static List <double> quadratic(Sums s, double[] X, double[] Y)
        {
            double[] coefficient = new double[3];
            double[,] sys_matrix = { { s.sum4(X), s.sum3(X), s.sum2(X) },
                                     { s.sum3(X), s.sum2(X), s.sum(X)  },
                                     { s.sum2(X), s.sum(X),  X.Length  } };
            double[] solution_row = { s.sum_x_x_mult_y(X, Y), s.sum_x_mult_y(X, Y), s.sum(Y) };

            alglib.rmatrixsolve(sys_matrix, 3, solution_row, out int t, out alglib.densesolverreport c, out coefficient);
            List <double> cof = new List <double>();

            cof.AddRange(coefficient);
            return(cof);
        }
        static List <double> exponential(Sums s, double[] X, double[] Y)
        {
            double[] coefficient = new double[2];
            double[,] sys_matrix = { { s.sum2(X), s.sum(X) },
                                     { s.sum(X),  X.Length } };
            double[] solution_row = { s.sum_y_on_log_x(X, Y), s.sum_log_x(Y) };

            alglib.rmatrixsolve(sys_matrix, 2, solution_row, out int t, out alglib.densesolverreport c, out coefficient);
            coefficient[1] = Math.Exp(coefficient[1]);
            List <double> cof = new List <double>();

            cof.AddRange(coefficient);
            return(cof);
        }