public virtual Vector[] Calculate_ElementNodal_DUDX()
        {
            double[] Ue   = Get_ElementNodal_U();
            Vector[] Xi   = Interpolator.Calculate_ElementNodal_Xi();
            int      NNPE = Xi.Length;

            Vector[] DUDX = new Vector[NNPE];
            for (int i = 0; i < NNPE; i++)
            {
                DUDX[i] = Interpolator.Interpolate_DerivativeOfVariable_WRTX(Xi[i], Ue, ElementNodes);
            }
            return(DUDX);
        }