Condensed generalized finite element method

Generalized or extended finite element methods (GFEM/XFEM) are in general badly conditioned and have numerous additional degrees of freedom (DOF) compared with the FEM due to introduction of enriched functions. In this paper, we develop an approach to establish a subspace of a conventional GFEM/XFEM approximation space using partition of unity (PU) techniques and local least-square procedures. The proposed GFEM is referred to as condensed GFEM (CGFEM), which (i) possesses as many DOFs as the preliminary FEM, (ii) enjoys similar approximation properties with the GFEM/XFEM, and (iii) is well-conditioned in the sense that its conditioning is of the same order as that of the FEM. The fundamental approximation properties of CGFEM are proven mathematically. The CGFEM is applied to a problem of high order polynomial approximations and a Poisson crack problem. The optimal convergence orders of the former are proven rigorously. Numerical experiments and comparisons with the conventional GFEM/XFEM and FEM are presented to verify the theory and effectiveness of CGFEM.