Supercloseness of finite element method for a singularly perturbed convection-diffusion problem on Bakhvalov-type mesh in 2D

For singularly perturbed convection-diffusion problems, supercloseness analysis of finite element method is still open on Bakhvalov-type meshes, especially in the case of 2D. The difficulties arise from the width of the mesh in the layer adjacent to the transition point, resulting in a suboptimal estimate for convergence. Existing analysis techniques cannot handle these difficulties well. To fill this gap, a novel interpolation is designed delicately for the first time for the smooth part of the solution, bringing about the optimal supercloseness result of almost order 2 under an energy norm for finite element method. Our theoretical result is uniformly in the singular perturbation parameter and is supported by the numerical experiments.