# Confusion between standard finite element and mass-lumping finite element methods

Consider the following equation, subject to homogeneous Neumann boundary condition. $$u_t = \Delta u + f(u).$$ The weak formulation is as follows: $$(u_t,w) = (\Delta u, w) + (f(u),w), \quad \forall w \in H^1(\Omega).$$ The standard finite element scheme is： $$(u_{h,t},w_h) = (\Delta_h u_h, w_h) + (f(u_h),w), \quad \forall w_h \in V^r_h(\Omega),$$ where the $$V^r_h$$ denotes the space of polynomials of degree at most r. And the mass lumping method scheme is $$(u_{h,t},w_h)_h = (\Delta_h u_h, w_h)_h + (f(u_h),w)_h, \forall w_h \in V^r_h(\Omega),$$

or

$$(u_{h,t},w_h)_h = (\Delta_h u_h, w_h) + (f(u_h),w)_h, \forall w_h \in V^r_h(\Omega),$$

where $$(\cdot, \cdot)_{h}$$ signifies the use of the mass lumping method for integration. The main difference between the above two mass lumping methods is the assembly of the stiffness matrix.

Defined $$P_h$$$$L^2(\Omega) \to V_h(\Omega)$$ denote the $$L_2$$-projection onto the space of $$V_h^r$$ , and $$\Pi_h: C(\bar{\Omega}) \to V^r_h$$ denote the interpolation by a function in $$V^r_h$$ through the nodes of mass lumping integration nodes. What I want to do is construct the error equation $$e_h = u_h - \Pi_h u$$, then $$\partial_t \Pi_h u(t) - \Delta_h \Pi_h u(t) = \Pi_h f(u) + \underbrace{\Pi_h \Delta u(t) - \Delta_h \Pi_h u (t)}_{g_h(u)}.$$

$$\partial_t \Pi_h u(t) - \Delta_h \Pi_h u(t) = \Pi_h f(u) + \underbrace{P_h \Delta u(t) - \Delta_h \Pi_h u (t)}_{g_h(u)}.$$

In the paper https://link.springer.com/article/10.1007/s10915-021-01746-y Lemma 2.4 proof that
$$(\Pi_h \Delta u - \Delta_h \Pi_h u , w_h) \leq h^{r+1}||u||_{2r+2} ||w_h||_1$$ Consider the $$g_h(u)$$ in the second equation: $$(g_h(u),w_h) = (P_h \Delta u - \Delta_h \Pi_h u, w_h) = (P_h \Delta u - \Delta u,w_h) + (\Delta u - \Delta_h \Pi_h u,w_h).$$

$$=(\Delta_h R_h u - \Delta u, w_h) + (\Delta u - \Delta_h \Pi_h u,w_h)$$

$$=(\Delta u - \Delta_h \Pi_h u,w_h).$$ where the $$R_h$$ is the Ritz projection. These two schemes seem to be equivalent, but when I use $$(\cdot,\cdot)_h$$ to integrate the stiffness matrix, the convergence order will be reduced in some cases. Did I go wrong somewhere?

• How exactly are $\Pi_h$ and $R_h$ defined? Nov 7, 2023 at 17:08
• @WolfgangBangerth I fixed my problem, now $\Pi_h$ is the Lagrange interpolation operator, which interpolation by a function in $V^r_h$ through the nodes of mass-lumping intergrate nodes. For example, in 1D case, it means interpolate a function through the Gauss-Lobatto nodes. And $P_h$ is a standard L2 projection. $R_h$ is the Ritz projection. Nov 8, 2023 at 3:16