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cholesky.c
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1990-10-04
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/* choldecomp - Cholesky decomposition routines. */
/* XLISP-STAT 2.1 Copyright (c) 1990, by Luke Tierney */
/* Additions to Xlisp 2.1, Copyright (c) 1989 by David Michael Betz */
/* You may give out copies of this software; for conditions see the */
/* file COPYING included with this distribution. */
#include "xlisp.h"
#include "osdef.h"
#ifdef ANSI
#include "xlsproto.h"
#else
#include "xlsfun.h"
#endif ANSI
/*
choldecomp(a, n)
RMatrix a;
int n;
{
int i, j, k;
for (i = 0; i < n; i++) {
for (k = 0; k < i; k++)
a[i][i] -= a[k][i] * a[k][i];
a[i][i] = (a[i][i] > 0) ? sqrt(a[i][i]) : 0.0;
for (j = i + 1; j < n; j++) {
for (k = 0; k < i; k++) a[i][j] -= a[k][i] * a[k][j];
a[i][j] = (a[i][i] > 0.0) ? a[i][j] / a[i][i] : 0.0;
}
}
for (i = 0; i < n; i++)
for (j = 0; j < i; j++)
a[i][j] = 0.0;
}
*/
/* macro in xlsdef.h JKL
static double Max(a, b)
double a, b;
{
return(a > b ? a : b);
}
*/
void choldecomp(a, n, maxoffl, maxadd)
RMatrix a;
int n;
double maxoffl, *maxadd;
{
double minl, minljj, minl2;
int i, j, k;
minl = pow(macheps(), 0.25) * maxoffl;
minl2 = 0.0;
if (maxoffl == 0.0) {
for (i = 0; i < n; i++)
maxoffl = Max(fabs(a[i][i]), maxoffl);
maxoffl = sqrt(maxoffl);
minl2 = sqrt(macheps()) * maxoffl;
}
*maxadd = 0.0;
for (j = 0; j < n; j++) {
for (i = 0; i < j; i++) a[j][j] -= a[j][i] * a[j][i];
minljj = 0.0;
for (i = j + 1; i < n; i++) {
a[i][j] = a[j][i];
for (k = 0; k < j; k++) a[i][j] -= a[i][k] * a[j][k];
minljj = Max(fabs(a[i][j]), minljj);
}
minljj = Max(minljj / maxoffl, minl);
if (a[j][j] > minljj * minljj) a[j][j] = sqrt(a[j][j]);
else {
if (minljj < minl2) minljj = minl2;
*maxadd = Max(*maxadd, minljj * minljj - a[j][j]);
a[j][j] = minljj;
}
for (i = j + 1; i < n; i++) a[i][j] /= a[j][j];
}
for (i = 0; i < n; i++)
for (j = i + 1; j < n; j++) a[i][j] = 0.0;
}