dlarra-f(3)

dlarra.f –

Synopsis

Functions/Subroutines

subroutine dlarra (N, D, E, E2, SPLTOL, TNRM, NSPLIT, ISPLIT, INFO)
DLARRA computes the splitting points with the specified threshold.

Function/Subroutine Documentation

subroutine dlarra (integerN, double precision, dimension( * )D, double precision, dimension( * )E, double precision, dimension( * )E2, double precisionSPLTOL, double precisionTNRM, integerNSPLIT, integer, dimension( * )ISPLIT, integerINFO)

DLARRA computes the splitting points with the specified threshold.

Purpose:

 Compute the splitting points with threshold SPLTOL.
 DLARRA sets any "small" off-diagonal elements to zero.

Parameters:

 N is INTEGER
 The order of the matrix. N > 0.

 D is DOUBLE PRECISION array, dimension (N)
 On entry, the N diagonal elements of the tridiagonal
 matrix T.

 E is DOUBLE PRECISION array, dimension (N)
 On entry, the first (N-1) entries contain the subdiagonal
 elements of the tridiagonal matrix T; E(N) need not be set.
 On exit, the entries E( ISPLIT( I ) ), 1 <= I <= NSPLIT,
 are set to zero, the other entries of E are untouched.

 E2 is DOUBLE PRECISION array, dimension (N)
 On entry, the first (N-1) entries contain the SQUARES of the
 subdiagonal elements of the tridiagonal matrix T;
 E2(N) need not be set.
 On exit, the entries E2( ISPLIT( I ) ),
 1 <= I <= NSPLIT, have been set to zero

SPLTOL

 SPLTOL is DOUBLE PRECISION
 The threshold for splitting. Two criteria can be used:
 SPLTOL<0 : criterion based on absolute off-diagonal value
 SPLTOL>0 : criterion that preserves relative accuracy

TNRM

 TNRM is DOUBLE PRECISION
 The norm of the matrix.

NSPLIT

 NSPLIT is INTEGER
 The number of blocks T splits into. 1 <= NSPLIT <= N.

ISPLIT

 ISPLIT is INTEGER array, dimension (N)
 The splitting points, at which T breaks up into blocks.
 The first block consists of rows/columns 1 to ISPLIT(1),
 the second of rows/columns ISPLIT(1)+1 through ISPLIT(2),
 etc., and the NSPLIT-th consists of rows/columns
 ISPLIT(NSPLIT-1)+1 through ISPLIT(NSPLIT)=N.

INFO

 INFO is INTEGER
 = 0: successful exit

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 136 of file dlarra.f.

Author

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