dlarrk (3)  Linux Manuals
NAME
dlarrk.f 
SYNOPSIS
Functions/Subroutines
subroutine dlarrk (N, IW, GL, GU, D, E2, PIVMIN, RELTOL, W, WERR, INFO)
DLARRK computes one eigenvalue of a symmetric tridiagonal matrix T to suitable accuracy.
Function/Subroutine Documentation
subroutine dlarrk (integerN, integerIW, double precisionGL, double precisionGU, double precision, dimension( * )D, double precision, dimension( * )E2, double precisionPIVMIN, double precisionRELTOL, double precisionW, double precisionWERR, integerINFO)
DLARRK computes one eigenvalue of a symmetric tridiagonal matrix T to suitable accuracy.
Purpose:

DLARRK computes one eigenvalue of a symmetric tridiagonal matrix T to suitable accuracy. This is an auxiliary code to be called from DSTEMR. To avoid overflow, the matrix must be scaled so that its largest element is no greater than overflow**(1/2) * underflow**(1/4) in absolute value, and for greatest accuracy, it should not be much smaller than that. See W. Kahan "Accurate Eigenvalues of a Symmetric Tridiagonal Matrix", Report CS41, Computer Science Dept., Stanford University, July 21, 1966.
Parameters:

N
N is INTEGER The order of the tridiagonal matrix T. N >= 0.
IWIW is INTEGER The index of the eigenvalues to be returned.
GLGL is DOUBLE PRECISION
GUGU is DOUBLE PRECISION An upper and a lower bound on the eigenvalue.
DD is DOUBLE PRECISION array, dimension (N) The n diagonal elements of the tridiagonal matrix T.
E2E2 is DOUBLE PRECISION array, dimension (N1) The (n1) squared offdiagonal elements of the tridiagonal matrix T.
PIVMINPIVMIN is DOUBLE PRECISION The minimum pivot allowed in the Sturm sequence for T.
RELTOLRELTOL is DOUBLE PRECISION The minimum relative width of an interval. When an interval is narrower than RELTOL times the larger (in magnitude) endpoint, then it is considered to be sufficiently small, i.e., converged. Note: this should always be at least radix*machine epsilon.
WW is DOUBLE PRECISION
WERRWERR is DOUBLE PRECISION The error bound on the corresponding eigenvalue approximation in W.
INFOINFO is INTEGER = 0: Eigenvalue converged = 1: Eigenvalue did NOT converge
Internal Parameters:

FUDGE DOUBLE PRECISION, default = 2 A "fudge factor" to widen the Gershgorin intervals.
Author:

Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Date:
 September 2012
Definition at line 145 of file dlarrk.f.
Author
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