262  prevlastv = 1
263  DO i = k, 1, -1
264  IF( tau( i ).EQ.zero ) THEN
265 *
266 * H(i) = I
267 *
268  DO j = i, k
269  t( j, i ) = zero
270  END DO
271  ELSE
272 *
273 * general case
274 *
275  IF( i.LT.k ) THEN
276  IF( lsame( storev, 'C' ) ) THEN
277 * Skip any leading zeros.
278  DO lastv = 1, i-1
279  IF( v( lastv, i ).NE.zero ) exit
280  END DO
281  DO j = i+1, k
282  t( j, i ) = -tau( i ) * v( n-k+i , j )
283  END DO
284  j = max( lastv, prevlastv )
285 *
286 * T(i+1:k,i) = -tau(i) * V(j:n-k+i,i+1:k)**T * V(j:n-k+i,i)
287 *
288  CALL sgemv( 'Transpose', n-k+i-j, k-i, -tau( i ),
289  $ v( j, i+1 ), ldv, v( j, i ), 1, one,
290  $ t( i+1, i ), 1 )
291  ELSE
292 * Skip any leading zeros.
293  DO lastv = 1, i-1
294  IF( v( i, lastv ).NE.zero ) exit
295  END DO
296  DO j = i+1, k
297  t( j, i ) = -tau( i ) * v( j, n-k+i )
298  END DO
299  j = max( lastv, prevlastv )
300 *
301 * T(i+1:k,i) = -tau(i) * V(i+1:k,j:n-k+i) * V(i,j:n-k+i)**T
302 *
303  CALL sgemv( 'No transpose', k-i, n-k+i-j,
304  $ -tau( i ), v( i+1, j ), ldv, v( i, j ), ldv,
305  $ one, t( i+1, i ), 1 )
306  END IF
307 *
308 * T(i+1:k,i) := T(i+1:k,i+1:k) * T(i+1:k,i)
309 *
310  CALL strmv( 'Lower', 'No transpose', 'Non-unit', k-i,
311  $ t( i+1, i+1 ), ldt, t( i+1, i ), 1 )
312  IF( i.GT.1 ) THEN
313  prevlastv = min( prevlastv, lastv )
314  ELSE
315  prevlastv = lastv
316  END IF
317  END IF
318  t( i, i ) = tau( i )
319  END IF
320  END DO
321  END IF
322  return
323 *
324 * End of SLARFT
325 *
326  END