515

RWTH Publication No: 822053 2021 IGPM515.pdf 
TITLE 
Optimal preconditioners for a Nitsche stabilized fictitious domain finite element method 
AUTHORS 
Sven Groß, Arnold Reusken 
ABSTRACT 
In this paper we consider a class of fictitious domain finite element methods known
from the literature. These methods use standard finite element spaces on a fixed unfitted triangulation
combined with the Nitsche technique and a ghost penalty stabilization. As a model problem
we consider the application of such a method to the Poisson equation. We introduce and analyze
a new class of preconditioners that is based on a subspace decomposition approach. The finite element
space is split into two subspaces, where one subspace is spanned by all nodal basis functions
corresponding to nodes on the boundary of the fictitious domain and the other space is spanned
by all remaining nodal basis functions. We will show that this splitting is stable, uniformly in the
discretization parameter and in the location of the problem boundary in the triangulation. We also
prove that the Galerkin discretization in the first subspace leads to a uniformly wellconditioned
matrix and that the Galerkin discretization in the second subspace is uniformly equivalent to a standard finite element discretization of a Poisson equation on the fictitious domain with homogeneous
Dirichlet boundary conditions. Results of numerical experiments that illustrate optimality of such a
preconditioner are included. 
KEYWORDS 
unfitted finite elements, CutFEM, Nitsche method, fictitious domain method, preconditioner 