/*
 *
 * Cholesky factorization of a symmetric, positive definite tridiagonal
 * matrix `A' according to `A=transpose(U)*U=L*transpose(L)'. On input, the
 * main diagonal of `A' is stored in the vector `d', and one of the secondary
 * diagonals is stored in the vector `e'. The operation works inplace.
 *
 */

unsigned int
cholfactridiag (unsigned int n, double *d, double *e)
{
  unsigned int i;
  double f;

  if (n == 0) {
    return (0);
    }
  if (isign3 (*(d + 0)) != 1) {
    return (1);
    }
  *(d + 0) = sqrt (*(d + 0));
  for (i = 1; i < n; i++) {
    *(e + i - 1) /= *(d + i - 1);
    f = *(d + i) - *(e + i - 1) * *(e + i - 1);
    if (isign3 (f) != 1) {
      return (1);
      }
    *(d + i) = sqrt (f);
    }
  return (0);
  }