# dlarrj.f - Man Page

## Synopsis

### Functions/Subroutines

subroutine **dlarrj** (**N**, D, E2, IFIRST, ILAST, RTOL, OFFSET, W, WERR, WORK, IWORK, PIVMIN, SPDIAM, INFO)**DLARRJ** performs refinement of the initial estimates of the eigenvalues of the matrix T.

## Function/Subroutine Documentation

### subroutine dlarrj (integer N, double precision, dimension( * ) D, double precision, dimension( * ) E2, integer IFIRST, integer ILAST, double precision RTOL, integer OFFSET, double precision, dimension( * ) W, double precision, dimension( * ) WERR, double precision, dimension( * ) WORK, integer, dimension( * ) IWORK, double precision PIVMIN, double precision SPDIAM, integer INFO)

**DLARRJ** performs refinement of the initial estimates of the eigenvalues of the matrix T.

**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.

**Parameters:***N*N is INTEGER The order of the matrix.

*D*D is DOUBLE PRECISION array, dimension (N) The N diagonal elements of T.

*E2*E2 is DOUBLE PRECISION array, dimension (N-1) The Squares of the (N-1) subdiagonal elements of T.

*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.

*RTOL*RTOL is 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*OFFSET is 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*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.

*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 for T.

*SPDIAM*SPDIAM is DOUBLE PRECISION The spectral diameter of T.

*INFO*INFO is INTEGER Error flag.

**Author:**Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

**Date:**June 2017

**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 170 of file dlarrj.f.

## Author

Generated automatically by Doxygen for LAPACK from the source code.

## Referenced By

The man page dlarrj(3) is an alias of dlarrj.f(3).