double aitkenneville (unsigned int n, double *xi, double *yi, double xo, double *w)
{
  /* xi[n], yi[n], w[n + n] */
  unsigned int i, j;
  for (i = 0; i < n; i++) {
    w[n - (i + 1)] = yi[i];
    w[n + n - (i + 1)] = xi[i];
    }
  for (i = 1; i < n; i++)
    for (j = i; j < n; j++)
      w[j - i] += (w[1 + j - i] - w[j - i]) * (xo - w[n + j - i]) / (w[n + j] - w[n + j - i]);
  return (w[0]);
  }