      SUBROUTINE CHEEVR( JOBZ, RANGE, UPLO, N, A, LDA, VL, VU, IL, IU,
     $                   ABSTOL, M, W, Z, LDZ, ISUPPZ, WORK, LWORK,
     $                   RWORK, LRWORK, IWORK, LIWORK, INFO )
*
*  -- LAPACK driver routine (version 3.0) --
*     Univ. of Tennessee, Univ. of California Berkeley, NAG Ltd.,
*     Courant Institute, Argonne National Lab, and Rice University
*     March 20, 2000
*
*     .. Scalar Arguments ..
      CHARACTER          JOBZ, RANGE, UPLO
      INTEGER            IL, INFO, IU, LDA, LDZ, LIWORK, LRWORK, LWORK,
     $                   M, N
      REAL               ABSTOL, VL, VU
*     ..
*     .. Array Arguments ..
      INTEGER            ISUPPZ( * ), IWORK( * )
      REAL               RWORK( * ), W( * )
      COMPLEX            A( LDA, * ), WORK( * ), Z( LDZ, * )
*     ..
*
*  Purpose
*  =======
*
*  CHEEVR computes selected eigenvalues and, optionally, eigenvectors
*  of a complex Hermitian matrix T.  Eigenvalues and eigenvectors can
*  be selected by specifying either a range of values or a range of
*  indices for the desired eigenvalues.
*
*  Whenever possible, CHEEVR calls CSTEGR to compute the
*  eigenspectrum using Relatively Robust Representations.  CSTEGR
*  computes eigenvalues by the dqds algorithm, while orthogonal
*  eigenvectors are computed from various "good" L D L^T representations
*  (also known as Relatively Robust Representations). Gram-Schmidt
*  orthogonalization is avoided as far as possible. More specifically,
*  the various steps of the algorithm are as follows. For the i-th
*  unreduced block of T,
*     (a) Compute T - sigma_i = L_i D_i L_i^T, such that L_i D_i L_i^T
*          is a relatively robust representation,
*     (b) Compute the eigenvalues, lambda_j, of L_i D_i L_i^T to high
*         relative accuracy by the dqds algorithm,
*     (c) If there is a cluster of close eigenvalues, "choose" sigma_i
*         close to the cluster, and go to step (a),
*     (d) Given the approximate eigenvalue lambda_j of L_i D_i L_i^T,
*         compute the corresponding eigenvector by forming a
*         rank-revealing twisted factorization.
*  The desired accuracy of the output can be specified by the input
*  parameter ABSTOL.
*
*  For more details, see "A new O(n^2) algorithm for the symmetric
*  tridiagonal eigenvalue/eigenvector problem", by Inderjit Dhillon,
*  Computer Science Division Technical Report No. UCB//CSD-97-971,
*  UC Berkeley, May 1997.
*
*
*  Note 1 : CHEEVR calls CSTEGR when the full spectrum is requested
*  on machines which conform to the ieee-754 floating point standard.
*  CHEEVR calls SSTEBZ and CSTEIN on non-ieee machines and
*  when partial spectrum requests are made.
*
*  Normal execution of CSTEGR may create NaNs and infinities and
*  hence may abort due to a floating point exception in environments
*  which do not handle NaNs and infinities in the ieee standard default
*  manner.
*
*  Arguments
*  =========
*
*  JOBZ    (input) CHARACTER*1
*          = 'N':  Compute eigenvalues only;
*          = 'V':  Compute eigenvalues and eigenvectors.
*
*  RANGE   (input) CHARACTER*1
*          = 'A': all eigenvalues will be found.
*          = 'V': all eigenvalues in the half-open interval (VL,VU]
*                 will be found.
*          = 'I': the IL-th through IU-th eigenvalues will be found.
********** For RANGE = 'V' or 'I' and IU - IL < N - 1, SSTEBZ and
********** CSTEIN are called
*
*  UPLO    (input) CHARACTER*1
*          = 'U':  Upper triangle of A is stored;
*          = 'L':  Lower triangle of A is stored.
*
*  N       (input) INTEGER
*          The order of the matrix A.  N >= 0.
*
*  A       (input/output) COMPLEX array, dimension (LDA, N)
*          On entry, the Hermitian matrix A.  If UPLO = 'U', the
*          leading N-by-N upper triangular part of A contains the
*          upper triangular part of the matrix A.  If UPLO = 'L',
*          the leading N-by-N lower triangular part of A contains
*          the lower triangular part of the matrix A.
*          On exit, the lower triangle (if UPLO='L') or the upper
*          triangle (if UPLO='U') of A, including the diagonal, is
*          destroyed.
*
*  LDA     (input) INTEGER
*          The leading dimension of the array A.  LDA >= max(1,N).
*
*  VL      (input) REAL
*  VU      (input) REAL
*          If RANGE='V', the lower and upper bounds of the interval to
*          be searched for eigenvalues. VL < VU.
*          Not referenced if RANGE = 'A' or 'I'.
*
*  IL      (input) INTEGER
*  IU      (input) INTEGER
*          If RANGE='I', the indices (in ascending order) of the
*          smallest and largest eigenvalues to be returned.
*          1 <= IL <= IU <= N, if N > 0; IL = 1 and IU = 0 if N = 0.
*          Not referenced if RANGE = 'A' or 'V'.
*
*  ABSTOL  (input) REAL
*          The absolute error tolerance for the eigenvalues.
*          An approximate eigenvalue is accepted as converged
*          when it is determined to lie in an interval [a,b]
*          of width less than or equal to
*
*                  ABSTOL + EPS *   max( |a|,|b| ) ,
*
*          where EPS is the machine precision.  If ABSTOL is less than
*          or equal to zero, then  EPS*|T|  will be used in its place,
*          where |T| is the 1-norm of the tridiagonal matrix obtained
*          by reducing A to tridiagonal form.
*
*          See "Computing Small Singular Values of Bidiagonal Matrices
*          with Guaranteed High Relative Accuracy," by Demmel and
*          Kahan, LAPACK Working Note #3.
*
*          If high relative accuracy is important, set ABSTOL to
*          SLAMCH( 'Safe minimum' ).  Doing so will guarantee that
*          eigenvalues are computed to high relative accuracy when
*          possible in future releases.  The current code does not
*          make any guarantees about high relative accuracy, but
*          furutre releases will. See J. Barlow and J. Demmel,
*          "Computing Accurate Eigensystems of Scaled Diagonally
*          Dominant Matrices", LAPACK Working Note #7, for a discussion
*          of which matrices define their eigenvalues to high relative
*          accuracy.
*
*  M       (output) INTEGER
*          The total number of eigenvalues found.  0 <= M <= N.
*          If RANGE = 'A', M = N, and if RANGE = 'I', M = IU-IL+1.
*
*  W       (output) REAL array, dimension (N)
*          The first M elements contain the selected eigenvalues in
*          ascending order.
*
*  Z       (output) COMPLEX array, dimension (LDZ, max(1,M))
*          If JOBZ = 'V', then if INFO = 0, the first M columns of Z
*          contain the orthonormal eigenvectors of the matrix A
*          corresponding to the selected eigenvalues, with the i-th
*          column of Z holding the eigenvector associated with W(i).
*          If JOBZ = 'N', then Z is not referenced.
*          Note: the user must ensure that at least max(1,M) columns are
*          supplied in the array Z; if RANGE = 'V', the exact value of M
*          is not known in advance and an upper bound must be used.
*
*  LDZ     (input) INTEGER
*          The leading dimension of the array Z.  LDZ >= 1, and if
*          JOBZ = 'V', LDZ >= max(1,N).
*
*  ISUPPZ  (output) INTEGER ARRAY, dimension ( 2*max(1,M) )
*          The support of the eigenvectors in Z, i.e., the indices
*          indicating the nonzero elements in Z. The i-th eigenvector
*          is nonzero only in elements ISUPPZ( 2*i-1 ) through
*          ISUPPZ( 2*i ).
********** Implemented only for RANGE = 'A' or 'I' and IU - IL = N - 1
*
*  WORK    (workspace/output) COMPLEX array, dimension (LWORK)
*          On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
*
*  LWORK   (input) INTEGER
*          The length of the array WORK.  LWORK >= max(1,2*N).
*          For optimal efficiency, LWORK >= (NB+1)*N,
*          where NB is the max of the blocksize for CHETRD and for
*          CUNMTR as returned by ILAENV.
*
*          If LWORK = -1, then a workspace query is assumed; the routine
*          only calculates the optimal size of the WORK array, returns
*          this value as the first entry of the WORK array, and no error
*          message related to LWORK is issued by XERBLA.
*
*  RWORK   (workspace/output) REAL array, dimension (LRWORK)
*          On exit, if INFO = 0, RWORK(1) returns the optimal
*          (and minimal) LRWORK.
*
* LRWORK  (input) INTEGER
*         The length of the array RWORK.  LRWORK >= max(1,24*N).
*
*         If LRWORK = -1, then a workspace query is assumed; the routine
*         only calculates the optimal size of the RWORK array, returns
*         this value as the first entry of the RWORK array, and no error
*         message related to LRWORK is issued by XERBLA.
*
*  IWORK   (workspace/output) INTEGER array, dimension (LIWORK)
*          On exit, if INFO = 0, IWORK(1) returns the optimal
*          (and minimal) LIWORK.
*
* LIWORK  (input) INTEGER
*         The dimension of the array IWORK.  LIWORK >= max(1,10*N).
*
*         If LIWORK = -1, then a workspace query is assumed; the
*         routine only calculates the optimal size of the IWORK array,
*         returns this value as the first entry of the IWORK array, and
*         no error message related to LIWORK is issued by XERBLA.
*
*  INFO    (output) INTEGER
*          = 0:  successful exit
*          < 0:  if INFO = -i, the i-th argument had an illegal value
*          > 0:  Internal error
*
*  Further Details
*  ===============
*
*  Based on contributions by
*     Inderjit Dhillon, IBM Almaden, USA
*     Osni Marques, LBNL/NERSC, USA
*     Ken Stanley, Computer Science Division, University of
*       California at Berkeley, USA
*
* =====================================================================
*
*     .. Parameters ..
      REAL               ZERO, ONE
      PARAMETER          ( ZERO = 0.0E+0, ONE = 1.0E+0 )
*     ..
*     .. Local Scalars ..
      LOGICAL            ALLEIG, INDEIG, LOWER, LQUERY, VALEIG, WANTZ
      CHARACTER          ORDER
      INTEGER            I, IEEEOK, IINFO, IMAX, INDIBL, INDIFL, INDISP,
     $                   INDIWO, INDRD, INDRDD, INDRE, INDREE, INDRWK,
     $                   INDTAU, INDWK, INDWKN, ISCALE, ITMP1, J, JJ,
     $                   LIWMIN, LLWORK, LLWRKN, LRWMIN, LWKOPT, LWMIN,
     $                   NB, NSPLIT
      REAL               ABSTLL, ANRM, BIGNUM, EPS, RMAX, RMIN, SAFMIN,
     $                   SIGMA, SMLNUM, TMP1, VLL, VUU
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      INTEGER            ILAENV
      REAL               CLANSY, SLAMCH
      EXTERNAL           LSAME, ILAENV, CLANSY, SLAMCH
*     ..
*     .. External Subroutines ..
      EXTERNAL           CHETRD, CSSCAL, CSTEGR, CSTEIN, CSWAP, CUNMTR,
     $                   SCOPY, SSCAL, SSTEBZ, SSTERF, XERBLA
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          MAX, MIN, REAL, SQRT
*     ..
*     .. Executable Statements ..
*
*     Test the input parameters.
*
      IEEEOK = ILAENV( 10, 'CHEEVR', 'N', 1, 2, 3, 4 )
*
      LOWER = LSAME( UPLO, 'L' )
      WANTZ = LSAME( JOBZ, 'V' )
      ALLEIG = LSAME( RANGE, 'A' )
      VALEIG = LSAME( RANGE, 'V' )
      INDEIG = LSAME( RANGE, 'I' )
*
      LQUERY = ( ( LWORK.EQ.-1 ) .OR. ( LRWORK.EQ.-1 ) .OR.
     $         ( LIWORK.EQ.-1 ) )
*
      LRWMIN = MAX( 1, 24*N )
      LIWMIN = MAX( 1, 10*N )
      LWMIN = MAX( 1, 2*N )
*
      INFO = 0
      IF( .NOT.( WANTZ .OR. LSAME( JOBZ, 'N' ) ) ) THEN
         INFO = -1
      ELSE IF( .NOT.( ALLEIG .OR. VALEIG .OR. INDEIG ) ) THEN
         INFO = -2
      ELSE IF( .NOT.( LOWER .OR. LSAME( UPLO, 'U' ) ) ) THEN
         INFO = -3
      ELSE IF( N.LT.0 ) THEN
         INFO = -4
      ELSE IF( LDA.LT.MAX( 1, N ) ) THEN
         INFO = -6
      ELSE
         IF( VALEIG ) THEN
            IF( N.GT.0 .AND. VU.LE.VL )
     $         INFO = -8
         ELSE IF( INDEIG ) THEN
            IF( IL.LT.1 .OR. IL.GT.MAX( 1, N ) ) THEN
               INFO = -9
            ELSE IF( IU.LT.MIN( N, IL ) .OR. IU.GT.N ) THEN
               INFO = -10
            END IF
         END IF
      END IF
      IF( INFO.EQ.0 ) THEN
         IF( LDZ.LT.1 .OR. ( WANTZ .AND. LDZ.LT.N ) ) THEN
            INFO = -15
         ELSE IF( LWORK.LT.LWMIN .AND. .NOT.LQUERY ) THEN
            INFO = -18
         ELSE IF( LRWORK.LT.LRWMIN .AND. .NOT.LQUERY ) THEN
            INFO = -20
         ELSE IF( LIWORK.LT.LIWMIN .AND. .NOT.LQUERY ) THEN
            INFO = -22
         END IF
      END IF
*
      IF( INFO.EQ.0 ) THEN
         NB = ILAENV( 1, 'CHETRD', UPLO, N, -1, -1, -1 )
         NB = MAX( NB, ILAENV( 1, 'CUNMTR', UPLO, N, -1, -1, -1 ) )
         LWKOPT = MAX( ( NB+1 )*N, LWMIN )
         WORK( 1 ) = LWKOPT
         RWORK( 1 ) = LRWMIN
         IWORK( 1 ) = LIWMIN
      END IF
*
      IF( INFO.NE.0 ) THEN
         CALL XERBLA( 'CHEEVR', -INFO )
         RETURN
      ELSE IF( LQUERY ) THEN
         RETURN
      END IF
*
*     Quick return if possible
*
      M = 0
      IF( N.EQ.0 ) THEN
         WORK( 1 ) = 1
         RETURN
      END IF
*
      IF( N.EQ.1 ) THEN
         WORK( 1 ) = 7
         IF( ALLEIG .OR. INDEIG ) THEN
            M = 1
            W( 1 ) = REAL( A( 1, 1 ) )
         ELSE
            IF( VL.LT.REAL( A( 1, 1 ) ) .AND. VU.GE.REAL( A( 1, 1 ) ) )
     $           THEN
               M = 1
               W( 1 ) = REAL( A( 1, 1 ) )
            END IF
         END IF
         IF( WANTZ )
     $      Z( 1, 1 ) = ONE
         RETURN
      END IF
*
*     Get machine constants.
*
      SAFMIN = SLAMCH( 'Safe minimum' )
      EPS = SLAMCH( 'Precision' )
      SMLNUM = SAFMIN / EPS
      BIGNUM = ONE / SMLNUM
      RMIN = SQRT( SMLNUM )
      RMAX = MIN( SQRT( BIGNUM ), ONE / SQRT( SQRT( SAFMIN ) ) )
*
*     Scale matrix to allowable range, if necessary.
*
      ISCALE = 0
      ABSTLL = ABSTOL
      VLL = VL
      VUU = VU
      ANRM = CLANSY( 'M', UPLO, N, A, LDA, RWORK )
      IF( ANRM.GT.ZERO .AND. ANRM.LT.RMIN ) THEN
         ISCALE = 1
         SIGMA = RMIN / ANRM
      ELSE IF( ANRM.GT.RMAX ) THEN
         ISCALE = 1
         SIGMA = RMAX / ANRM
      END IF
      IF( ISCALE.EQ.1 ) THEN
         IF( LOWER ) THEN
            DO 10 J = 1, N
               CALL CSSCAL( N-J+1, SIGMA, A( J, J ), 1 )
   10       CONTINUE
         ELSE
            DO 20 J = 1, N
               CALL CSSCAL( J, SIGMA, A( 1, J ), 1 )
   20       CONTINUE
         END IF
         IF( ABSTOL.GT.0 )
     $      ABSTLL = ABSTOL*SIGMA
         IF( VALEIG ) THEN
            VLL = VL*SIGMA
            VUU = VU*SIGMA
         END IF
      END IF
*
*     Call CHETRD to reduce Hermitian matrix to tridiagonal form.
*
      INDTAU = 1
      INDWK = INDTAU + N
*
      INDRE = 1
      INDRD = INDRE + N
      INDREE = INDRD + N
      INDRDD = INDREE + N
      INDRWK = INDRDD + N
      LLWORK = LWORK - INDWK + 1
      CALL CHETRD( UPLO, N, A, LDA, RWORK( INDRD ), RWORK( INDRE ),
     $             WORK( INDTAU ), WORK( INDWK ), LLWORK, IINFO )
*
*     If all eigenvalues are desired
*     then call SSTERF or CSTEGR and CUNMTR.
*
      IF( ( ALLEIG .OR. ( INDEIG .AND. IL.EQ.1 .AND. IU.EQ.N ) ) .AND.
     $    IEEEOK.EQ.1 ) THEN
         IF( .NOT.WANTZ ) THEN
            CALL SCOPY( N, RWORK( INDRD ), 1, W, 1 )
            CALL SCOPY( N-1, RWORK( INDRE ), 1, RWORK( INDREE ), 1 )
            CALL SSTERF( N, W, RWORK( INDREE ), INFO )
         ELSE
            CALL SCOPY( N-1, RWORK( INDRE ), 1, RWORK( INDREE ), 1 )
            CALL SCOPY( N, RWORK( INDRD ), 1, RWORK( INDRDD ), 1 )
*
            CALL CSTEGR( JOBZ, 'A', N, RWORK( INDRDD ),
     $                   RWORK( INDREE ), VL, VU, IL, IU, ABSTOL, M, W,
     $                   Z, LDZ, ISUPPZ, RWORK( INDRWK ), LWORK, IWORK,
     $                   LIWORK, INFO )
*
*
*
*        Apply unitary matrix used in reduction to tridiagonal
*        form to eigenvectors returned by CSTEIN.
*
            IF( WANTZ .AND. INFO.EQ.0 ) THEN
               INDWKN = INDWK
               LLWRKN = LWORK - INDWKN + 1
               CALL CUNMTR( 'L', UPLO, 'N', N, M, A, LDA,
     $                      WORK( INDTAU ), Z, LDZ, WORK( INDWKN ),
     $                      LLWRKN, IINFO )
            END IF
         END IF
*
*
         IF( INFO.EQ.0 ) THEN
            M = N
            GO TO 30
         END IF
         INFO = 0
      END IF
*
*     Otherwise, call SSTEBZ and, if eigenvectors are desired, CSTEIN.
*     Also call SSTEBZ and CSTEIN if CSTEGR fails.
*
      IF( WANTZ ) THEN
         ORDER = 'B'
      ELSE
         ORDER = 'E'
      END IF
      INDIFL = 1
      INDIBL = INDIFL + N
      INDISP = INDIBL + N
      INDIWO = INDISP + N
      CALL SSTEBZ( RANGE, ORDER, N, VLL, VUU, IL, IU, ABSTLL,
     $             RWORK( INDRD ), RWORK( INDRE ), M, NSPLIT, W,
     $             IWORK( INDIBL ), IWORK( INDISP ), RWORK( INDRWK ),
     $             IWORK( INDIWO ), INFO )
*
      IF( WANTZ ) THEN
         CALL CSTEIN( N, RWORK( INDRD ), RWORK( INDRE ), M, W,
     $                IWORK( INDIBL ), IWORK( INDISP ), Z, LDZ,
     $                RWORK( INDRWK ), IWORK( INDIWO ), IWORK( INDIFL ),
     $                INFO )
*
*        Apply unitary matrix used in reduction to tridiagonal
*        form to eigenvectors returned by CSTEIN.
*
         INDWKN = INDWK
         LLWRKN = LWORK - INDWKN + 1
         CALL CUNMTR( 'L', UPLO, 'N', N, M, A, LDA, WORK( INDTAU ), Z,
     $                LDZ, WORK( INDWKN ), LLWRKN, IINFO )
      END IF
*
*     If matrix was scaled, then rescale eigenvalues appropriately.
*
   30 CONTINUE
      IF( ISCALE.EQ.1 ) THEN
         IF( INFO.EQ.0 ) THEN
            IMAX = M
         ELSE
            IMAX = INFO - 1
         END IF
         CALL SSCAL( IMAX, ONE / SIGMA, W, 1 )
      END IF
*
*     If eigenvalues are not in order, then sort them, along with
*     eigenvectors.
*
      IF( WANTZ ) THEN
         DO 50 J = 1, M - 1
            I = 0
            TMP1 = W( J )
            DO 40 JJ = J + 1, M
               IF( W( JJ ).LT.TMP1 ) THEN
                  I = JJ
                  TMP1 = W( JJ )
               END IF
   40       CONTINUE
*
            IF( I.NE.0 ) THEN
               ITMP1 = IWORK( INDIBL+I-1 )
               W( I ) = W( J )
               IWORK( INDIBL+I-1 ) = IWORK( INDIBL+J-1 )
               W( J ) = TMP1
               IWORK( INDIBL+J-1 ) = ITMP1
               CALL CSWAP( N, Z( 1, I ), 1, Z( 1, J ), 1 )
            END IF
   50    CONTINUE
      END IF
*
*     Set WORK(1) to optimal workspace size.
*
      WORK( 1 ) = LWKOPT
      RWORK( 1 ) = LRWMIN
      IWORK( 1 ) = LIWMIN
*
      RETURN
*
*     End of CHEEVR
*
      END
