          SUBROUTINE SSTEIN( N, D, E, M, W, IBLOCK, ISPLIT, Z, LDZ, WORK,
         $                   IWORK, IFAIL, INFO )
    *
    *  -- LAPACK routine (version 3.1) --
    *     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
    *     November 2006
    *
    *     .. Scalar Arguments ..
          INTEGER            INFO, LDZ, M, N
    *     ..
    *     .. Array Arguments ..
          INTEGER            IBLOCK( * ), IFAIL( * ), ISPLIT( * ),
         $                   IWORK( * )
          REAL               D( * ), E( * ), W( * ), WORK( * ), Z( LDZ, * )
    *     ..
    *
    *  Purpose
    *  =======
    *
    *  SSTEIN computes the eigenvectors of a real symmetric tridiagonal
    *  matrix T corresponding to specified eigenvalues, using inverse
    *  iteration.
    *
    *  The maximum number of iterations allowed for each eigenvector is
    *  specified by an internal parameter MAXITS (currently set to 5).
    *
    *  Arguments
    *  =========
    *
    *  N       (input) INTEGER
    *          The order of the matrix.  N >= 0.
    *
    *  D       (input) REAL array, dimension (N)
    *          The n diagonal elements of the tridiagonal matrix T.
    *
    *  E       (input) REAL array, dimension (N-1)
    *          The (n-1) subdiagonal elements of the tridiagonal matrix
    *          T, in elements 1 to N-1.
    *
    *  M       (input) INTEGER
    *          The number of eigenvectors to be found.  0 <= M <= N.
    *
    *  W       (input) REAL array, dimension (N)
    *          The first M elements of W contain the eigenvalues for
    *          which eigenvectors are to be computed.  The eigenvalues
    *          should be grouped by split-off block and ordered from
    *          smallest to largest within the block.  ( The output array
    *          W from SSTEBZ with ORDER = 'B' is expected here. )
    *
    *  IBLOCK  (input) INTEGER array, dimension (N)
    *          The submatrix indices associated with the corresponding
    *          eigenvalues in W; IBLOCK(i)=1 if eigenvalue W(i) belongs to
    *          the first submatrix from the top, =2 if W(i) belongs to
    *          the second submatrix, etc.  ( The output array IBLOCK
    *          from SSTEBZ is expected here. )
    *
    *  ISPLIT  (input) INTEGER array, dimension (N)
    *          The splitting points, at which T breaks up into submatrices.
    *          The first submatrix consists of rows/columns 1 to
    *          ISPLIT( 1 ), the second of rows/columns ISPLIT( 1 )+1
    *          through ISPLIT( 2 ), etc.
    *          ( The output array ISPLIT from SSTEBZ is expected here. )
    *
    *  Z       (output) REAL array, dimension (LDZ, M)
    *          The computed eigenvectors.  The eigenvector associated
    *          with the eigenvalue W(i) is stored in the i-th column of
    *          Z.  Any vector which fails to converge is set to its current
    *          iterate after MAXITS iterations.
    *
    *  LDZ     (input) INTEGER
    *          The leading dimension of the array Z.  LDZ >= max(1,N).
    *
    *  WORK    (workspace) REAL array, dimension (5*N)
    *
    *  IWORK   (workspace) INTEGER array, dimension (N)
    *
    *  IFAIL   (output) INTEGER array, dimension (M)
    *          On normal exit, all elements of IFAIL are zero.
    *          If one or more eigenvectors fail to converge after
    *          MAXITS iterations, then their indices are stored in
    *          array IFAIL.
    *
    *  INFO    (output) INTEGER
    *          = 0: successful exit.
    *          < 0: if INFO = -i, the i-th argument had an illegal value
    *          > 0: if INFO = i, then i eigenvectors failed to converge
    *               in MAXITS iterations.  Their indices are stored in
    *               array IFAIL.
    *
    *  Internal Parameters
    *  ===================
    *
    *  MAXITS  INTEGER, default = 5
    *          The maximum number of iterations performed.
    *
    *  EXTRA   INTEGER, default = 2
    *          The number of iterations performed after norm growth
    *          criterion is satisfied, should be at least 1.
    *
    *  =====================================================================
    *
    *     .. Parameters ..
          REAL               ZERO, ONE, TEN, ODM3, ODM1
          PARAMETER          ( ZERO = 0.0E+0, ONE = 1.0E+0, TEN = 1.0E+1,
         $                   ODM3 = 1.0E-3, ODM1 = 1.0E-1 )
          INTEGER            MAXITS, EXTRA
          PARAMETER          ( MAXITS = 5, EXTRA = 2 )
    *     ..
    *     .. Local Scalars ..
          INTEGER            B1, BLKSIZ, BN, GPIND, I, IINFO, INDRV1,
         $                   INDRV2, INDRV3, INDRV4, INDRV5, ITS, J, J1,
         $                   JBLK, JMAX, NBLK, NRMCHK
          REAL               CTR, EPS, EPS1, NRM, ONENRM, ORTOL, PERTOL,
         $                   SCL, SEP, STPCRT, TOL, XJ, XJM
    *     ..
    *     .. Local Arrays ..
          INTEGER            ISEED( 4 )
    *     ..
    *     .. External Functions ..
          INTEGER            ISAMAX
          REAL               SASUM, SDOT, SLAMCH, SNRM2
          EXTERNAL           ISAMAX, SASUM, SDOT, SLAMCH, SNRM2
    *     ..
    *     .. External Subroutines ..
          EXTERNAL           SAXPY, SCOPY, SLAGTF, SLAGTS, SLARNV, SSCAL,
         $                   XERBLA
    *     ..
    *     .. Intrinsic Functions ..
          INTRINSIC          ABS, MAX, SQRT
    *     ..
    *     .. Executable Statements ..
    *
    *     Test the input parameters.
    *
          INFO = 0
          DO 10 I = 1, M
             IFAIL( I ) = 0
       10 CONTINUE
    *
          IF( N.LT.0 ) THEN
             INFO = -1
          ELSE IF( M.LT.0 .OR. M.GT.N ) THEN
             INFO = -4
          ELSE IF( LDZ.LT.MAX( 1, N ) ) THEN
             INFO = -9
          ELSE
             DO 20 J = 2, M
                IF( IBLOCK( J ).LT.IBLOCK( J-1 ) ) THEN
                   INFO = -6
                   GO TO 30
                END IF
                IF( IBLOCK( J ).EQ.IBLOCK( J-1 ) .AND. W( J ).LT.W( J-1 ) )
         $           THEN
                   INFO = -5
                   GO TO 30
                END IF
       20    CONTINUE
       30    CONTINUE
          END IF
    *
          IF( INFO.NE.0 ) THEN
             CALL XERBLA( 'SSTEIN', -INFO )
             RETURN
          END IF
    *
    *     Quick return if possible
    *
          IF( N.EQ.0 .OR. M.EQ.0 ) THEN
             RETURN
          ELSE IF( N.EQ.1 ) THEN
             Z( 1, 1 ) = ONE
             RETURN
          END IF
    *
    *     Get machine constants.
    *
          EPS = SLAMCH( 'Precision' )
    *
    *     Initialize seed for random number generator SLARNV.
    *
          DO 40 I = 1, 4
             ISEED( I ) = 1
       40 CONTINUE
    *
    *     Initialize pointers.
    *
          INDRV1 = 0
          INDRV2 = INDRV1 + N
          INDRV3 = INDRV2 + N
          INDRV4 = INDRV3 + N
          INDRV5 = INDRV4 + N
    *
    *     Compute eigenvectors of matrix blocks.
    *
          J1 = 1
          DO 160 NBLK = 1, IBLOCK( M )
    *
    *        Find starting and ending indices of block nblk.
    *
             IF( NBLK.EQ.1 ) THEN
                B1 = 1
             ELSE
                B1 = ISPLIT( NBLK-1 ) + 1
             END IF
             BN = ISPLIT( NBLK )
             BLKSIZ = BN - B1 + 1
             IF( BLKSIZ.EQ.1 )
         $      GO TO 60
             GPIND = B1
    *
    *        Compute reorthogonalization criterion and stopping criterion.
    *
             ONENRM = ABS( D( B1 ) ) + ABS( E( B1 ) )
             ONENRM = MAX( ONENRM, ABS( D( BN ) )+ABS( E( BN-1 ) ) )
             DO 50 I = B1 + 1, BN - 1
                ONENRM = MAX( ONENRM, ABS( D( I ) )+ABS( E( I-1 ) )+
         $               ABS( E( I ) ) )
       50    CONTINUE
             ORTOL = ODM3*ONENRM
    *
             STPCRT = SQRT( ODM1 / BLKSIZ )
    *
    *        Loop through eigenvalues of block nblk.
    *
       60    CONTINUE
             JBLK = 0
             DO 150 J = J1, M
                IF( IBLOCK( J ).NE.NBLK ) THEN
                   J1 = J
                   GO TO 160
                END IF
                JBLK = JBLK + 1
                XJ = W( J )
    *
    *           Skip all the work if the block size is one.
    *
                IF( BLKSIZ.EQ.1 ) THEN
                   WORK( INDRV1+1 ) = ONE
                   GO TO 120
                END IF
    *
    *           If eigenvalues j and j-1 are too close, add a relatively
    *           small perturbation.
    *
                IF( JBLK.GT.1 ) THEN
                   EPS1 = ABS( EPS*XJ )
                   PERTOL = TEN*EPS1
                   SEP = XJ - XJM
                   IF( SEP.LT.PERTOL )
         $            XJ = XJM + PERTOL
                END IF
    *
                ITS = 0
                NRMCHK = 0
    *
    *           Get random starting vector.
    *
                CALL SLARNV( 2, ISEED, BLKSIZ, WORK( INDRV1+1 ) )
    *
    *           Copy the matrix T so it won't be destroyed in factorization.
    *
                CALL SCOPY( BLKSIZ, D( B1 ), 1, WORK( INDRV4+1 ), 1 )
                CALL SCOPY( BLKSIZ-1, E( B1 ), 1, WORK( INDRV2+2 ), 1 )
                CALL SCOPY( BLKSIZ-1, E( B1 ), 1, WORK( INDRV3+1 ), 1 )
    *
    *           Compute LU factors with partial pivoting  ( PT = LU )
    *
                TOL = ZERO
                CALL SLAGTF( BLKSIZ, WORK( INDRV4+1 ), XJ, WORK( INDRV2+2 ),
         $                   WORK( INDRV3+1 ), TOL, WORK( INDRV5+1 ), IWORK,
         $                   IINFO )
    *
    *           Update iteration count.
    *
       70       CONTINUE
                ITS = ITS + 1
                IF( ITS.GT.MAXITS )
         $         GO TO 100
    *
    *           Normalize and scale the righthand side vector Pb.
    *
                SCL = BLKSIZ*ONENRM*MAX( EPS,
         $            ABS( WORK( INDRV4+BLKSIZ ) ) ) /
         $            SASUM( BLKSIZ, WORK( INDRV1+1 ), 1 )
                CALL SSCAL( BLKSIZ, SCL, WORK( INDRV1+1 ), 1 )
    *
    *           Solve the system LU = Pb.
    *
                CALL SLAGTS( -1, BLKSIZ, WORK( INDRV4+1 ), WORK( INDRV2+2 ),
         $                   WORK( INDRV3+1 ), WORK( INDRV5+1 ), IWORK,
         $                   WORK( INDRV1+1 ), TOL, IINFO )
    *
    *           Reorthogonalize by modified Gram-Schmidt if eigenvalues are
    *           close enough.
    *
                IF( JBLK.EQ.1 )
         $         GO TO 90
                IF( ABS( XJ-XJM ).GT.ORTOL )
         $         GPIND = J
                IF( GPIND.NE.J ) THEN
                   DO 80 I = GPIND, J - 1
                      CTR = -SDOT( BLKSIZ, WORK( INDRV1+1 ), 1, Z( B1, I ),
         $                  1 )
                      CALL SAXPY( BLKSIZ, CTR, Z( B1, I ), 1,
         $                        WORK( INDRV1+1 ), 1 )
       80          CONTINUE
                END IF
    *
    *           Check the infinity norm of the iterate.
    *
       90       CONTINUE
                JMAX = ISAMAX( BLKSIZ, WORK( INDRV1+1 ), 1 )
                NRM = ABS( WORK( INDRV1+JMAX ) )
    *
    *           Continue for additional iterations after norm reaches
    *           stopping criterion.
    *
                IF( NRM.LT.STPCRT )
         $         GO TO 70
                NRMCHK = NRMCHK + 1
                IF( NRMCHK.LT.EXTRA+1 )
         $         GO TO 70
    *
                GO TO 110
    *
    *           If stopping criterion was not satisfied, update info and
    *           store eigenvector number in array ifail.
    *
      100       CONTINUE
                INFO = INFO + 1
                IFAIL( INFO ) = J
    *
    *           Accept iterate as jth eigenvector.
    *
      110       CONTINUE
                SCL = ONE / SNRM2( BLKSIZ, WORK( INDRV1+1 ), 1 )
                JMAX = ISAMAX( BLKSIZ, WORK( INDRV1+1 ), 1 )
                IF( WORK( INDRV1+JMAX ).LT.ZERO )
         $         SCL = -SCL
                CALL SSCAL( BLKSIZ, SCL, WORK( INDRV1+1 ), 1 )
      120       CONTINUE
                DO 130 I = 1, N
                   Z( I, J ) = ZERO
      130       CONTINUE
                DO 140 I = 1, BLKSIZ
                   Z( B1+I-1, J ) = WORK( INDRV1+I )
      140       CONTINUE
    *
    *           Save the shift to check eigenvalue spacing at next
    *           iteration.
    *
                XJM = XJ
    *
      150    CONTINUE
      160 CONTINUE
    *
          RETURN
    *
    *     End of SSTEIN
    *
          END