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/* dgttrf.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 dgttrf_(integer *n, doublereal *dl, doublereal *d__,
doublereal *du, doublereal *du2, integer *ipiv, integer *info)
{
/* System generated locals */
integer i__1;
doublereal d__1, d__2;
/* Local variables */
integer i__;
doublereal fact, temp;
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 */
/* ======= */
/* DGTTRF computes an LU factorization of a real tridiagonal matrix A */
/* using elimination with partial pivoting and row interchanges. */
/* The factorization has the form */
/* A = L * U */
/* where L is a product of permutation and unit lower bidiagonal */
/* matrices and U is upper triangular with nonzeros in only the main */
/* diagonal and first two superdiagonals. */
/* Arguments */
/* ========= */
/* N (input) INTEGER */
/* The order of the matrix A. */
/* DL (input/output) DOUBLE PRECISION array, dimension (N-1) */
/* On entry, DL must contain the (n-1) sub-diagonal elements of */
/* A. */
/* On exit, DL is overwritten by the (n-1) multipliers that */
/* define the matrix L from the LU factorization of A. */
/* D (input/output) DOUBLE PRECISION array, dimension (N) */
/* On entry, D must contain the diagonal elements of A. */
/* On exit, D is overwritten by the n diagonal elements of the */
/* upper triangular matrix U from the LU factorization of A. */
/* DU (input/output) DOUBLE PRECISION array, dimension (N-1) */
/* On entry, DU must contain the (n-1) super-diagonal elements */
/* of A. */
/* On exit, DU is overwritten by the (n-1) elements of the first */
/* super-diagonal of U. */
/* DU2 (output) DOUBLE PRECISION array, dimension (N-2) */
/* On exit, DU2 is overwritten by the (n-2) elements of the */
/* second super-diagonal of U. */
/* IPIV (output) INTEGER array, dimension (N) */
/* The pivot indices; for 1 <= i <= n, row i of the matrix was */
/* interchanged with row IPIV(i). IPIV(i) will always be either */
/* i or i+1; IPIV(i) = i indicates a row interchange was not */
/* required. */
/* INFO (output) INTEGER */
/* = 0: successful exit */
/* < 0: if INFO = -k, the k-th argument had an illegal value */
/* > 0: if INFO = k, U(k,k) is exactly zero. The factorization */
/* has been completed, but the factor U is exactly */
/* singular, and division by zero will occur if it is used */
/* to solve a system of equations. */
/* ===================================================================== */
/* .. Parameters .. */
/* .. */
/* .. Local Scalars .. */
/* .. */
/* .. Intrinsic Functions .. */
/* .. */
/* .. External Subroutines .. */
/* .. */
/* .. Executable Statements .. */
/* Parameter adjustments */
--ipiv;
--du2;
--du;
--d__;
--dl;
/* Function Body */
*info = 0;
if (*n < 0) {
*info = -1;
i__1 = -(*info);
xerbla_("DGTTRF", &i__1);
return 0;
}
/* Quick return if possible */
if (*n == 0) {
return 0;
}
/* Initialize IPIV(i) = i and DU2(I) = 0 */
i__1 = *n;
for (i__ = 1; i__ <= i__1; ++i__) {
ipiv[i__] = i__;
/* L10: */
}
i__1 = *n - 2;
for (i__ = 1; i__ <= i__1; ++i__) {
du2[i__] = 0.;
/* L20: */
}
i__1 = *n - 2;
for (i__ = 1; i__ <= i__1; ++i__) {
if ((d__1 = d__[i__], abs(d__1)) >= (d__2 = dl[i__], abs(d__2))) {
/* No row interchange required, eliminate DL(I) */
if (d__[i__] != 0.) {
fact = dl[i__] / d__[i__];
dl[i__] = fact;
d__[i__ + 1] -= fact * du[i__];
}
} else {
/* Interchange rows I and I+1, eliminate DL(I) */
fact = d__[i__] / dl[i__];
d__[i__] = dl[i__];
dl[i__] = fact;
temp = du[i__];
du[i__] = d__[i__ + 1];
d__[i__ + 1] = temp - fact * d__[i__ + 1];
du2[i__] = du[i__ + 1];
du[i__ + 1] = -fact * du[i__ + 1];
ipiv[i__] = i__ + 1;
}
/* L30: */
}
if (*n > 1) {
i__ = *n - 1;
if ((d__1 = d__[i__], abs(d__1)) >= (d__2 = dl[i__], abs(d__2))) {
if (d__[i__] != 0.) {
fact = dl[i__] / d__[i__];
dl[i__] = fact;
d__[i__ + 1] -= fact * du[i__];
}
} else {
fact = d__[i__] / dl[i__];
d__[i__] = dl[i__];
dl[i__] = fact;
temp = du[i__];
du[i__] = d__[i__ + 1];
d__[i__ + 1] = temp - fact * d__[i__ + 1];
ipiv[i__] = i__ + 1;
}
}
/* Check for a zero on the diagonal of U. */
i__1 = *n;
for (i__ = 1; i__ <= i__1; ++i__) {
if (d__[i__] == 0.) {
*info = i__;
goto L50;
}
/* L40: */
}
L50:
return 0;
/* End of DGTTRF */
} /* dgttrf_ */
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