LAPACK: SRC/ssbevx.f Source File

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00001       SUBROUTINE SSBEVX( JOBZ, RANGE, UPLO, N, KD, AB, LDAB, Q, LDQ, VL,
00002      $                   VU, IL, IU, ABSTOL, M, W, Z, LDZ, WORK, IWORK,
00003      $                   IFAIL, INFO )
00004 *
00005 *  -- LAPACK driver routine (version 3.2) --
00006 *  -- LAPACK is a software package provided by Univ. of Tennessee,    --
00007 *  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
00008 *     November 2006
00009 *
00010 *     .. Scalar Arguments ..
00011       CHARACTER          JOBZ, RANGE, UPLO
00012       INTEGER            IL, INFO, IU, KD, LDAB, LDQ, LDZ, M, N
00013       REAL               ABSTOL, VL, VU
00014 *     ..
00015 *     .. Array Arguments ..
00016       INTEGER            IFAIL( * ), IWORK( * )
00017       REAL               AB( LDAB, * ), Q( LDQ, * ), W( * ), WORK( * ),
00018      $                   Z( LDZ, * )
00019 *     ..
00020 *
00021 *  Purpose
00022 *  =======
00023 *
00024 *  SSBEVX computes selected eigenvalues and, optionally, eigenvectors
00025 *  of a real symmetric band matrix A.  Eigenvalues and eigenvectors can
00026 *  be selected by specifying either a range of values or a range of
00027 *  indices for the desired eigenvalues.
00028 *
00029 *  Arguments
00030 *  =========
00031 *
00032 *  JOBZ    (input) CHARACTER*1
00033 *          = 'N':  Compute eigenvalues only;
00034 *          = 'V':  Compute eigenvalues and eigenvectors.
00035 *
00036 *  RANGE   (input) CHARACTER*1
00037 *          = 'A': all eigenvalues will be found;
00038 *          = 'V': all eigenvalues in the half-open interval (VL,VU]
00039 *                 will be found;
00040 *          = 'I': the IL-th through IU-th eigenvalues will be found.
00041 *
00042 *  UPLO    (input) CHARACTER*1
00043 *          = 'U':  Upper triangle of A is stored;
00044 *          = 'L':  Lower triangle of A is stored.
00045 *
00046 *  N       (input) INTEGER
00047 *          The order of the matrix A.  N >= 0.
00048 *
00049 *  KD      (input) INTEGER
00050 *          The number of superdiagonals of the matrix A if UPLO = 'U',
00051 *          or the number of subdiagonals if UPLO = 'L'.  KD >= 0.
00052 *
00053 *  AB      (input/output) REAL array, dimension (LDAB, N)
00054 *          On entry, the upper or lower triangle of the symmetric band
00055 *          matrix A, stored in the first KD+1 rows of the array.  The
00056 *          j-th column of A is stored in the j-th column of the array AB
00057 *          as follows:
00058 *          if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j;
00059 *          if UPLO = 'L', AB(1+i-j,j)    = A(i,j) for j<=i<=min(n,j+kd).
00060 *
00061 *          On exit, AB is overwritten by values generated during the
00062 *          reduction to tridiagonal form.  If UPLO = 'U', the first
00063 *          superdiagonal and the diagonal of the tridiagonal matrix T
00064 *          are returned in rows KD and KD+1 of AB, and if UPLO = 'L',
00065 *          the diagonal and first subdiagonal of T are returned in the
00066 *          first two rows of AB.
00067 *
00068 *  LDAB    (input) INTEGER
00069 *          The leading dimension of the array AB.  LDAB >= KD + 1.
00070 *
00071 *  Q       (output) REAL array, dimension (LDQ, N)
00072 *          If JOBZ = 'V', the N-by-N orthogonal matrix used in the
00073 *                         reduction to tridiagonal form.
00074 *          If JOBZ = 'N', the array Q is not referenced.
00075 *
00076 *  LDQ     (input) INTEGER
00077 *          The leading dimension of the array Q.  If JOBZ = 'V', then
00078 *          LDQ >= max(1,N).
00079 *
00080 *  VL      (input) REAL
00081 *  VU      (input) REAL
00082 *          If RANGE='V', the lower and upper bounds of the interval to
00083 *          be searched for eigenvalues. VL < VU.
00084 *          Not referenced if RANGE = 'A' or 'I'.
00085 *
00086 *  IL      (input) INTEGER
00087 *  IU      (input) INTEGER
00088 *          If RANGE='I', the indices (in ascending order) of the
00089 *          smallest and largest eigenvalues to be returned.
00090 *          1 <= IL <= IU <= N, if N > 0; IL = 1 and IU = 0 if N = 0.
00091 *          Not referenced if RANGE = 'A' or 'V'.
00092 *
00093 *  ABSTOL  (input) REAL
00094 *          The absolute error tolerance for the eigenvalues.
00095 *          An approximate eigenvalue is accepted as converged
00096 *          when it is determined to lie in an interval [a,b]
00097 *          of width less than or equal to
00098 *
00099 *                  ABSTOL + EPS *   max( |a|,|b| ) ,
00100 *
00101 *          where EPS is the machine precision.  If ABSTOL is less than
00102 *          or equal to zero, then  EPS*|T|  will be used in its place,
00103 *          where |T| is the 1-norm of the tridiagonal matrix obtained
00104 *          by reducing AB to tridiagonal form.
00105 *
00106 *          Eigenvalues will be computed most accurately when ABSTOL is
00107 *          set to twice the underflow threshold 2*SLAMCH('S'), not zero.
00108 *          If this routine returns with INFO>0, indicating that some
00109 *          eigenvectors did not converge, try setting ABSTOL to
00110 *          2*SLAMCH('S').
00111 *
00112 *          See "Computing Small Singular Values of Bidiagonal Matrices
00113 *          with Guaranteed High Relative Accuracy," by Demmel and
00114 *          Kahan, LAPACK Working Note #3.
00115 *
00116 *  M       (output) INTEGER
00117 *          The total number of eigenvalues found.  0 <= M <= N.
00118 *          If RANGE = 'A', M = N, and if RANGE = 'I', M = IU-IL+1.
00119 *
00120 *  W       (output) REAL array, dimension (N)
00121 *          The first M elements contain the selected eigenvalues in
00122 *          ascending order.
00123 *
00124 *  Z       (output) REAL array, dimension (LDZ, max(1,M))
00125 *          If JOBZ = 'V', then if INFO = 0, the first M columns of Z
00126 *          contain the orthonormal eigenvectors of the matrix A
00127 *          corresponding to the selected eigenvalues, with the i-th
00128 *          column of Z holding the eigenvector associated with W(i).
00129 *          If an eigenvector fails to converge, then that column of Z
00130 *          contains the latest approximation to the eigenvector, and the
00131 *          index of the eigenvector is returned in IFAIL.
00132 *          If JOBZ = 'N', then Z is not referenced.
00133 *          Note: the user must ensure that at least max(1,M) columns are
00134 *          supplied in the array Z; if RANGE = 'V', the exact value of M
00135 *          is not known in advance and an upper bound must be used.
00136 *
00137 *  LDZ     (input) INTEGER
00138 *          The leading dimension of the array Z.  LDZ >= 1, and if
00139 *          JOBZ = 'V', LDZ >= max(1,N).
00140 *
00141 *  WORK    (workspace) REAL array, dimension (7*N)
00142 *
00143 *  IWORK   (workspace) INTEGER array, dimension (5*N)
00144 *
00145 *  IFAIL   (output) INTEGER array, dimension (N)
00146 *          If JOBZ = 'V', then if INFO = 0, the first M elements of
00147 *          IFAIL are zero.  If INFO > 0, then IFAIL contains the
00148 *          indices of the eigenvectors that failed to converge.
00149 *          If JOBZ = 'N', then IFAIL is not referenced.
00150 *
00151 *  INFO    (output) INTEGER
00152 *          = 0:  successful exit.
00153 *          < 0:  if INFO = -i, the i-th argument had an illegal value.
00154 *          > 0:  if INFO = i, then i eigenvectors failed to converge.
00155 *                Their indices are stored in array IFAIL.
00156 *
00157 *  =====================================================================
00158 *
00159 *     .. Parameters ..
00160       REAL               ZERO, ONE
00161       PARAMETER          ( ZERO = 0.0E0, ONE = 1.0E0 )
00162 *     ..
00163 *     .. Local Scalars ..
00164       LOGICAL            ALLEIG, INDEIG, LOWER, TEST, VALEIG, WANTZ
00165       CHARACTER          ORDER
00166       INTEGER            I, IINFO, IMAX, INDD, INDE, INDEE, INDIBL,
00167      $                   INDISP, INDIWO, INDWRK, ISCALE, ITMP1, J, JJ,
00168      $                   NSPLIT
00169       REAL               ABSTLL, ANRM, BIGNUM, EPS, RMAX, RMIN, SAFMIN,
00170      $                   SIGMA, SMLNUM, TMP1, VLL, VUU
00171 *     ..
00172 *     .. External Functions ..
00173       LOGICAL            LSAME
00174       REAL               SLAMCH, SLANSB
00175       EXTERNAL           LSAME, SLAMCH, SLANSB
00176 *     ..
00177 *     .. External Subroutines ..
00178       EXTERNAL           SCOPY, SGEMV, SLACPY, SLASCL, SSBTRD, SSCAL,
00179      $                   SSTEBZ, SSTEIN, SSTEQR, SSTERF, SSWAP, XERBLA
00180 *     ..
00181 *     .. Intrinsic Functions ..
00182       INTRINSIC          MAX, MIN, SQRT
00183 *     ..
00184 *     .. Executable Statements ..
00185 *
00186 *     Test the input parameters.
00187 *
00188       WANTZ = LSAME( JOBZ, 'V' )
00189       ALLEIG = LSAME( RANGE, 'A' )
00190       VALEIG = LSAME( RANGE, 'V' )
00191       INDEIG = LSAME( RANGE, 'I' )
00192       LOWER = LSAME( UPLO, 'L' )
00193 *
00194       INFO = 0
00195       IF( .NOT.( WANTZ .OR. LSAME( JOBZ, 'N' ) ) ) THEN
00196          INFO = -1
00197       ELSE IF( .NOT.( ALLEIG .OR. VALEIG .OR. INDEIG ) ) THEN
00198          INFO = -2
00199       ELSE IF( .NOT.( LOWER .OR. LSAME( UPLO, 'U' ) ) ) THEN
00200          INFO = -3
00201       ELSE IF( N.LT.0 ) THEN
00202          INFO = -4
00203       ELSE IF( KD.LT.0 ) THEN
00204          INFO = -5
00205       ELSE IF( LDAB.LT.KD+1 ) THEN
00206          INFO = -7
00207       ELSE IF( WANTZ .AND. LDQ.LT.MAX( 1, N ) ) THEN
00208          INFO = -9
00209       ELSE
00210          IF( VALEIG ) THEN
00211             IF( N.GT.0 .AND. VU.LE.VL )
00212      $         INFO = -11
00213          ELSE IF( INDEIG ) THEN
00214             IF( IL.LT.1 .OR. IL.GT.MAX( 1, N ) ) THEN
00215                INFO = -12
00216             ELSE IF( IU.LT.MIN( N, IL ) .OR. IU.GT.N ) THEN
00217                INFO = -13
00218             END IF
00219          END IF
00220       END IF
00221       IF( INFO.EQ.0 ) THEN
00222          IF( LDZ.LT.1 .OR. ( WANTZ .AND. LDZ.LT.N ) )
00223      $     INFO = -18
00224       END IF
00225 *
00226       IF( INFO.NE.0 ) THEN
00227          CALL XERBLA( 'SSBEVX', -INFO )
00228          RETURN
00229       END IF
00230 *
00231 *     Quick return if possible
00232 *
00233       M = 0
00234       IF( N.EQ.0 )
00235      $   RETURN
00236 *
00237       IF( N.EQ.1 ) THEN
00238          M = 1
00239          IF( LOWER ) THEN
00240             TMP1 = AB( 1, 1 )
00241          ELSE
00242             TMP1 = AB( KD+1, 1 )
00243          END IF
00244          IF( VALEIG ) THEN
00245             IF( .NOT.( VL.LT.TMP1 .AND. VU.GE.TMP1 ) )
00246      $         M = 0
00247          END IF
00248          IF( M.EQ.1 ) THEN
00249             W( 1 ) = TMP1
00250             IF( WANTZ )
00251      $         Z( 1, 1 ) = ONE
00252          END IF
00253          RETURN
00254       END IF
00255 *
00256 *     Get machine constants.
00257 *
00258       SAFMIN = SLAMCH( 'Safe minimum' )
00259       EPS = SLAMCH( 'Precision' )
00260       SMLNUM = SAFMIN / EPS
00261       BIGNUM = ONE / SMLNUM
00262       RMIN = SQRT( SMLNUM )
00263       RMAX = MIN( SQRT( BIGNUM ), ONE / SQRT( SQRT( SAFMIN ) ) )
00264 *
00265 *     Scale matrix to allowable range, if necessary.
00266 *
00267       ISCALE = 0
00268       ABSTLL = ABSTOL
00269       IF ( VALEIG ) THEN
00270          VLL = VL
00271          VUU = VU
00272       ELSE
00273          VLL = ZERO
00274          VUU = ZERO
00275       ENDIF
00276       ANRM = SLANSB( 'M', UPLO, N, KD, AB, LDAB, WORK )
00277       IF( ANRM.GT.ZERO .AND. ANRM.LT.RMIN ) THEN
00278          ISCALE = 1
00279          SIGMA = RMIN / ANRM
00280       ELSE IF( ANRM.GT.RMAX ) THEN
00281          ISCALE = 1
00282          SIGMA = RMAX / ANRM
00283       END IF
00284       IF( ISCALE.EQ.1 ) THEN
00285          IF( LOWER ) THEN
00286             CALL SLASCL( 'B', KD, KD, ONE, SIGMA, N, N, AB, LDAB, INFO )
00287          ELSE
00288             CALL SLASCL( 'Q', KD, KD, ONE, SIGMA, N, N, AB, LDAB, INFO )
00289          END IF
00290          IF( ABSTOL.GT.0 )
00291      $      ABSTLL = ABSTOL*SIGMA
00292          IF( VALEIG ) THEN
00293             VLL = VL*SIGMA
00294             VUU = VU*SIGMA
00295          END IF
00296       END IF
00297 *
00298 *     Call SSBTRD to reduce symmetric band matrix to tridiagonal form.
00299 *
00300       INDD = 1
00301       INDE = INDD + N
00302       INDWRK = INDE + N
00303       CALL SSBTRD( JOBZ, UPLO, N, KD, AB, LDAB, WORK( INDD ),
00304      $             WORK( INDE ), Q, LDQ, WORK( INDWRK ), IINFO )
00305 *
00306 *     If all eigenvalues are desired and ABSTOL is less than or equal
00307 *     to zero, then call SSTERF or SSTEQR.  If this fails for some
00308 *     eigenvalue, then try SSTEBZ.
00309 *
00310       TEST = .FALSE.
00311       IF (INDEIG) THEN
00312          IF (IL.EQ.1 .AND. IU.EQ.N) THEN
00313             TEST = .TRUE.
00314          END IF
00315       END IF
00316       IF ((ALLEIG .OR. TEST) .AND. (ABSTOL.LE.ZERO)) THEN
00317          CALL SCOPY( N, WORK( INDD ), 1, W, 1 )
00318          INDEE = INDWRK + 2*N
00319          IF( .NOT.WANTZ ) THEN
00320             CALL SCOPY( N-1, WORK( INDE ), 1, WORK( INDEE ), 1 )
00321             CALL SSTERF( N, W, WORK( INDEE ), INFO )
00322          ELSE
00323             CALL SLACPY( 'A', N, N, Q, LDQ, Z, LDZ )
00324             CALL SCOPY( N-1, WORK( INDE ), 1, WORK( INDEE ), 1 )
00325             CALL SSTEQR( JOBZ, N, W, WORK( INDEE ), Z, LDZ,
00326      $                   WORK( INDWRK ), INFO )
00327             IF( INFO.EQ.0 ) THEN
00328                DO 10 I = 1, N
00329                   IFAIL( I ) = 0
00330    10          CONTINUE
00331             END IF
00332          END IF
00333          IF( INFO.EQ.0 ) THEN
00334             M = N
00335             GO TO 30
00336          END IF
00337          INFO = 0
00338       END IF
00339 *
00340 *     Otherwise, call SSTEBZ and, if eigenvectors are desired, SSTEIN.
00341 *
00342       IF( WANTZ ) THEN
00343          ORDER = 'B'
00344       ELSE
00345          ORDER = 'E'
00346       END IF
00347       INDIBL = 1
00348       INDISP = INDIBL + N
00349       INDIWO = INDISP + N
00350       CALL SSTEBZ( RANGE, ORDER, N, VLL, VUU, IL, IU, ABSTLL,
00351      $             WORK( INDD ), WORK( INDE ), M, NSPLIT, W,
00352      $             IWORK( INDIBL ), IWORK( INDISP ), WORK( INDWRK ),
00353      $             IWORK( INDIWO ), INFO )
00354 *
00355       IF( WANTZ ) THEN
00356          CALL SSTEIN( N, WORK( INDD ), WORK( INDE ), M, W,
00357      $                IWORK( INDIBL ), IWORK( INDISP ), Z, LDZ,
00358      $                WORK( INDWRK ), IWORK( INDIWO ), IFAIL, INFO )
00359 *
00360 *        Apply orthogonal matrix used in reduction to tridiagonal
00361 *        form to eigenvectors returned by SSTEIN.
00362 *
00363          DO 20 J = 1, M
00364             CALL SCOPY( N, Z( 1, J ), 1, WORK( 1 ), 1 )
00365             CALL SGEMV( 'N', N, N, ONE, Q, LDQ, WORK, 1, ZERO,
00366      $                  Z( 1, J ), 1 )
00367    20    CONTINUE
00368       END IF
00369 *
00370 *     If matrix was scaled, then rescale eigenvalues appropriately.
00371 *
00372    30 CONTINUE
00373       IF( ISCALE.EQ.1 ) THEN
00374          IF( INFO.EQ.0 ) THEN
00375             IMAX = M
00376          ELSE
00377             IMAX = INFO - 1
00378          END IF
00379          CALL SSCAL( IMAX, ONE / SIGMA, W, 1 )
00380       END IF
00381 *
00382 *     If eigenvalues are not in order, then sort them, along with
00383 *     eigenvectors.
00384 *
00385       IF( WANTZ ) THEN
00386          DO 50 J = 1, M - 1
00387             I = 0
00388             TMP1 = W( J )
00389             DO 40 JJ = J + 1, M
00390                IF( W( JJ ).LT.TMP1 ) THEN
00391                   I = JJ
00392                   TMP1 = W( JJ )
00393                END IF
00394    40       CONTINUE
00395 *
00396             IF( I.NE.0 ) THEN
00397                ITMP1 = IWORK( INDIBL+I-1 )
00398                W( I ) = W( J )
00399                IWORK( INDIBL+I-1 ) = IWORK( INDIBL+J-1 )
00400                W( J ) = TMP1
00401                IWORK( INDIBL+J-1 ) = ITMP1
00402                CALL SSWAP( N, Z( 1, I ), 1, Z( 1, J ), 1 )
00403                IF( INFO.NE.0 ) THEN
00404                   ITMP1 = IFAIL( I )
00405                   IFAIL( I ) = IFAIL( J )
00406                   IFAIL( J ) = ITMP1
00407                END IF
00408             END IF
00409    50    CONTINUE
00410       END IF
00411 *
00412       RETURN
00413 *
00414 *     End of SSBEVX
00415 *
00416       END