SLARRB (3)  Linux Manuals
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( IFIRSTOFFSET ) through W( ILASTOFFSET ), 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 midpoints and semiwidths in the arrays W and WERR respectively.
Parameters:

N
N is INTEGER The order of the matrix.
DD is REAL array, dimension (N) The N diagonal elements of the diagonal matrix D.
LLDLLD is REAL array, dimension (N1) The (N1) elements L(i)*L(i)*D(i).
IFIRSTIFIRST is INTEGER The index of the first eigenvalue to be computed.
ILASTILAST is INTEGER The index of the last eigenvalue to be computed.
RTOL1RTOL1 is REAL
RTOL2RTOL2 is REAL Tolerance for the convergence of the bisection intervals. An interval [LEFT,RIGHT] has converged if RIGHTLEFT.LT.MAX( RTOL1*GAP, RTOL2*MAX(LEFT,RIGHT) ) where GAP is the (estimated) distance to the nearest eigenvalue.
OFFSETOFFSET is INTEGER Offset for the arrays W, WGAP and WERR, i.e., the IFIRSTOFFSET through ILASTOFFSET elements of these arrays are to be used.
WW is REAL array, dimension (N) On input, W( IFIRSTOFFSET ) through W( ILASTOFFSET ) are estimates of the eigenvalues of L D L^T indexed IFIRST throug ILAST. On output, these estimates are refined.
WGAPWGAP is REAL array, dimension (N1) On input, the (estimated) gaps between consecutive eigenvalues of L D L^T, i.e., WGAP(IOFFSET) is the gap between eigenvalues I and I+1. Note that if IFIRST.EQ.ILAST then WGAP(IFIRSTOFFSET) must be set to ZERO. On output, these gaps are refined.
WERRWERR is REAL array, dimension (N) On input, WERR( IFIRSTOFFSET ) through WERR( ILASTOFFSET ) are the errors in the estimates of the corresponding elements in W. On output, these errors are refined.
WORKWORK is REAL array, dimension (2*N) Workspace.
IWORKIWORK is INTEGER array, dimension (2*N) Workspace.
PIVMINPIVMIN is REAL The minimum pivot in the Sturm sequence.
SPDIAMSPDIAM is REAL The spectral diameter of the matrix.
TWISTTWIST 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)
INFOINFO 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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