dlarra.f(3)

NAME
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

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

    D

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

    E

              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

              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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