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/* dpttrf.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 "f2c.h"
#include "blaswrap.h"
/* Subroutine */ int dpttrf_(integer *n, doublereal *d__, doublereal *e,
integer *info)
{
/* System generated locals */
integer i__1;
/* Local variables */
integer i__, i4;
doublereal ei;
extern /* Subroutine */ int xerbla_(char *, integer *);
/* -- LAPACK routine (version 3.2) -- */
/* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd.. */
/* November 2006 */
/* .. Scalar Arguments .. */
/* .. */
/* .. Array Arguments .. */
/* .. */
/* Purpose */
/* ======= */
/* DPTTRF computes the L*D*L' factorization of a real symmetric */
/* positive definite tridiagonal matrix A. The factorization may also */
/* be regarded as having the form A = U'*D*U. */
/* Arguments */
/* ========= */
/* N (input) INTEGER */
/* The order of the matrix A. N >= 0. */
/* D (input/output) DOUBLE PRECISION array, dimension (N) */
/* On entry, the n diagonal elements of the tridiagonal matrix */
/* A. On exit, the n diagonal elements of the diagonal matrix */
/* D from the L*D*L' factorization of A. */
/* E (input/output) DOUBLE PRECISION array, dimension (N-1) */
/* On entry, the (n-1) subdiagonal elements of the tridiagonal */
/* matrix A. On exit, the (n-1) subdiagonal elements of the */
/* unit bidiagonal factor L from the L*D*L' factorization of A. */
/* E can also be regarded as the superdiagonal of the unit */
/* bidiagonal factor U from the U'*D*U factorization of A. */
/* INFO (output) INTEGER */
/* = 0: successful exit */
/* < 0: if INFO = -k, the k-th argument had an illegal value */
/* > 0: if INFO = k, the leading minor of order k is not */
/* positive definite; if k < N, the factorization could not */
/* be completed, while if k = N, the factorization was */
/* completed, but D(N) <= 0. */
/* ===================================================================== */
/* .. Parameters .. */
/* .. */
/* .. Local Scalars .. */
/* .. */
/* .. External Subroutines .. */
/* .. */
/* .. Intrinsic Functions .. */
/* .. */
/* .. Executable Statements .. */
/* Test the input parameters. */
/* Parameter adjustments */
--e;
--d__;
/* Function Body */
*info = 0;
if (*n < 0) {
*info = -1;
i__1 = -(*info);
xerbla_("DPTTRF", &i__1);
return 0;
}
/* Quick return if possible */
if (*n == 0) {
return 0;
}
/* Compute the L*D*L' (or U'*D*U) factorization of A. */
i4 = (*n - 1) % 4;
i__1 = i4;
for (i__ = 1; i__ <= i__1; ++i__) {
if (d__[i__] <= 0.) {
*info = i__;
goto L30;
}
ei = e[i__];
e[i__] = ei / d__[i__];
d__[i__ + 1] -= e[i__] * ei;
/* L10: */
}
i__1 = *n - 4;
for (i__ = i4 + 1; i__ <= i__1; i__ += 4) {
/* Drop out of the loop if d(i) <= 0: the matrix is not positive */
/* definite. */
if (d__[i__] <= 0.) {
*info = i__;
goto L30;
}
/* Solve for e(i) and d(i+1). */
ei = e[i__];
e[i__] = ei / d__[i__];
d__[i__ + 1] -= e[i__] * ei;
if (d__[i__ + 1] <= 0.) {
*info = i__ + 1;
goto L30;
}
/* Solve for e(i+1) and d(i+2). */
ei = e[i__ + 1];
e[i__ + 1] = ei / d__[i__ + 1];
d__[i__ + 2] -= e[i__ + 1] * ei;
if (d__[i__ + 2] <= 0.) {
*info = i__ + 2;
goto L30;
}
/* Solve for e(i+2) and d(i+3). */
ei = e[i__ + 2];
e[i__ + 2] = ei / d__[i__ + 2];
d__[i__ + 3] -= e[i__ + 2] * ei;
if (d__[i__ + 3] <= 0.) {
*info = i__ + 3;
goto L30;
}
/* Solve for e(i+3) and d(i+4). */
ei = e[i__ + 3];
e[i__ + 3] = ei / d__[i__ + 3];
d__[i__ + 4] -= e[i__ + 3] * ei;
/* L20: */
}
/* Check d(n) for positive definiteness. */
if (d__[*n] <= 0.) {
*info = *n;
}
L30:
return 0;
/* End of DPTTRF */
} /* dpttrf_ */
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