Low-order dPG-FEM for an elliptic PDE

This paper introduces a novel lowest-order discontinuous Petrov–Galerkin (dPG) finite element method (FEM) for the Poisson model problem. The ultra-weak formulation allows for piecewise constant and affine ansatz functions and for piecewise affine and lowest-order Raviart–Thomas test functions. This lowest-order discretization for the Poisson model problem allows for a direct proof of the discrete inf–sup condition and a complete a priori and a posteriori error analysis. Numerical experiments investigate the performance of the method and underline the quasi-optimal convergence.