docs: static public double F8F2(double[] x) {
  
      double sum = 0.0;
  
      for (int i = 1; i < x.length; i++) {
        sum += F8(F2(x[i - 1], x[i]));
      }
      sum += F8(F2(x[x.length - 1], x[0]));
  
      return (sum);
    }