SRC/dlarra.f(3)

NAME
SRC/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 (integer n, double precision, dimension( * ) d, double precision, dimension( * ) e, double precision, dimension( * ) e2, double precision spltol, double precision tnrm, integer nsplit, integer, dimension( * ) isplit, integer info)

DLARRA computes the splitting points with the specified threshold.

Purpose:

    !>
    !> Compute the splitting points with threshold SPLTOL.
    !> DLARRA sets any  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.

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 134 of file dlarra.f.

Author

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