1: *> \brief \b DLARZT
2: *
3: * =========== DOCUMENTATION ===========
4: *
5: * Online html documentation available at
6: * http://www.netlib.org/lapack/explore-html/
7: *
8: *> \htmlonly
9: *> Download DLARZT + dependencies
10: *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/dlarzt.f">
11: *> [TGZ]</a>
12: *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/dlarzt.f">
13: *> [ZIP]</a>
14: *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/dlarzt.f">
15: *> [TXT]</a>
16: *> \endhtmlonly
17: *
18: * Definition:
19: * ===========
20: *
21: * SUBROUTINE DLARZT( 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: * DOUBLE PRECISION T( LDT, * ), TAU( * ), V( LDV, * )
29: * ..
30: *
31: *
32: *> \par Purpose:
33: * =============
34: *>
35: *> \verbatim
36: *>
37: *> DLARZT forms the triangular factor T of a real block reflector
38: *> H of order > n, which is defined as a product of k elementary
39: *> reflectors.
40: *>
41: *> If DIRECT = 'F', H = H(1) H(2) . . . H(k) and T is upper triangular;
42: *>
43: *> If DIRECT = 'B', H = H(k) . . . H(2) H(1) and T is lower triangular.
44: *>
45: *> If STOREV = 'C', the vector which defines the elementary reflector
46: *> H(i) is stored in the i-th column of the array V, and
47: *>
48: *> H = I - V * T * V**T
49: *>
50: *> If STOREV = 'R', the vector which defines the elementary reflector
51: *> H(i) is stored in the i-th row of the array V, and
52: *>
53: *> H = I - V**T * T * V
54: *>
55: *> Currently, only STOREV = 'R' and DIRECT = 'B' are supported.
56: *> \endverbatim
57: *
58: * Arguments:
59: * ==========
60: *
61: *> \param[in] DIRECT
62: *> \verbatim
63: *> DIRECT is CHARACTER*1
64: *> Specifies the order in which the elementary reflectors are
65: *> multiplied to form the block reflector:
66: *> = 'F': H = H(1) H(2) . . . H(k) (Forward, not supported yet)
67: *> = 'B': H = H(k) . . . H(2) H(1) (Backward)
68: *> \endverbatim
69: *>
70: *> \param[in] STOREV
71: *> \verbatim
72: *> STOREV is CHARACTER*1
73: *> Specifies how the vectors which define the elementary
74: *> reflectors are stored (see also Further Details):
75: *> = 'C': columnwise (not supported yet)
76: *> = 'R': rowwise
77: *> \endverbatim
78: *>
79: *> \param[in] N
80: *> \verbatim
81: *> N is INTEGER
82: *> The order of the block reflector H. N >= 0.
83: *> \endverbatim
84: *>
85: *> \param[in] K
86: *> \verbatim
87: *> K is INTEGER
88: *> The order of the triangular factor T (= the number of
89: *> elementary reflectors). K >= 1.
90: *> \endverbatim
91: *>
92: *> \param[in,out] V
93: *> \verbatim
94: *> V is DOUBLE PRECISION array, dimension
95: *> (LDV,K) if STOREV = 'C'
96: *> (LDV,N) if STOREV = 'R'
97: *> The matrix V. See further details.
98: *> \endverbatim
99: *>
100: *> \param[in] LDV
101: *> \verbatim
102: *> LDV is INTEGER
103: *> The leading dimension of the array V.
104: *> If STOREV = 'C', LDV >= max(1,N); if STOREV = 'R', LDV >= K.
105: *> \endverbatim
106: *>
107: *> \param[in] TAU
108: *> \verbatim
109: *> TAU is DOUBLE PRECISION array, dimension (K)
110: *> TAU(i) must contain the scalar factor of the elementary
111: *> reflector H(i).
112: *> \endverbatim
113: *>
114: *> \param[out] T
115: *> \verbatim
116: *> T is DOUBLE PRECISION array, dimension (LDT,K)
117: *> The k by k triangular factor T of the block reflector.
118: *> If DIRECT = 'F', T is upper triangular; if DIRECT = 'B', T is
119: *> lower triangular. The rest of the array is not used.
120: *> \endverbatim
121: *>
122: *> \param[in] LDT
123: *> \verbatim
124: *> LDT is INTEGER
125: *> The leading dimension of the array T. LDT >= K.
126: *> \endverbatim
127: *
128: * Authors:
129: * ========
130: *
131: *> \author Univ. of Tennessee
132: *> \author Univ. of California Berkeley
133: *> \author Univ. of Colorado Denver
134: *> \author NAG Ltd.
135: *
136: *> \date November 2011
137: *
138: *> \ingroup doubleOTHERcomputational
139: *
140: *> \par Contributors:
141: * ==================
142: *>
143: *> A. Petitet, Computer Science Dept., Univ. of Tenn., Knoxville, USA
144: *
145: *> \par Further Details:
146: * =====================
147: *>
148: *> \verbatim
149: *>
150: *> The shape of the matrix V and the storage of the vectors which define
151: *> the H(i) is best illustrated by the following example with n = 5 and
152: *> k = 3. The elements equal to 1 are not stored; the corresponding
153: *> array elements are modified but restored on exit. The rest of the
154: *> array is not used.
155: *>
156: *> DIRECT = 'F' and STOREV = 'C': DIRECT = 'F' and STOREV = 'R':
157: *>
158: *> ______V_____
159: *> ( v1 v2 v3 ) / \
160: *> ( v1 v2 v3 ) ( v1 v1 v1 v1 v1 . . . . 1 )
161: *> V = ( v1 v2 v3 ) ( v2 v2 v2 v2 v2 . . . 1 )
162: *> ( v1 v2 v3 ) ( v3 v3 v3 v3 v3 . . 1 )
163: *> ( v1 v2 v3 )
164: *> . . .
165: *> . . .
166: *> 1 . .
167: *> 1 .
168: *> 1
169: *>
170: *> DIRECT = 'B' and STOREV = 'C': DIRECT = 'B' and STOREV = 'R':
171: *>
172: *> ______V_____
173: *> 1 / \
174: *> . 1 ( 1 . . . . v1 v1 v1 v1 v1 )
175: *> . . 1 ( . 1 . . . v2 v2 v2 v2 v2 )
176: *> . . . ( . . 1 . . v3 v3 v3 v3 v3 )
177: *> . . .
178: *> ( v1 v2 v3 )
179: *> ( v1 v2 v3 )
180: *> V = ( v1 v2 v3 )
181: *> ( v1 v2 v3 )
182: *> ( v1 v2 v3 )
183: *> \endverbatim
184: *>
185: * =====================================================================
186: SUBROUTINE DLARZT( DIRECT, STOREV, N, K, V, LDV, TAU, T, LDT )
187: *
188: * -- LAPACK computational routine (version 3.4.0) --
189: * -- LAPACK is a software package provided by Univ. of Tennessee, --
190: * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
191: * November 2011
192: *
193: * .. Scalar Arguments ..
194: CHARACTER DIRECT, STOREV
195: INTEGER K, LDT, LDV, N
196: * ..
197: * .. Array Arguments ..
198: DOUBLE PRECISION T( LDT, * ), TAU( * ), V( LDV, * )
199: * ..
200: *
201: * =====================================================================
202: *
203: * .. Parameters ..
204: DOUBLE PRECISION ZERO
205: PARAMETER ( ZERO = 0.0D+0 )
206: * ..
207: * .. Local Scalars ..
208: INTEGER I, INFO, J
209: * ..
210: * .. External Subroutines ..
211: EXTERNAL DGEMV, DTRMV, XERBLA
212: * ..
213: * .. External Functions ..
214: LOGICAL LSAME
215: EXTERNAL LSAME
216: * ..
217: * .. Executable Statements ..
218: *
219: * Check for currently supported options
220: *
221: INFO = 0
222: IF( .NOT.LSAME( DIRECT, 'B' ) ) THEN
223: INFO = -1
224: ELSE IF( .NOT.LSAME( STOREV, 'R' ) ) THEN
225: INFO = -2
226: END IF
227: IF( INFO.NE.0 ) THEN
228: CALL XERBLA( 'DLARZT', -INFO )
229: RETURN
230: END IF
231: *
232: DO 20 I = K, 1, -1
233: IF( TAU( I ).EQ.ZERO ) THEN
234: *
235: * H(i) = I
236: *
237: DO 10 J = I, K
238: T( J, I ) = ZERO
239: 10 CONTINUE
240: ELSE
241: *
242: * general case
243: *
244: IF( I.LT.K ) THEN
245: *
246: * T(i+1:k,i) = - tau(i) * V(i+1:k,1:n) * V(i,1:n)**T
247: *
248: CALL DGEMV( 'No transpose', K-I, N, -TAU( I ),
249: $ V( I+1, 1 ), LDV, V( I, 1 ), LDV, ZERO,
250: $ T( I+1, I ), 1 )
251: *
252: * T(i+1:k,i) = T(i+1:k,i+1:k) * T(i+1:k,i)
253: *
254: CALL DTRMV( 'Lower', 'No transpose', 'Non-unit', K-I,
255: $ T( I+1, I+1 ), LDT, T( I+1, I ), 1 )
256: END IF
257: T( I, I ) = TAU( I )
258: END IF
259: 20 CONTINUE
260: RETURN
261: *
262: * End of DLARZT
263: *
264: END
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