SRC/dlarrb.f(3)            Library Functions Manual            SRC/dlarrb.f(3)

NAME
       SRC/dlarrb.f

SYNOPSIS
   Functions/Subroutines
       subroutine dlarrb (n, d, lld, ifirst, ilast, rtol1, rtol2, offset, w,
           wgap, werr, work, iwork, pivmin, spdiam, twist, info)
           DLARRB provides limited bisection to locate eigenvalues for more
           accuracy.

Function/Subroutine Documentation
   subroutine dlarrb (integer n, double precision, dimension( * ) d, double
       precision, dimension( * ) lld, integer ifirst, integer ilast, double
       precision rtol1, double precision rtol2, integer offset, double
       precision, dimension( * ) w, double precision, dimension( * ) wgap,
       double precision, dimension( * ) werr, double precision, dimension( * )
       work, integer, dimension( * ) iwork, double precision pivmin, double
       precision spdiam, integer twist, integer info)
       DLARRB provides limited bisection to locate eigenvalues for more
       accuracy.

       Purpose:


            Given the relatively robust representation(RRR) L D L^T, DLARRB
            does 'limited' bisection to refine the eigenvalues of L D L^T,
            W( IFIRST-OFFSET ) through W( ILAST-OFFSET ), to more accuracy. Initial
            guesses for these eigenvalues are input in W, the corresponding estimate
            of the error in these guesses and their gaps are input in WERR
            and WGAP, respectively. During bisection, intervals
            [left, right] are maintained by storing their mid-points and
            semi-widths in the arrays W and WERR respectively.

       Parameters
           N

                     N is INTEGER
                     The order of the matrix.

           D

                     D is DOUBLE PRECISION array, dimension (N)
                     The N diagonal elements of the diagonal matrix D.

           LLD

                     LLD is DOUBLE PRECISION array, dimension (N-1)
                     The (N-1) elements L(i)*L(i)*D(i).

           IFIRST

                     IFIRST is INTEGER
                     The index of the first eigenvalue to be computed.

           ILAST

                     ILAST is INTEGER
                     The index of the last eigenvalue to be computed.

           RTOL1

                     RTOL1 is DOUBLE PRECISION

           RTOL2

                     RTOL2 is DOUBLE PRECISION
                     Tolerance for the convergence of the bisection intervals.
                     An interval [LEFT,RIGHT] has converged if
                     RIGHT-LEFT < MAX( RTOL1*GAP, RTOL2*MAX(|LEFT|,|RIGHT|) )
                     where GAP is the (estimated) distance to the nearest
                     eigenvalue.

           OFFSET

                     OFFSET is INTEGER
                     Offset for the arrays W, WGAP and WERR, i.e., the IFIRST-OFFSET
                     through ILAST-OFFSET elements of these arrays are to be used.

           W

                     W is DOUBLE PRECISION array, dimension (N)
                     On input, W( IFIRST-OFFSET ) through W( ILAST-OFFSET ) are
                     estimates of the eigenvalues of L D L^T indexed IFIRST through
                     ILAST.
                     On output, these estimates are refined.

           WGAP

                     WGAP is DOUBLE PRECISION array, dimension (N-1)
                     On input, the (estimated) gaps between consecutive
                     eigenvalues of L D L^T, i.e., WGAP(I-OFFSET) is the gap between
                     eigenvalues I and I+1. Note that if IFIRST = ILAST
                     then WGAP(IFIRST-OFFSET) must be set to ZERO.
                     On output, these gaps are refined.

           WERR

                     WERR is DOUBLE PRECISION array, dimension (N)
                     On input, WERR( IFIRST-OFFSET ) through WERR( ILAST-OFFSET ) are
                     the errors in the estimates of the corresponding elements in W.
                     On output, these errors are refined.

           WORK

                     WORK is DOUBLE PRECISION array, dimension (2*N)
                     Workspace.

           IWORK

                     IWORK is INTEGER array, dimension (2*N)
                     Workspace.

           PIVMIN

                     PIVMIN is DOUBLE PRECISION
                     The minimum pivot in the Sturm sequence.

           SPDIAM

                     SPDIAM is DOUBLE PRECISION
                     The spectral diameter of the matrix.

           TWIST

                     TWIST is INTEGER
                     The twist index for the twisted factorization that is used
                     for the negcount.
                     TWIST = N: Compute negcount from L D L^T - LAMBDA I = L+ D+ L+^T
                     TWIST = 1: Compute negcount from L D L^T - LAMBDA I = U- D- U-^T
                     TWIST = R: Compute negcount from L D L^T - LAMBDA I = N(r) D(r) N(r)

           INFO

                     INFO is INTEGER
                     Error flag.

       Author
           Univ. of Tennessee

           Univ. of California Berkeley

           Univ. of Colorado Denver

           NAG Ltd.

       Contributors:
           Beresford Parlett, University of California, Berkeley, USA
            Jim Demmel, University of California, Berkeley, USA
            Inderjit Dhillon, University of Texas, Austin, USA
            Osni Marques, LBNL/NERSC, USA
            Christof Voemel, University of California, Berkeley, USA

       Definition at line 193 of file dlarrb.f.

Author
       Generated automatically by Doxygen for LAPACK from the source code.

LAPACK                          Version 3.12.0                 SRC/dlarrb.f(3)