public override OOPTools_Math.Vector IntegralArgument_Fe(OOPTools_Math.Vector Xi)
        {
            int    NNPE = ElementNodes.Length;
            double Det_Jac;
            Vector X;
            Vector N;
            Vector B = new Vector(2);

            Interpolator.Calculate_X_N_DetJacobian(Xi, ElementNodes, out X, out N, out Det_Jac);
            B = b(X);
            Vector F_Arg = new Vector(NNPE * 2);

            for (int i = 0; i < NNPE; i++)
            {
                F_Arg.Values[2 * i]     = N.Values[i] * B.Values[0] * rho(X); //nodes at x direction
                F_Arg.Values[2 * i + 1] = N.Values[i] * B.Values[1] * rho(X); //nodes at y direction
            }
            return(F_Arg);
        }