LAPACK  3.5.0
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slarfy.f
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1 *> \brief \b SLARFY
2 *
3 * =========== DOCUMENTATION ===========
4 *
5 * Online html documentation available at
6 * http://www.netlib.org/lapack/explore-html/
7 *
8 * Definition:
9 * ===========
10 *
11 * SUBROUTINE SLARFY( UPLO, N, V, INCV, TAU, C, LDC, WORK )
12 *
13 * .. Scalar Arguments ..
14 * CHARACTER UPLO
15 * INTEGER INCV, LDC, N
16 * REAL TAU
17 * ..
18 * .. Array Arguments ..
19 * REAL C( LDC, * ), V( * ), WORK( * )
20 * ..
21 *
22 *
23 *> \par Purpose:
24 * =============
25 *>
26 *> \verbatim
27 *>
28 *> SLARFY applies an elementary reflector, or Householder matrix, H,
29 *> to an n x n symmetric matrix C, from both the left and the right.
30 *>
31 *> H is represented in the form
32 *>
33 *> H = I - tau * v * v'
34 *>
35 *> where tau is a scalar and v is a vector.
36 *>
37 *> If tau is zero, then H is taken to be the unit matrix.
38 *> \endverbatim
39 *
40 * Arguments:
41 * ==========
42 *
43 *> \param[in] UPLO
44 *> \verbatim
45 *> UPLO is CHARACTER*1
46 *> Specifies whether the upper or lower triangular part of the
47 *> symmetric matrix C is stored.
48 *> = 'U': Upper triangle
49 *> = 'L': Lower triangle
50 *> \endverbatim
51 *>
52 *> \param[in] N
53 *> \verbatim
54 *> N is INTEGER
55 *> The number of rows and columns of the matrix C. N >= 0.
56 *> \endverbatim
57 *>
58 *> \param[in] V
59 *> \verbatim
60 *> V is REAL array, dimension
61 *> (1 + (N-1)*abs(INCV))
62 *> The vector v as described above.
63 *> \endverbatim
64 *>
65 *> \param[in] INCV
66 *> \verbatim
67 *> INCV is INTEGER
68 *> The increment between successive elements of v. INCV must
69 *> not be zero.
70 *> \endverbatim
71 *>
72 *> \param[in] TAU
73 *> \verbatim
74 *> TAU is REAL
75 *> The value tau as described above.
76 *> \endverbatim
77 *>
78 *> \param[in,out] C
79 *> \verbatim
80 *> C is REAL array, dimension (LDC, N)
81 *> On entry, the matrix C.
82 *> On exit, C is overwritten by H * C * H'.
83 *> \endverbatim
84 *>
85 *> \param[in] LDC
86 *> \verbatim
87 *> LDC is INTEGER
88 *> The leading dimension of the array C. LDC >= max( 1, N ).
89 *> \endverbatim
90 *>
91 *> \param[out] WORK
92 *> \verbatim
93 *> WORK is REAL array, dimension (N)
94 *> \endverbatim
95 *
96 * Authors:
97 * ========
98 *
99 *> \author Univ. of Tennessee
100 *> \author Univ. of California Berkeley
101 *> \author Univ. of Colorado Denver
102 *> \author NAG Ltd.
103 *
104 *> \date November 2011
105 *
106 *> \ingroup single_eig
107 *
108 * =====================================================================
109  SUBROUTINE slarfy( UPLO, N, V, INCV, TAU, C, LDC, WORK )
110 *
111 * -- LAPACK test routine (version 3.4.0) --
112 * -- LAPACK is a software package provided by Univ. of Tennessee, --
113 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
114 * November 2011
115 *
116 * .. Scalar Arguments ..
117  CHARACTER uplo
118  INTEGER incv, ldc, n
119  REAL tau
120 * ..
121 * .. Array Arguments ..
122  REAL c( ldc, * ), v( * ), work( * )
123 * ..
124 *
125 * =====================================================================
126 *
127 * .. Parameters ..
128  REAL one, zero, half
129  parameter( one = 1.0e+0, zero = 0.0e+0, half = 0.5e+0 )
130 * ..
131 * .. Local Scalars ..
132  REAL alpha
133 * ..
134 * .. External Subroutines ..
135  EXTERNAL saxpy, ssymv, ssyr2
136 * ..
137 * .. External Functions ..
138  REAL sdot
139  EXTERNAL sdot
140 * ..
141 * .. Executable Statements ..
142 *
143  IF( tau.EQ.zero )
144  $ RETURN
145 *
146 * Form w:= C * v
147 *
148  CALL ssymv( uplo, n, one, c, ldc, v, incv, zero, work, 1 )
149 *
150  alpha = -half*tau*sdot( n, work, 1, v, incv )
151  CALL saxpy( n, alpha, v, incv, work, 1 )
152 *
153 * C := C - v * w' - w * v'
154 *
155  CALL ssyr2( uplo, n, -tau, v, incv, work, 1, c, ldc )
156 *
157  RETURN
158 *
159 * End of SLARFY
160 *
161  END