          SUBROUTINE DSTERF( N, D, E, INFO )
    *
    *  -- LAPACK routine (version 3.1) --
    *     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
    *     November 2006
    *
    *     .. Scalar Arguments ..
          INTEGER            INFO, N
    *     ..
    *     .. Array Arguments ..
          DOUBLE PRECISION   D( * ), E( * )
    *     ..
    *
    *  Purpose
    *  =======
    *
    *  DSTERF computes all eigenvalues of a symmetric tridiagonal matrix
    *  using the Pal-Walker-Kahan variant of the QL or QR algorithm.
    *
    *  Arguments
    *  =========
    *
    *  N       (input) INTEGER
    *          The order of the matrix.  N >= 0.
    *
    *  D       (input/output) DOUBLE PRECISION array, dimension (N)
    *          On entry, the n diagonal elements of the tridiagonal matrix.
    *          On exit, if INFO = 0, the eigenvalues in ascending order.
    *
    *  E       (input/output) DOUBLE PRECISION array, dimension (N-1)
    *          On entry, the (n-1) subdiagonal elements of the tridiagonal
    *          matrix.
    *          On exit, E has been destroyed.
    *
    *  INFO    (output) INTEGER
    *          = 0:  successful exit
    *          < 0:  if INFO = -i, the i-th argument had an illegal value
    *          > 0:  the algorithm failed to find all of the eigenvalues in
    *                a total of 30*N iterations; if INFO = i, then i
    *                elements of E have not converged to zero.
    *
    *  =====================================================================
    *
    *     .. Parameters ..
          DOUBLE PRECISION   ZERO, ONE, TWO, THREE
          PARAMETER          ( ZERO = 0.0D0, ONE = 1.0D0, TWO = 2.0D0,
         $                   THREE = 3.0D0 )
          INTEGER            MAXIT
          PARAMETER          ( MAXIT = 30 )
    *     ..
    *     .. Local Scalars ..
          INTEGER            I, ISCALE, JTOT, L, L1, LEND, LENDSV, LSV, M,
         $                   NMAXIT
          DOUBLE PRECISION   ALPHA, ANORM, BB, C, EPS, EPS2, GAMMA, OLDC,
         $                   OLDGAM, P, R, RT1, RT2, RTE, S, SAFMAX, SAFMIN,
         $                   SIGMA, SSFMAX, SSFMIN
    *     ..
    *     .. External Functions ..
          DOUBLE PRECISION   DLAMCH, DLANST, DLAPY2
          EXTERNAL           DLAMCH, DLANST, DLAPY2
    *     ..
    *     .. External Subroutines ..
          EXTERNAL           DLAE2, DLASCL, DLASRT, XERBLA
    *     ..
    *     .. Intrinsic Functions ..
          INTRINSIC          ABS, SIGN, SQRT
    *     ..
    *     .. Executable Statements ..
    *
    *     Test the input parameters.
    *
          INFO = 0
    *
    *     Quick return if possible
    *
          IF( N.LT.0 ) THEN
             INFO = -1
             CALL XERBLA( 'DSTERF', -INFO )
             RETURN
          END IF
          IF( N.LE.1 )
         $   RETURN
    *
    *     Determine the unit roundoff for this environment.
    *
          EPS = DLAMCH( 'E' )
          EPS2 = EPS**2
          SAFMIN = DLAMCH( 'S' )
          SAFMAX = ONE / SAFMIN
          SSFMAX = SQRT( SAFMAX ) / THREE
          SSFMIN = SQRT( SAFMIN ) / EPS2
    *
    *     Compute the eigenvalues of the tridiagonal matrix.
    *
          NMAXIT = N*MAXIT
          SIGMA = ZERO
          JTOT = 0
    *
    *     Determine where the matrix splits and choose QL or QR iteration
    *     for each block, according to whether top or bottom diagonal
    *     element is smaller.
    *
          L1 = 1
    *
       10 CONTINUE
          IF( L1.GT.N )
         $   GO TO 170
          IF( L1.GT.1 )
         $   E( L1-1 ) = ZERO
          DO 20 M = L1, N - 1
             IF( ABS( E( M ) ).LE.( SQRT( ABS( D( M ) ) )*SQRT( ABS( D( M+
         $       1 ) ) ) )*EPS ) THEN
                E( M ) = ZERO
                GO TO 30
             END IF
       20 CONTINUE
          M = N
    *
       30 CONTINUE
          L = L1
          LSV = L
          LEND = M
          LENDSV = LEND
          L1 = M + 1
          IF( LEND.EQ.L )
         $   GO TO 10
    *
    *     Scale submatrix in rows and columns L to LEND
    *
          ANORM = DLANST( 'I', LEND-L+1, D( L ), E( L ) )
          ISCALE = 0
          IF( ANORM.GT.SSFMAX ) THEN
             ISCALE = 1
             CALL DLASCL( 'G', 0, 0, ANORM, SSFMAX, LEND-L+1, 1, D( L ), N,
         $                INFO )
             CALL DLASCL( 'G', 0, 0, ANORM, SSFMAX, LEND-L, 1, E( L ), N,
         $                INFO )
          ELSE IF( ANORM.LT.SSFMIN ) THEN
             ISCALE = 2
             CALL DLASCL( 'G', 0, 0, ANORM, SSFMIN, LEND-L+1, 1, D( L ), N,
         $                INFO )
             CALL DLASCL( 'G', 0, 0, ANORM, SSFMIN, LEND-L, 1, E( L ), N,
         $                INFO )
          END IF
    *
          DO 40 I = L, LEND - 1
             E( I ) = E( I )**2
       40 CONTINUE
    *
    *     Choose between QL and QR iteration
    *
          IF( ABS( D( LEND ) ).LT.ABS( D( L ) ) ) THEN
             LEND = LSV
             L = LENDSV
          END IF
    *
          IF( LEND.GE.L ) THEN
    *
    *        QL Iteration
    *
    *        Look for small subdiagonal element.
    *
       50    CONTINUE
             IF( L.NE.LEND ) THEN
                DO 60 M = L, LEND - 1
                   IF( ABS( E( M ) ).LE.EPS2*ABS( D( M )*D( M+1 ) ) )
         $            GO TO 70
       60       CONTINUE
             END IF
             M = LEND
    *
       70    CONTINUE
             IF( M.LT.LEND )
         $      E( M ) = ZERO
             P = D( L )
             IF( M.EQ.L )
         $      GO TO 90
    *
    *        If remaining matrix is 2 by 2, use DLAE2 to compute its
    *        eigenvalues.
    *
             IF( M.EQ.L+1 ) THEN
                RTE = SQRT( E( L ) )
                CALL DLAE2( D( L ), RTE, D( L+1 ), RT1, RT2 )
                D( L ) = RT1
                D( L+1 ) = RT2
                E( L ) = ZERO
                L = L + 2
                IF( L.LE.LEND )
         $         GO TO 50
                GO TO 150
             END IF
    *
             IF( JTOT.EQ.NMAXIT )
         $      GO TO 150
             JTOT = JTOT + 1
    *
    *        Form shift.
    *
             RTE = SQRT( E( L ) )
             SIGMA = ( D( L+1 )-P ) / ( TWO*RTE )
             R = DLAPY2( SIGMA, ONE )
             SIGMA = P - ( RTE / ( SIGMA+SIGN( R, SIGMA ) ) )
    *
             C = ONE
             S = ZERO
             GAMMA = D( M ) - SIGMA
             P = GAMMA*GAMMA
    *
    *        Inner loop
    *
             DO 80 I = M - 1, L, -1
                BB = E( I )
                R = P + BB
                IF( I.NE.M-1 )
         $         E( I+1 ) = S*R
                OLDC = C
                C = P / R
                S = BB / R
                OLDGAM = GAMMA
                ALPHA = D( I )
                GAMMA = C*( ALPHA-SIGMA ) - S*OLDGAM
                D( I+1 ) = OLDGAM + ( ALPHA-GAMMA )
                IF( C.NE.ZERO ) THEN
                   P = ( GAMMA*GAMMA ) / C
                ELSE
                   P = OLDC*BB
                END IF
       80    CONTINUE
    *
             E( L ) = S*P
             D( L ) = SIGMA + GAMMA
             GO TO 50
    *
    *        Eigenvalue found.
    *
       90    CONTINUE
             D( L ) = P
    *
             L = L + 1
             IF( L.LE.LEND )
         $      GO TO 50
             GO TO 150
    *
          ELSE
    *
    *        QR Iteration
    *
    *        Look for small superdiagonal element.
    *
      100    CONTINUE
             DO 110 M = L, LEND + 1, -1
                IF( ABS( E( M-1 ) ).LE.EPS2*ABS( D( M )*D( M-1 ) ) )
         $         GO TO 120
      110    CONTINUE
             M = LEND
    *
      120    CONTINUE
             IF( M.GT.LEND )
         $      E( M-1 ) = ZERO
             P = D( L )
             IF( M.EQ.L )
         $      GO TO 140
    *
    *        If remaining matrix is 2 by 2, use DLAE2 to compute its
    *        eigenvalues.
    *
             IF( M.EQ.L-1 ) THEN
                RTE = SQRT( E( L-1 ) )
                CALL DLAE2( D( L ), RTE, D( L-1 ), RT1, RT2 )
                D( L ) = RT1
                D( L-1 ) = RT2
                E( L-1 ) = ZERO
                L = L - 2
                IF( L.GE.LEND )
         $         GO TO 100
                GO TO 150
             END IF
    *
             IF( JTOT.EQ.NMAXIT )
         $      GO TO 150
             JTOT = JTOT + 1
    *
    *        Form shift.
    *
             RTE = SQRT( E( L-1 ) )
             SIGMA = ( D( L-1 )-P ) / ( TWO*RTE )
             R = DLAPY2( SIGMA, ONE )
             SIGMA = P - ( RTE / ( SIGMA+SIGN( R, SIGMA ) ) )
    *
             C = ONE
             S = ZERO
             GAMMA = D( M ) - SIGMA
             P = GAMMA*GAMMA
    *
    *        Inner loop
    *
             DO 130 I = M, L - 1
                BB = E( I )
                R = P + BB
                IF( I.NE.M )
         $         E( I-1 ) = S*R
                OLDC = C
                C = P / R
                S = BB / R
                OLDGAM = GAMMA
                ALPHA = D( I+1 )
                GAMMA = C*( ALPHA-SIGMA ) - S*OLDGAM
                D( I ) = OLDGAM + ( ALPHA-GAMMA )
                IF( C.NE.ZERO ) THEN
                   P = ( GAMMA*GAMMA ) / C
                ELSE
                   P = OLDC*BB
                END IF
      130    CONTINUE
    *
             E( L-1 ) = S*P
             D( L ) = SIGMA + GAMMA
             GO TO 100
    *
    *        Eigenvalue found.
    *
      140    CONTINUE
             D( L ) = P
    *
             L = L - 1
             IF( L.GE.LEND )
         $      GO TO 100
             GO TO 150
    *
          END IF
    *
    *     Undo scaling if necessary
    *
      150 CONTINUE
          IF( ISCALE.EQ.1 )
         $   CALL DLASCL( 'G', 0, 0, SSFMAX, ANORM, LENDSV-LSV+1, 1,
         $                D( LSV ), N, INFO )
          IF( ISCALE.EQ.2 )
         $   CALL DLASCL( 'G', 0, 0, SSFMIN, ANORM, LENDSV-LSV+1, 1,
         $                D( LSV ), N, INFO )
    *
    *     Check for no convergence to an eigenvalue after a total
    *     of N*MAXIT iterations.
    *
          IF( JTOT.LT.NMAXIT )
         $   GO TO 10
          DO 160 I = 1, N - 1
             IF( E( I ).NE.ZERO )
         $      INFO = INFO + 1
      160 CONTINUE
          GO TO 180
    *
    *     Sort eigenvalues in increasing order.
    *
      170 CONTINUE
          CALL DLASRT( 'I', N, D, INFO )
    *
      180 CONTINUE
          RETURN
    *
    *     End of DSTERF
    *
          END