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dlarra.f
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1 *> \brief \b DLARRA computes the splitting points with the specified threshold.
2 *
3 * =========== DOCUMENTATION ===========
4 *
5 * Online html documentation available at
6 * http://www.netlib.org/lapack/explore-html/
7 *
8 *> \htmlonly
9 *> Download DLARRA + dependencies
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11 *> [TGZ]</a>
12 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/dlarra.f">
13 *> [ZIP]</a>
14 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/dlarra.f">
15 *> [TXT]</a>
16 *> \endhtmlonly
17 *
18 * Definition:
19 * ===========
20 *
21 * SUBROUTINE DLARRA( N, D, E, E2, SPLTOL, TNRM,
22 * NSPLIT, ISPLIT, INFO )
23 *
24 * .. Scalar Arguments ..
25 * INTEGER INFO, N, NSPLIT
26 * DOUBLE PRECISION SPLTOL, TNRM
27 * ..
28 * .. Array Arguments ..
29 * INTEGER ISPLIT( * )
30 * DOUBLE PRECISION D( * ), E( * ), E2( * )
31 * ..
32 *
33 *
34 *> \par Purpose:
35 * =============
36 *>
37 *> \verbatim
38 *>
39 *> Compute the splitting points with threshold SPLTOL.
40 *> DLARRA sets any "small" off-diagonal elements to zero.
41 *> \endverbatim
42 *
43 * Arguments:
44 * ==========
45 *
46 *> \param[in] N
47 *> \verbatim
48 *> N is INTEGER
49 *> The order of the matrix. N > 0.
50 *> \endverbatim
51 *>
52 *> \param[in] D
53 *> \verbatim
54 *> D is DOUBLE PRECISION array, dimension (N)
55 *> On entry, the N diagonal elements of the tridiagonal
56 *> matrix T.
57 *> \endverbatim
58 *>
59 *> \param[in,out] E
60 *> \verbatim
61 *> E is DOUBLE PRECISION array, dimension (N)
62 *> On entry, the first (N-1) entries contain the subdiagonal
63 *> elements of the tridiagonal matrix T; E(N) need not be set.
64 *> On exit, the entries E( ISPLIT( I ) ), 1 <= I <= NSPLIT,
65 *> are set to zero, the other entries of E are untouched.
66 *> \endverbatim
67 *>
68 *> \param[in,out] E2
69 *> \verbatim
70 *> E2 is DOUBLE PRECISION array, dimension (N)
71 *> On entry, the first (N-1) entries contain the SQUARES of the
72 *> subdiagonal elements of the tridiagonal matrix T;
73 *> E2(N) need not be set.
74 *> On exit, the entries E2( ISPLIT( I ) ),
75 *> 1 <= I <= NSPLIT, have been set to zero
76 *> \endverbatim
77 *>
78 *> \param[in] SPLTOL
79 *> \verbatim
80 *> SPLTOL is DOUBLE PRECISION
81 *> The threshold for splitting. Two criteria can be used:
82 *> SPLTOL<0 : criterion based on absolute off-diagonal value
83 *> SPLTOL>0 : criterion that preserves relative accuracy
84 *> \endverbatim
85 *>
86 *> \param[in] TNRM
87 *> \verbatim
88 *> TNRM is DOUBLE PRECISION
89 *> The norm of the matrix.
90 *> \endverbatim
91 *>
92 *> \param[out] NSPLIT
93 *> \verbatim
94 *> NSPLIT is INTEGER
95 *> The number of blocks T splits into. 1 <= NSPLIT <= N.
96 *> \endverbatim
97 *>
98 *> \param[out] ISPLIT
99 *> \verbatim
100 *> ISPLIT is INTEGER array, dimension (N)
101 *> The splitting points, at which T breaks up into blocks.
102 *> The first block consists of rows/columns 1 to ISPLIT(1),
103 *> the second of rows/columns ISPLIT(1)+1 through ISPLIT(2),
104 *> etc., and the NSPLIT-th consists of rows/columns
105 *> ISPLIT(NSPLIT-1)+1 through ISPLIT(NSPLIT)=N.
106 *> \endverbatim
107 *>
108 *> \param[out] INFO
109 *> \verbatim
110 *> INFO is INTEGER
111 *> = 0: successful exit
112 *> \endverbatim
113 *
114 * Authors:
115 * ========
116 *
117 *> \author Univ. of Tennessee
118 *> \author Univ. of California Berkeley
119 *> \author Univ. of Colorado Denver
120 *> \author NAG Ltd.
121 *
122 *> \date September 2012
123 *
124 *> \ingroup auxOTHERauxiliary
125 *
126 *> \par Contributors:
127 * ==================
128 *>
129 *> Beresford Parlett, University of California, Berkeley, USA \n
130 *> Jim Demmel, University of California, Berkeley, USA \n
131 *> Inderjit Dhillon, University of Texas, Austin, USA \n
132 *> Osni Marques, LBNL/NERSC, USA \n
133 *> Christof Voemel, University of California, Berkeley, USA
134 *
135 * =====================================================================
136  SUBROUTINE dlarra( N, D, E, E2, SPLTOL, TNRM,
137  $ nsplit, isplit, info )
138 *
139 * -- LAPACK auxiliary routine (version 3.4.2) --
140 * -- LAPACK is a software package provided by Univ. of Tennessee, --
141 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
142 * September 2012
143 *
144 * .. Scalar Arguments ..
145  INTEGER info, n, nsplit
146  DOUBLE PRECISION spltol, tnrm
147 * ..
148 * .. Array Arguments ..
149  INTEGER isplit( * )
150  DOUBLE PRECISION d( * ), e( * ), e2( * )
151 * ..
152 *
153 * =====================================================================
154 *
155 * .. Parameters ..
156  DOUBLE PRECISION zero
157  parameter( zero = 0.0d0 )
158 * ..
159 * .. Local Scalars ..
160  INTEGER i
161  DOUBLE PRECISION eabs, tmp1
162 
163 * ..
164 * .. Intrinsic Functions ..
165  INTRINSIC abs
166 * ..
167 * .. Executable Statements ..
168 *
169  info = 0
170 
171 * Compute splitting points
172  nsplit = 1
173  IF(spltol.LT.zero) THEN
174 * Criterion based on absolute off-diagonal value
175  tmp1 = abs(spltol)* tnrm
176  DO 9 i = 1, n-1
177  eabs = abs( e(i) )
178  IF( eabs .LE. tmp1) THEN
179  e(i) = zero
180  e2(i) = zero
181  isplit( nsplit ) = i
182  nsplit = nsplit + 1
183  END IF
184  9 CONTINUE
185  ELSE
186 * Criterion that guarantees relative accuracy
187  DO 10 i = 1, n-1
188  eabs = abs( e(i) )
189  IF( eabs .LE. spltol * sqrt(abs(d(i)))*sqrt(abs(d(i+1))) )
190  $ THEN
191  e(i) = zero
192  e2(i) = zero
193  isplit( nsplit ) = i
194  nsplit = nsplit + 1
195  END IF
196  10 CONTINUE
197  ENDIF
198  isplit( nsplit ) = n
199 
200  RETURN
201 *
202 * End of DLARRA
203 *
204  END