A Dichotomy of Finite Element Spaces and Its Application to an Energy-Conservative Scheme for the Regularized Long Wave Equation

Abstract

SIAM Journal on Numerical Analysis, Volume 64, Issue 5, Page 2079-2099, October 2026.
Abstract. Certain energy-conservative Galerkin discretizations for nonlinear dispersive wave equations have revealed an unusual convergence behavior: optimal convergence is attained when continuous Lagrange finite element spaces of odd polynomial degree are employed, whereas the use of even-degree polynomials leads to reduced convergence rate. The present work demonstrates that this behavior is intrinsic to the structure of the finite element spaces themselves. In particular, it is shown to be closely connected to the standard [math]-projection of derivatives, which possesses a super-approximation property exclusively for odd polynomial degrees. We also examine the implications of this feature for an energy-conservative Galerkin approximation of the regularized long-wave equation where the energy is a cubic functional. Although the resulting scheme conserves both mass and energy, we further show that the impulse is approximated with high accuracy, and we establish a priori error bounds for the associated semidiscrete formulation.