---
name: "slarrb"
section: "3lapack"
platform: "linux"
package: "liblapack-doc"
version: "3.2.1-8"
release: "Debian 6.0 (Squeeze)"
platforms: ["linux"]
url: "https://new.carta.tech/slarrb/3lapack"
---

# slarrb(3lapack)

the relatively robust representation(RRR) L D L^T, SLARRB does "limited" bisection to refine the eigenvalues of L D L^T,

## NAME

SLARRB - the relatively robust representation(RRR) L D L^T, SLARRB does "limited" bisection to refine the eigenvalues of L D L^T,

## SYNOPSIS

- SUBROUTINE SLARRB(

  N, D, LLD, IFIRST, ILAST, RTOL1, RTOL2, OFFSET, W, WGAP, WERR, WORK, IWORK, PIVMIN, SPDIAM, TWIST, INFO )

- INTEGER

  IFIRST, ILAST, INFO, N, OFFSET, TWIST

- REAL

  PIVMIN, RTOL1, RTOL2, SPDIAM

- INTEGER

  IWORK( \* )

- REAL

  D( \* ), LLD( \* ), W( \* ), WERR( \* ), WGAP( \* ), WORK( \* )

## 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.\

## ARGUMENTS

- N (input) INTEGER

  The order of the matrix.

- D (input) REAL array, dimension (N)

  The N diagonal elements of the diagonal matrix D.

- LLD (input) REAL array, dimension (N-1)

  The (N-1) elements L(i)\*L(i)\*D(i).

- IFIRST (input) INTEGER

  The index of the first eigenvalue to be computed.

- ILAST (input) INTEGER

  The index of the last eigenvalue to be computed.

- RTOL1 (input) REAL

  RTOL2   (input) 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 (input) 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 (input/output) 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 (input/output) 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 (input/output) 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 (workspace) REAL array, dimension (2\*N)

  Workspace.

- IWORK (workspace) INTEGER array, dimension (2\*N)

  Workspace.

- PIVMIN (input) DOUBLE PRECISION

  The minimum pivot in the Sturm sequence.

- SPDIAM (input) DOUBLE PRECISION

  The spectral diameter of the matrix.

- TWIST (input) 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 (output) INTEGER

  Error flag.

## FURTHER DETAILS

Based on contributions by\
\
    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