Provided by: liblapack-doc_3.3.1-1_all

**NAME**

LAPACK-3 - the initial eigenvalue approximations of T, DLARRJ does bisection to refine the eigenvalues of T,

**SYNOPSIS**

SUBROUTINE DLARRJ( N, D, E2, IFIRST, ILAST, RTOL, OFFSET, W, WERR, WORK, IWORK, PIVMIN, SPDIAM, INFO ) INTEGER IFIRST, ILAST, INFO, N, OFFSET DOUBLE PRECISION PIVMIN, RTOL, SPDIAM INTEGER IWORK( * ) DOUBLE PRECISION D( * ), E2( * ), W( * ), WERR( * ), WORK( * )

**PURPOSE**

Given the initial eigenvalue approximations of T, DLARRJ does bisection to refine the eigenvalues of 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 in WERR. 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) DOUBLE PRECISION array, dimension (N) The N diagonal elements of T. E2 (input) DOUBLE PRECISION array, dimension (N-1) The Squares of the (N-1) subdiagonal elements of T. IFIRST (input) INTEGER The index of the first eigenvalue to be computed. ILAST (input) INTEGER The index of the last eigenvalue to be computed. RTOL (input) DOUBLE PRECISION Tolerance for the convergence of the bisection intervals. An interval [LEFT,RIGHT] has converged if RIGHT-LEFT.LT.RTOL*MAX(|LEFT|,|RIGHT|). OFFSET (input) INTEGER Offset for the arrays W and WERR, i.e., the IFIRST-OFFSET through ILAST-OFFSET elements of these arrays are to be used. W (input/output) 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. WERR (input/output) 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 (workspace) DOUBLE PRECISION array, dimension (2*N) Workspace. IWORK (workspace) INTEGER array, dimension (2*N) Workspace. PIVMIN (input) DOUBLE PRECISION The minimum pivot in the Sturm sequence for T. SPDIAM (input) DOUBLE PRECISION The spectral diameter of T. 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 LAPACK auxiliary routine (version 3.2.2) April 2011 DLARRJ(3lapack)