NAME

       slarrb.f -

SYNOPSIS

   Functions/Subroutines
       subroutine slarrb (N, D, LLD, IFIRST, ILAST, RTOL1, RTOL2, OFFSET, W, WGAP, WERR, WORK,
           IWORK, PIVMIN, SPDIAM, TWIST, INFO)
           SLARRB provides limited bisection to locate eigenvalues for more accuracy.

Function/Subroutine Documentation

   subroutine slarrb (integerN, real, dimension( * )D, real, dimension( * )LLD, integerIFIRST,
       integerILAST, realRTOL1, realRTOL2, integerOFFSET, real, dimension( * )W, real, dimension(
       * )WGAP, real, dimension( * )WERR, real, dimension( * )WORK, integer, dimension( * )IWORK,
       realPIVMIN, realSPDIAM, integerTWIST, integerINFO)
       SLARRB provides limited bisection to locate eigenvalues for more accuracy.

       Purpose:

            Given the relatively robust representation(RRR) L D L^T, SLARRB
            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 REAL array, dimension (N)
                     The N diagonal elements of the diagonal matrix D.

           LLD

                     LLD is REAL 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 REAL

           RTOL2

                     RTOL2 is REAL
                     Tolerance for the convergence of the bisection intervals.
                     An interval [LEFT,RIGHT] has converged if
                     RIGHT-LEFT.LT.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 REAL array, dimension (N)
                     On input, W( IFIRST-OFFSET ) through W( ILAST-OFFSET ) are
                     estimates of the eigenvalues of L D L^T indexed IFIRST throug
                     ILAST.
                     On output, these estimates are refined.

           WGAP

                     WGAP is REAL 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.EQ.ILAST
                     then WGAP(IFIRST-OFFSET) must be set to ZERO.
                     On output, these gaps are refined.

           WERR

                     WERR is REAL 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 REAL array, dimension (2*N)
                     Workspace.

           IWORK

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

           PIVMIN

                     PIVMIN is REAL
                     The minimum pivot in the Sturm sequence.

           SPDIAM

                     SPDIAM is REAL
                     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.

       Date:
           September 2012

       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 195 of file slarrb.f.

Author

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