A superconvergent ultra-weak local discontinuous Galerkin method for nonlinear fourth-order boundary-value problems

In this paper, we focus on constructing and analyzing a superconvergent ultra-weak local discontinuous Galerkin (UWLDG) method for the one-dimensional nonlinear fourth-order equation of the form - u((4)) = f(x, u). We combine the advantages of the local discontinuous Galerkin (LDG) method and the ultra-weak discontinuous Galerkin (UWDG) method. First, we rewrite the fourth-order equation into a second-order system, then we apply the UWDG method to the system. Optimal error estimates for the solution and its second derivative in the L-2-norm are established on regular meshes. More precisely, we use special projections to prove optimal error estimates with order p + 1 in the L-2-norm for the solution and for the auxiliary variable approximating the second derivative of the solution, when piecewise polynomials of degree at most p and mesh size h are used. We then show that the UWLDG solutions are superconvergent with order p + 2 toward special projections of the exact solutions. Our proofs are valid for arbitrary regular meshes using P-p polynomials with p >= 2. Finally, various numerical examples are presented to demonstrate the accuracy and capability of our method.