DLAED6(3f)

NAME
DLAED6 - the positive or negative root (closest to the origin) of z(1) z(2) z(3) f(x) = rho + --------- + ---------- + --------- d(1)-x d(2)-x d(3)-x It is assumed that if ORGATI = .true

SYNOPSIS

SUBROUTINE DLAED6(
    KNITER, ORGATI, RHO, D, Z, FINIT, TAU, INFO )

LOGICAL
    ORGATI

INTEGER
    INFO, KNITER

DOUBLE
    PRECISION FINIT, RHO, TAU

DOUBLE
    PRECISION D( 3 ), Z( 3 )

PURPOSE
DLAED6 computes the positive or negative root (closest to the origin) of
                 z(1) z(2) z(3) f(x) = rho + --------- + ---------- + ---------
                d(1)-x d(2)-x d(3)-x
      otherwise it is between d(1) and d(2)
This routine will be called by DLAED4 when necessary. In most cases, the root sought is the smallest in magnitude, though it might not be in some extremely rare situations.

ARGUMENTS

KNITER       (input) INTEGER
    Refer to DLAED4 for its significance.

ORGATI       (input) LOGICAL
    If ORGATI is true, the needed root is between d(2) and d(3); otherwise it is between d(1) and d(2). See DLAED4 for further details.

RHO          (input) DOUBLE PRECISION
    Refer to the equation f(x) above.

D            (input) DOUBLE PRECISION array, dimension (3)
    D satisfies d(1) < d(2) < d(3).

Z            (input) DOUBLE PRECISION array, dimension (3)
    Each of the elements in z must be positive.

FINIT        (input) DOUBLE PRECISION
    The value of f at 0. It is more accurate than the one evaluated inside this routine (if someone wants to do so).

TAU          (output) DOUBLE PRECISION
    The root of the equation f(x).

INFO         (output) INTEGER
    = 0: successful exit
    > 0: if INFO = 1, failure to converge

FURTHER DETAILS
30/06/99: Based on contributions by
   Ren-Cang Li, Computer Science Division, University of California
   at Berkeley, USA
10/02/03: This version has a few statements commented out for thread safety (machine parameters are computed on each entry). SJH. 05/10/06: Modified from a new version of Ren-Cang Li, use
   Gragg-Thornton-Warner cubic convergent scheme for better stability.