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/* dlarra.f -- translated by f2c (version 20061008).
You must link the resulting object file with libf2c:
on Microsoft Windows system, link with libf2c.lib;
on Linux or Unix systems, link with .../path/to/libf2c.a -lm
or, if you install libf2c.a in a standard place, with -lf2c -lm
-- in that order, at the end of the command line, as in
cc *.o -lf2c -lm
Source for libf2c is in /netlib/f2c/libf2c.zip, e.g.,
http://www.netlib.org/f2c/libf2c.zip
*/
#include "clapack.h"
/* Subroutine */ int dlarra_(integer *n, doublereal *d__, doublereal *e,
doublereal *e2, doublereal *spltol, doublereal *tnrm, integer *nsplit,
integer *isplit, integer *info)
{
/* System generated locals */
integer i__1;
doublereal d__1, d__2;
/* Builtin functions */
double sqrt(doublereal);
/* Local variables */
integer i__;
doublereal tmp1, eabs;
/* -- LAPACK auxiliary routine (version 3.2) -- */
/* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd.. */
/* November 2006 */
/* .. Scalar Arguments .. */
/* .. */
/* .. Array Arguments .. */
/* .. */
/* Purpose */
/* ======= */
/* Compute the splitting points with threshold SPLTOL. */
/* DLARRA sets any "small" off-diagonal elements to zero. */
/* Arguments */
/* ========= */
/* N (input) INTEGER */
/* The order of the matrix. N > 0. */
/* D (input) DOUBLE PRECISION array, dimension (N) */
/* On entry, the N diagonal elements of the tridiagonal */
/* matrix T. */
/* E (input/output) 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 (input/output) 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 (input) 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 (input) DOUBLE PRECISION */
/* The norm of the matrix. */
/* NSPLIT (output) INTEGER */
/* The number of blocks T splits into. 1 <= NSPLIT <= N. */
/* ISPLIT (output) 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 (output) INTEGER */
/* = 0: successful exit */
/* Further Details */
/* =============== */
/* Based on contributions by */
/* 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 */
/* ===================================================================== */
/* .. Parameters .. */
/* .. */
/* .. Local Scalars .. */
/* .. */
/* .. Intrinsic Functions .. */
/* .. */
/* .. Executable Statements .. */
/* Parameter adjustments */
--isplit;
--e2;
--e;
--d__;
/* Function Body */
*info = 0;
/* Compute splitting points */
*nsplit = 1;
if (*spltol < 0.) {
/* Criterion based on absolute off-diagonal value */
tmp1 = abs(*spltol) * *tnrm;
i__1 = *n - 1;
for (i__ = 1; i__ <= i__1; ++i__) {
eabs = (d__1 = e[i__], abs(d__1));
if (eabs <= tmp1) {
e[i__] = 0.;
e2[i__] = 0.;
isplit[*nsplit] = i__;
++(*nsplit);
}
/* L9: */
}
} else {
/* Criterion that guarantees relative accuracy */
i__1 = *n - 1;
for (i__ = 1; i__ <= i__1; ++i__) {
eabs = (d__1 = e[i__], abs(d__1));
if (eabs <= *spltol * sqrt((d__1 = d__[i__], abs(d__1))) * sqrt((
d__2 = d__[i__ + 1], abs(d__2)))) {
e[i__] = 0.;
e2[i__] = 0.;
isplit[*nsplit] = i__;
++(*nsplit);
}
/* L10: */
}
}
isplit[*nsplit] = *n;
return 0;
/* End of DLARRA */
} /* dlarra_ */