LAPACK  3.10.1 LAPACK: Linear Algebra PACKage
sstev.f
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1 *> \brief <b> SSTEV computes the eigenvalues and, optionally, the left and/or right eigenvectors for OTHER matrices</b>
2 *
3 * =========== DOCUMENTATION ===========
4 *
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17 *
18 * Definition:
19 * ===========
20 *
21 * SUBROUTINE SSTEV( JOBZ, N, D, E, Z, LDZ, WORK, INFO )
22 *
23 * .. Scalar Arguments ..
24 * CHARACTER JOBZ
25 * INTEGER INFO, LDZ, N
26 * ..
27 * .. Array Arguments ..
28 * REAL D( * ), E( * ), WORK( * ), Z( LDZ, * )
29 * ..
30 *
31 *
32 *> \par Purpose:
33 * =============
34 *>
35 *> \verbatim
36 *>
37 *> SSTEV computes all eigenvalues and, optionally, eigenvectors of a
38 *> real symmetric tridiagonal matrix A.
39 *> \endverbatim
40 *
41 * Arguments:
42 * ==========
43 *
44 *> \param[in] JOBZ
45 *> \verbatim
46 *> JOBZ is CHARACTER*1
47 *> = 'N': Compute eigenvalues only;
48 *> = 'V': Compute eigenvalues and eigenvectors.
49 *> \endverbatim
50 *>
51 *> \param[in] N
52 *> \verbatim
53 *> N is INTEGER
54 *> The order of the matrix. N >= 0.
55 *> \endverbatim
56 *>
57 *> \param[in,out] D
58 *> \verbatim
59 *> D is REAL array, dimension (N)
60 *> On entry, the n diagonal elements of the tridiagonal matrix
61 *> A.
62 *> On exit, if INFO = 0, the eigenvalues in ascending order.
63 *> \endverbatim
64 *>
65 *> \param[in,out] E
66 *> \verbatim
67 *> E is REAL array, dimension (N-1)
68 *> On entry, the (n-1) subdiagonal elements of the tridiagonal
69 *> matrix A, stored in elements 1 to N-1 of E.
70 *> On exit, the contents of E are destroyed.
71 *> \endverbatim
72 *>
73 *> \param[out] Z
74 *> \verbatim
75 *> Z is REAL array, dimension (LDZ, N)
76 *> If JOBZ = 'V', then if INFO = 0, Z contains the orthonormal
77 *> eigenvectors of the matrix A, with the i-th column of Z
78 *> holding the eigenvector associated with D(i).
79 *> If JOBZ = 'N', then Z is not referenced.
80 *> \endverbatim
81 *>
82 *> \param[in] LDZ
83 *> \verbatim
84 *> LDZ is INTEGER
85 *> The leading dimension of the array Z. LDZ >= 1, and if
86 *> JOBZ = 'V', LDZ >= max(1,N).
87 *> \endverbatim
88 *>
89 *> \param[out] WORK
90 *> \verbatim
91 *> WORK is REAL array, dimension (max(1,2*N-2))
92 *> If JOBZ = 'N', WORK is not referenced.
93 *> \endverbatim
94 *>
95 *> \param[out] INFO
96 *> \verbatim
97 *> INFO is INTEGER
98 *> = 0: successful exit
99 *> < 0: if INFO = -i, the i-th argument had an illegal value
100 *> > 0: if INFO = i, the algorithm failed to converge; i
101 *> off-diagonal elements of E did not converge to zero.
102 *> \endverbatim
103 *
104 * Authors:
105 * ========
106 *
107 *> \author Univ. of Tennessee
108 *> \author Univ. of California Berkeley
109 *> \author Univ. of Colorado Denver
110 *> \author NAG Ltd.
111 *
112 *> \ingroup realOTHEReigen
113 *
114 * =====================================================================
115  SUBROUTINE sstev( JOBZ, N, D, E, Z, LDZ, WORK, INFO )
116 *
117 * -- LAPACK driver routine --
118 * -- LAPACK is a software package provided by Univ. of Tennessee, --
119 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
120 *
121 * .. Scalar Arguments ..
122  CHARACTER JOBZ
123  INTEGER INFO, LDZ, N
124 * ..
125 * .. Array Arguments ..
126  REAL D( * ), E( * ), WORK( * ), Z( LDZ, * )
127 * ..
128 *
129 * =====================================================================
130 *
131 * .. Parameters ..
132  REAL ZERO, ONE
133  parameter( zero = 0.0e0, one = 1.0e0 )
134 * ..
135 * .. Local Scalars ..
136  LOGICAL WANTZ
137  INTEGER IMAX, ISCALE
138  REAL BIGNUM, EPS, RMAX, RMIN, SAFMIN, SIGMA, SMLNUM,
139  \$ TNRM
140 * ..
141 * .. External Functions ..
142  LOGICAL LSAME
143  REAL SLAMCH, SLANST
144  EXTERNAL lsame, slamch, slanst
145 * ..
146 * .. External Subroutines ..
147  EXTERNAL sscal, ssteqr, ssterf, xerbla
148 * ..
149 * .. Intrinsic Functions ..
150  INTRINSIC sqrt
151 * ..
152 * .. Executable Statements ..
153 *
154 * Test the input parameters.
155 *
156  wantz = lsame( jobz, 'V' )
157 *
158  info = 0
159  IF( .NOT.( wantz .OR. lsame( jobz, 'N' ) ) ) THEN
160  info = -1
161  ELSE IF( n.LT.0 ) THEN
162  info = -2
163  ELSE IF( ldz.LT.1 .OR. ( wantz .AND. ldz.LT.n ) ) THEN
164  info = -6
165  END IF
166 *
167  IF( info.NE.0 ) THEN
168  CALL xerbla( 'SSTEV ', -info )
169  RETURN
170  END IF
171 *
172 * Quick return if possible
173 *
174  IF( n.EQ.0 )
175  \$ RETURN
176 *
177  IF( n.EQ.1 ) THEN
178  IF( wantz )
179  \$ z( 1, 1 ) = one
180  RETURN
181  END IF
182 *
183 * Get machine constants.
184 *
185  safmin = slamch( 'Safe minimum' )
186  eps = slamch( 'Precision' )
187  smlnum = safmin / eps
188  bignum = one / smlnum
189  rmin = sqrt( smlnum )
190  rmax = sqrt( bignum )
191 *
192 * Scale matrix to allowable range, if necessary.
193 *
194  iscale = 0
195  tnrm = slanst( 'M', n, d, e )
196  IF( tnrm.GT.zero .AND. tnrm.LT.rmin ) THEN
197  iscale = 1
198  sigma = rmin / tnrm
199  ELSE IF( tnrm.GT.rmax ) THEN
200  iscale = 1
201  sigma = rmax / tnrm
202  END IF
203  IF( iscale.EQ.1 ) THEN
204  CALL sscal( n, sigma, d, 1 )
205  CALL sscal( n-1, sigma, e( 1 ), 1 )
206  END IF
207 *
208 * For eigenvalues only, call SSTERF. For eigenvalues and
209 * eigenvectors, call SSTEQR.
210 *
211  IF( .NOT.wantz ) THEN
212  CALL ssterf( n, d, e, info )
213  ELSE
214  CALL ssteqr( 'I', n, d, e, z, ldz, work, info )
215  END IF
216 *
217 * If matrix was scaled, then rescale eigenvalues appropriately.
218 *
219  IF( iscale.EQ.1 ) THEN
220  IF( info.EQ.0 ) THEN
221  imax = n
222  ELSE
223  imax = info - 1
224  END IF
225  CALL sscal( imax, one / sigma, d, 1 )
226  END IF
227 *
228  RETURN
229 *
230 * End of SSTEV
231 *
232  END
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
subroutine ssteqr(COMPZ, N, D, E, Z, LDZ, WORK, INFO)
SSTEQR
Definition: ssteqr.f:131
subroutine ssterf(N, D, E, INFO)
SSTERF
Definition: ssterf.f:86
subroutine sstev(JOBZ, N, D, E, Z, LDZ, WORK, INFO)
SSTEV computes the eigenvalues and, optionally, the left and/or right eigenvectors for OTHER matrices
Definition: sstev.f:116
subroutine sscal(N, SA, SX, INCX)
SSCAL
Definition: sscal.f:79