LAPACK  3.10.0
LAPACK: Linear Algebra PACKage
slartgs.f
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1 *> \brief \b SLARTGS generates a plane rotation designed to introduce a bulge in implicit QR iteration for the bidiagonal SVD problem.
2 *
3 * =========== DOCUMENTATION ===========
4 *
5 * Online html documentation available at
6 * http://www.netlib.org/lapack/explore-html/
7 *
8 *> \htmlonly
9 *> Download SLARTGS + dependencies
10 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/slartgs.f">
11 *> [TGZ]</a>
12 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/slartgs.f">
13 *> [ZIP]</a>
14 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/slartgs.f">
15 *> [TXT]</a>
16 *> \endhtmlonly
17 *
18 * Definition:
19 * ===========
20 *
21 * SUBROUTINE SLARTGS( X, Y, SIGMA, CS, SN )
22 *
23 * .. Scalar Arguments ..
24 * REAL CS, SIGMA, SN, X, Y
25 * ..
26 *
27 *
28 *> \par Purpose:
29 * =============
30 *>
31 *> \verbatim
32 *>
33 *> SLARTGS generates a plane rotation designed to introduce a bulge in
34 *> Golub-Reinsch-style implicit QR iteration for the bidiagonal SVD
35 *> problem. X and Y are the top-row entries, and SIGMA is the shift.
36 *> The computed CS and SN define a plane rotation satisfying
37 *>
38 *> [ CS SN ] . [ X^2 - SIGMA ] = [ R ],
39 *> [ -SN CS ] [ X * Y ] [ 0 ]
40 *>
41 *> with R nonnegative. If X^2 - SIGMA and X * Y are 0, then the
42 *> rotation is by PI/2.
43 *> \endverbatim
44 *
45 * Arguments:
46 * ==========
47 *
48 *> \param[in] X
49 *> \verbatim
50 *> X is REAL
51 *> The (1,1) entry of an upper bidiagonal matrix.
52 *> \endverbatim
53 *>
54 *> \param[in] Y
55 *> \verbatim
56 *> Y is REAL
57 *> The (1,2) entry of an upper bidiagonal matrix.
58 *> \endverbatim
59 *>
60 *> \param[in] SIGMA
61 *> \verbatim
62 *> SIGMA is REAL
63 *> The shift.
64 *> \endverbatim
65 *>
66 *> \param[out] CS
67 *> \verbatim
68 *> CS is REAL
69 *> The cosine of the rotation.
70 *> \endverbatim
71 *>
72 *> \param[out] SN
73 *> \verbatim
74 *> SN is REAL
75 *> The sine of the rotation.
76 *> \endverbatim
77 *
78 * Authors:
79 * ========
80 *
81 *> \author Univ. of Tennessee
82 *> \author Univ. of California Berkeley
83 *> \author Univ. of Colorado Denver
84 *> \author NAG Ltd.
85 *
86 *> \ingroup auxOTHERcomputational
87 *
88 * =====================================================================
89  SUBROUTINE slartgs( X, Y, SIGMA, CS, SN )
90 *
91 * -- LAPACK computational routine --
92 * -- LAPACK is a software package provided by Univ. of Tennessee, --
93 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
94 *
95 * .. Scalar Arguments ..
96  REAL CS, SIGMA, SN, X, Y
97 * ..
98 *
99 * ===================================================================
100 *
101 * .. Parameters ..
102  REAL NEGONE, ONE, ZERO
103  parameter( negone = -1.0e0, one = 1.0e0, zero = 0.0e0 )
104 * ..
105 * .. Local Scalars ..
106  REAL R, S, THRESH, W, Z
107 * ..
108 * .. External Subroutines ..
109  EXTERNAL slartgp
110 * ..
111 * .. External Functions ..
112  REAL SLAMCH
113  EXTERNAL slamch
114 * .. Executable Statements ..
115 *
116  thresh = slamch('E')
117 *
118 * Compute the first column of B**T*B - SIGMA^2*I, up to a scale
119 * factor.
120 *
121  IF( (sigma .EQ. zero .AND. abs(x) .LT. thresh) .OR.
122  $ (abs(x) .EQ. sigma .AND. y .EQ. zero) ) THEN
123  z = zero
124  w = zero
125  ELSE IF( sigma .EQ. zero ) THEN
126  IF( x .GE. zero ) THEN
127  z = x
128  w = y
129  ELSE
130  z = -x
131  w = -y
132  END IF
133  ELSE IF( abs(x) .LT. thresh ) THEN
134  z = -sigma*sigma
135  w = zero
136  ELSE
137  IF( x .GE. zero ) THEN
138  s = one
139  ELSE
140  s = negone
141  END IF
142  z = s * (abs(x)-sigma) * (s+sigma/x)
143  w = s * y
144  END IF
145 *
146 * Generate the rotation.
147 * CALL SLARTGP( Z, W, CS, SN, R ) might seem more natural;
148 * reordering the arguments ensures that if Z = 0 then the rotation
149 * is by PI/2.
150 *
151  CALL slartgp( w, z, sn, cs, r )
152 *
153  RETURN
154 *
155 * End SLARTGS
156 *
157  END
158 
subroutine slartgp(F, G, CS, SN, R)
SLARTGP generates a plane rotation so that the diagonal is nonnegative.
Definition: slartgp.f:95
subroutine slartgs(X, Y, SIGMA, CS, SN)
SLARTGS generates a plane rotation designed to introduce a bulge in implicit QR iteration for the bid...
Definition: slartgs.f:90