1*> \brief <b> CHBEV computes the eigenvalues and, optionally, the left and/or right eigenvectors for OTHER matrices</b>
2*
3*  =========== DOCUMENTATION ===========
4*
5* Online html documentation available at
6*            http://www.netlib.org/lapack/explore-html/
7*
8*> \htmlonly
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14*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/chbev.f">
15*> [TXT]</a>
16*> \endhtmlonly
17*
18*  Definition:
19*  ===========
20*
21*       SUBROUTINE CHBEV( JOBZ, UPLO, N, KD, AB, LDAB, W, Z, LDZ, WORK,
22*                         RWORK, INFO )
23*
24*       .. Scalar Arguments ..
25*       CHARACTER          JOBZ, UPLO
26*       INTEGER            INFO, KD, LDAB, LDZ, N
27*       ..
28*       .. Array Arguments ..
29*       REAL               RWORK( * ), W( * )
30*       COMPLEX            AB( LDAB, * ), WORK( * ), Z( LDZ, * )
31*       ..
32*
33*
34*> \par Purpose:
35*  =============
36*>
37*> \verbatim
38*>
39*> CHBEV computes all the eigenvalues and, optionally, eigenvectors of
40*> a complex Hermitian band matrix A.
41*> \endverbatim
42*
43*  Arguments:
44*  ==========
45*
46*> \param[in] JOBZ
47*> \verbatim
48*>          JOBZ is CHARACTER*1
49*>          = 'N':  Compute eigenvalues only;
50*>          = 'V':  Compute eigenvalues and eigenvectors.
51*> \endverbatim
52*>
53*> \param[in] UPLO
54*> \verbatim
55*>          UPLO is CHARACTER*1
56*>          = 'U':  Upper triangle of A is stored;
57*>          = 'L':  Lower triangle of A is stored.
58*> \endverbatim
59*>
60*> \param[in] N
61*> \verbatim
62*>          N is INTEGER
63*>          The order of the matrix A.  N >= 0.
64*> \endverbatim
65*>
66*> \param[in] KD
67*> \verbatim
68*>          KD is INTEGER
69*>          The number of superdiagonals of the matrix A if UPLO = 'U',
70*>          or the number of subdiagonals if UPLO = 'L'.  KD >= 0.
71*> \endverbatim
72*>
73*> \param[in,out] AB
74*> \verbatim
75*>          AB is COMPLEX array, dimension (LDAB, N)
76*>          On entry, the upper or lower triangle of the Hermitian band
77*>          matrix A, stored in the first KD+1 rows of the array.  The
78*>          j-th column of A is stored in the j-th column of the array AB
79*>          as follows:
80*>          if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j;
81*>          if UPLO = 'L', AB(1+i-j,j)    = A(i,j) for j<=i<=min(n,j+kd).
82*>
83*>          On exit, AB is overwritten by values generated during the
84*>          reduction to tridiagonal form.  If UPLO = 'U', the first
85*>          superdiagonal and the diagonal of the tridiagonal matrix T
86*>          are returned in rows KD and KD+1 of AB, and if UPLO = 'L',
87*>          the diagonal and first subdiagonal of T are returned in the
88*>          first two rows of AB.
89*> \endverbatim
90*>
91*> \param[in] LDAB
92*> \verbatim
93*>          LDAB is INTEGER
94*>          The leading dimension of the array AB.  LDAB >= KD + 1.
95*> \endverbatim
96*>
97*> \param[out] W
98*> \verbatim
99*>          W is REAL array, dimension (N)
100*>          If INFO = 0, the eigenvalues in ascending order.
101*> \endverbatim
102*>
103*> \param[out] Z
104*> \verbatim
105*>          Z is COMPLEX array, dimension (LDZ, N)
106*>          If JOBZ = 'V', then if INFO = 0, Z contains the orthonormal
107*>          eigenvectors of the matrix A, with the i-th column of Z
108*>          holding the eigenvector associated with W(i).
109*>          If JOBZ = 'N', then Z is not referenced.
110*> \endverbatim
111*>
112*> \param[in] LDZ
113*> \verbatim
114*>          LDZ is INTEGER
115*>          The leading dimension of the array Z.  LDZ >= 1, and if
116*>          JOBZ = 'V', LDZ >= max(1,N).
117*> \endverbatim
118*>
119*> \param[out] WORK
120*> \verbatim
121*>          WORK is COMPLEX array, dimension (N)
122*> \endverbatim
123*>
124*> \param[out] RWORK
125*> \verbatim
126*>          RWORK is REAL array, dimension (max(1,3*N-2))
127*> \endverbatim
128*>
129*> \param[out] INFO
130*> \verbatim
131*>          INFO is INTEGER
132*>          = 0:  successful exit.
133*>          < 0:  if INFO = -i, the i-th argument had an illegal value.
134*>          > 0:  if INFO = i, the algorithm failed to converge; i
135*>                off-diagonal elements of an intermediate tridiagonal
136*>                form did not converge to zero.
137*> \endverbatim
138*
139*  Authors:
140*  ========
141*
142*> \author Univ. of Tennessee
143*> \author Univ. of California Berkeley
144*> \author Univ. of Colorado Denver
145*> \author NAG Ltd.
146*
147*> \date December 2016
148*
149*> \ingroup complexOTHEReigen
150*
151*  =====================================================================
152      SUBROUTINE CHBEV( JOBZ, UPLO, N, KD, AB, LDAB, W, Z, LDZ, WORK,
153     $                  RWORK, INFO )
154*
155*  -- LAPACK driver routine (version 3.7.0) --
156*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
157*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
158*     December 2016
159*
160*     .. Scalar Arguments ..
161      CHARACTER          JOBZ, UPLO
162      INTEGER            INFO, KD, LDAB, LDZ, N
163*     ..
164*     .. Array Arguments ..
165      REAL               RWORK( * ), W( * )
166      COMPLEX            AB( LDAB, * ), WORK( * ), Z( LDZ, * )
167*     ..
168*
169*  =====================================================================
170*
171*     .. Parameters ..
172      REAL               ZERO, ONE
173      PARAMETER          ( ZERO = 0.0E0, ONE = 1.0E0 )
174*     ..
175*     .. Local Scalars ..
176      LOGICAL            LOWER, WANTZ
177      INTEGER            IINFO, IMAX, INDE, INDRWK, ISCALE
178      REAL               ANRM, BIGNUM, EPS, RMAX, RMIN, SAFMIN, SIGMA,
179     $                   SMLNUM
180*     ..
181*     .. External Functions ..
182      LOGICAL            LSAME
183      REAL               CLANHB, SLAMCH
184      EXTERNAL           LSAME, CLANHB, SLAMCH
185*     ..
186*     .. External Subroutines ..
187      EXTERNAL           CHBTRD, CLASCL, CSTEQR, SSCAL, SSTERF, XERBLA
188*     ..
189*     .. Intrinsic Functions ..
190      INTRINSIC          SQRT
191*     ..
192*     .. Executable Statements ..
193*
194*     Test the input parameters.
195*
196      WANTZ = LSAME( JOBZ, 'V' )
197      LOWER = LSAME( UPLO, 'L' )
198*
199      INFO = 0
200      IF( .NOT.( WANTZ .OR. LSAME( JOBZ, 'N' ) ) ) THEN
201         INFO = -1
202      ELSE IF( .NOT.( LOWER .OR. LSAME( UPLO, 'U' ) ) ) THEN
203         INFO = -2
204      ELSE IF( N.LT.0 ) THEN
205         INFO = -3
206      ELSE IF( KD.LT.0 ) THEN
207         INFO = -4
208      ELSE IF( LDAB.LT.KD+1 ) THEN
209         INFO = -6
210      ELSE IF( LDZ.LT.1 .OR. ( WANTZ .AND. LDZ.LT.N ) ) THEN
211         INFO = -9
212      END IF
213*
214      IF( INFO.NE.0 ) THEN
215         CALL XERBLA( 'CHBEV ', -INFO )
216         RETURN
217      END IF
218*
219*     Quick return if possible
220*
221      IF( N.EQ.0 )
222     $   RETURN
223*
224      IF( N.EQ.1 ) THEN
225         IF( LOWER ) THEN
226            W( 1 ) = AB( 1, 1 )
227         ELSE
228            W( 1 ) = AB( KD+1, 1 )
229         END IF
230         IF( WANTZ )
231     $      Z( 1, 1 ) = ONE
232         RETURN
233      END IF
234*
235*     Get machine constants.
236*
237      SAFMIN = SLAMCH( 'Safe minimum' )
238      EPS = SLAMCH( 'Precision' )
239      SMLNUM = SAFMIN / EPS
240      BIGNUM = ONE / SMLNUM
241      RMIN = SQRT( SMLNUM )
242      RMAX = SQRT( BIGNUM )
243*
244*     Scale matrix to allowable range, if necessary.
245*
246      ANRM = CLANHB( 'M', UPLO, N, KD, AB, LDAB, RWORK )
247      ISCALE = 0
248      IF( ANRM.GT.ZERO .AND. ANRM.LT.RMIN ) THEN
249         ISCALE = 1
250         SIGMA = RMIN / ANRM
251      ELSE IF( ANRM.GT.RMAX ) THEN
252         ISCALE = 1
253         SIGMA = RMAX / ANRM
254      END IF
255      IF( ISCALE.EQ.1 ) THEN
256         IF( LOWER ) THEN
257            CALL CLASCL( 'B', KD, KD, ONE, SIGMA, N, N, AB, LDAB, INFO )
258         ELSE
259            CALL CLASCL( 'Q', KD, KD, ONE, SIGMA, N, N, AB, LDAB, INFO )
260         END IF
261      END IF
262*
263*     Call CHBTRD to reduce Hermitian band matrix to tridiagonal form.
264*
265      INDE = 1
266      CALL CHBTRD( JOBZ, UPLO, N, KD, AB, LDAB, W, RWORK( INDE ), Z,
267     $             LDZ, WORK, IINFO )
268*
269*     For eigenvalues only, call SSTERF.  For eigenvectors, call CSTEQR.
270*
271      IF( .NOT.WANTZ ) THEN
272         CALL SSTERF( N, W, RWORK( INDE ), INFO )
273      ELSE
274         INDRWK = INDE + N
275         CALL CSTEQR( JOBZ, N, W, RWORK( INDE ), Z, LDZ,
276     $                RWORK( INDRWK ), INFO )
277      END IF
278*
279*     If matrix was scaled, then rescale eigenvalues appropriately.
280*
281      IF( ISCALE.EQ.1 ) THEN
282         IF( INFO.EQ.0 ) THEN
283            IMAX = N
284         ELSE
285            IMAX = INFO - 1
286         END IF
287         CALL SSCAL( IMAX, ONE / SIGMA, W, 1 )
288      END IF
289*
290      RETURN
291*
292*     End of CHBEV
293*
294      END
295