Nonconforming CrouzeixRaviar element approximation for Stekloff eigenvalues in inverse scattering ^{†}^{†}thanks: This work is supported by the National Natural Science Foundation of China (Grant Nos.11561014,11761022 ).
Abstract
In this paper, we use the nonconforming CrouzeixRaviart element method to solve a Stekloff eigenvalue problem arising in inverse scattering. The weak formulation corresponding to this problem is nonselfadjoint and does not satisfy elliptic condition, and its CrouzeixRaviart element discretization does not meet the Strang lemma condition. We use the standard duality techniques to prove an extension of Strang lemma. And we prove the convergence and error estimate of discrete eigenvalues and eigenfunctions using the spectral perturbation theory for compact operators. Finally, we present some numerical examples not only on uniform meshes but also in an adaptive refined meshes to show that the CrouzeixRaviart method is efficient for computing real and complex eigenvalues as expected.
Keywords:
Stekloff eigenvalueNonconforming CrouzeixRaviart elementStrang lemmaError estimates.∎
1 Introduction
Stekloff eigenvalue problems have important physical
background and many applications. For instance, they appear
in the analysis of stability of mechanical oscillators immersed
in a viscous fluid (see conca () and the references therein), in the study
of surface waves bergman (), in the study of the vibration modes
of a structure in contact with an incompressible fluid
bermudez () and in the analysis of the antiplane shearing on a system
of collinear
faults under slipdependent friction law bucur (). Hence, the finite element methods for solving these problems have attracted more and more scholars’ attention.
Till now, systematical and profound studies on the finite element approximation mainly
focus on Stekloff eigenvalue problems
which satisfy elliptic condition (see, e.g.,
alonso (); andreev (); armentano1 (); armentano2 (); bermudez (); bi (); bramble (); cao (); garau (); liq2 (); lim (); russo (); xie (); yang1 () and the references therein).
Recently Cakoni et al. cakoni1 () study a new Stekloff eigenvalue problem arising from the inverse scattering theory:
(1.1) 
where is a bounded domain in (),
is the outward normal derivative,
is the wavenumber and
is index of refraction that
is a bounded complex valued function with
and .
Note that the weak formulation of (1.1) (see (2.1)) does not satisfy elliptic condition.
Cakoni et al. cakoni1 ()
analyze the mathematics properties of (1.1) and use conforming finite element method to solve it.
Liu et al. liu () then study error estimates of conforming finite element eigenvalues for (1.1).
In this paper, we will study the nonconforming CrouzeixRaviart element
(CR element) approximation for the problem.
The CR element was first introduced by Crouzeix and Raviart in crouzeix () in 1973
to solve the stationary Stokes equation. It was also used to solve linear elasticity equations (see falk (); brenner2 ()),
the Laplace equation/eigenvalues (see armentano3 (); boffi (); brenner (); carstensen5 (); carstensen6 (); carstensen1 (); duran ()), Darcy’s equation ainsworth (),
Stekloff eigenvalue (seealonso (); bi (); liq2 (); russo (); yang1 ()) and so on.
The features of our work are as follows:

As we know, the convergence and error estimates of the nonconforming finite element method for an eigenvalue problem is based on the convergence and error estimates of the nonconforming finite element method for the corresponding source problem, and Strang lemma (see strang ()) is a fundamental analysis tool. However, the sesquilinear form in the CR element discretization here does not meet the Strang lemma condition. To overcome this difficulty, referring to §5.7 in brenner (), we use the standard duality techniques to prove an extension of Strang lemma (see Theorem 2). Based on the theorem, we prove the convergence and error estimates of the CR method for the corresponding source problem. The current paper, to our knowledge, is the first investigation of applying and extending Strang lemma to elliptic boundary value problem that the corresponding sesquilinear form is nonselfadjoint and not coercive.

Cakoni et al. cakoni1 () write (1.1) as an equivalent eigenvalue problem of the NeumanntoDirichlet operator . In this paper, we write the CR element approximation of (1.1) as an equivalent eigenvalue problem of the discrete operator , and prove converges in the sense of norm in , thus using BabuskaOsborn spectral approximation theory babuska () we prove first the convergence and error estimates of CR finite element eigenvalues and eigenfunctions for the problem (1.1).

We implement some numerical experiments not only on uniform meshes but also in adaptive refined meshes. It can be seen that the CR method is efficient for computing real and complex eigenvalues as expected. In addition, we discover, when the index of refraction is real and the corresponding eigenfunctions are singular, the CR element eigenvalues approximate the exact ones from above and conforming finite element eigenvalues approximate the exact ones from below, thus we get the upper and lower bounds of eigenvalues.
It should be pointed out that the theoretical analysis and conclusions in this paper are also valid for the extension CrouzeixRaviart element huj ().
In
this paper, regarding the basic theory of finite
element methods, we refer to babuska (); brenner (); ciarlet (); oden (); shiwang ().
Throughout this paper, denotes a positive constant
independent of , which may not be the same constant in different
places. For simplicity, we use the symbol to mean that
.
2 Preliminary
In this paper, we assume () is a polygonal () or polyhedron () domain.
Let denote the Sobolev
space with real order on , is the norm
on and , and
denotes the Sobolev space with real order
on with the norm .
Cakoni et al. cakoni1 () give the weak form of (1.1): Find , , such that
(2.1) 
where
The source problem associated with (1.1) is as follows: Find such that
(2.2) 
Consider the Neumann eigenvalue problem
(2.3) 
In this paper, we always assume is not an interior Neumann eigenvalue of (2.3). Under this assumption, according to cakoni1 () the NeumanntoDirichlet map can be defined as follows. Let , define by
(2.4) 
and , where denotes the restriction to . And (2.1) can be stated as the operator form:
(2.5) 
(2.1) and (2.5) are equivalent, namely, if is an eigenpair of (2.5), then is an eigenpair of (2.1),
; conversely, if is an
eigenpair of (2.1), then
is an eigenpair of (2.5), .
From cakoni1 () we know
is compact.
If is real, then is also selfadjoint.
Consider the dual problem of (2.1): Find
, such that
(2.6) 
The source problem associated with (2.6) is as follows: Find such that
(2.7) 
Define the corresponding NeumanntoDirichlet operator operator by
(2.8) 
and . Then (2.6) has the equivalent operator form:
(2.9) 
It can be proved that is the adjoint operator of in the sense of inner product . In fact, from (2.4) and (2.8) we have
Note that since is the adjoint operator of , the
primal and dual eigenvalues are connected via
.
Let be a regular simplex partition of
(see
ciarlet (), pp. 131). We denote
where is the diameter of element .
Let denote the set of all faces of elements . We split this set as follows: ,
with and being the sets of inner and boundary edges, respectively.
Let
be the CR element space defined on :
, is
continuous at the barycenters
of the faces of element .
The CR element approximation of
(2.1) is: Find , , such that
(2.10) 
where
.
Define ,
.
Evidently, is the norm on and it is easy to know that is not uniformly elliptic.
The CR element approximation of (2.2)
is: Find , such that
(2.11) 
Since is not an interior Neumann eigenvalue of (2.3), from spectral approximation theory we know that when is properly small also is not a CR element eigenvalue for (2.3). So the discrete source problem (2.11) is uniquely solvable. Thus, we can define the discrete operator , satisfying
(2.12) 
Let us denote by the function space defined on , which are restriction of functions in to . Define the discrete operator , satisfying . Then (2.12) has the equivalent operator form:
(2.13) 
namely, if is an eigenpair of (2.13), then is an eigenpair of (2.10),
; conversely, if is an
eigenpair of (2.10), then
is an eigenpair of (2.13), .
The nonconforming finite element approximation of
(2.6) is given by:
Find
, such that
(2.14) 
The CR element approximation of (2.7) is: Find , such that
(2.15) 
Define the discrete operator satisfying
(2.16) 
and denote , then (2.16) has the following equivalent operator form
(2.17) 
It can be proved that is the adjoint operator of
in the sense of inner product .
Hence, the primal and dual eigenvalues are connected via
.
We need the following regularity estimates which play an important role in our theoretical analysis.
Note that for , has a continuous extension, still denoted by , to .
Lemma 1
Proof
Since is not an interior Neumann eigenvalue of (2.3), there exists a unique solution to (2.2). Denote
Referring to the proof of (14.11) in ciarlet (), it is easy to verify that is a norm on that is equivalent to the norm . (2.3) can be rewritten as: Find , such that
(2.19) 
Since is not an interior Neumann eigenvalue for (2.3), is not an eigenvalue of (2.19). Define the map by
(2.20) 
Then (2.19) has the operator form:
And is compact, is not an eigenvalue of . So is bounded. Let be solution the following eqution:
(2.21) 
then we have . From (2.20) we obtain
which, together with (2.2) and (2.21), yields
Thus we have
and the proof is complete.
Lemma 2
Assume that is a polygonal with being the largest interior angle, and is the solution of (2.2). Let , then and
(2.22) 
let , then satisfying
(2.23) 
where , when , and when , and is a priori constant.
Proof
Consider the auxiliary boundary value problem:
(2.24)  
(2.25) 
Let and be the solution of (2.24) and (2.25), respectively, then it is easy to see that . Since , from classical regularity results (seedauge (), or Proposition 4.1 in alonso () and Proposition 4.4 in bermudez ()) we have
and from classical regularity result for the Laplace problem with homogeneous Neumann boundary condition we have
Thus we get
Substituting (2.18) into the above inequality we get (2.22) and (2.23).
Remark 1 (Regularity in ).
When is a polyhedron domain, regularity of the solution of the Neumann problem (2.24) has been discussed by many scholars.
Referring Theorem 4 in savare () and Remark 2.1 in garau (),
and using the argument of Lemma 2 in this paper,
we think the following regularity assumption is reasonable:
.
Assume that is a polyhedron domain, and is the solution of (2.2).
Let , then there is dependent on such that and
(2.26) 
It is easy to know that Lemmas 12 and Remark 1 are also valid for the dual problem (2.7).
3 The consistency term and the extension of Strang lemma
Define
Let
and be the solution of
(2.2) and (2.7), respectively. Define the
consistency terms: For any ,
(3.1)  
(3.2) 
In order to analyze error estimates of the consistency terms, we need the following trace inequalities.
Lemma 3
, , there holds
(3.3) 
Proof
The conclusion is followed by using the trace theorem on the reference element and the scaling argument. See, e.g., Lemma 2.2 in yang1 ().
Lemma 4
Let be the solution of (2.2), then there holds
(3.4) 
Proof
Inequality (3.4) is contained in the proof of Corollary 3.3 on page 1384
of bernardi (), see also Lemma 2.1 in caiz (). For the convenience of readers, we write the proof here.
For any ,
it is proven by going to a reference
element and using the inverse trace theorem that
there exists a lifting of such that , , ,
and
(3.5) 
From Green’s formula, (2.2), CauchySchwarz inequality, the definition of the dual norm and (3.5) we deduce
thus by the definition of the dual norm we obtain
This completes the proof of the lemma.
Based on the standard argument (see, for example
alonso (); liq2 (); yang1 ()), the following consistency error estimates will be
proved.
Theorem 3.1
Proof
Let denote the jump across an inner face . Then by Green’s formula we deduce
(3.8) 
Let be a ()face of , define
For , suppose that such that . Since is a linear function vanishing at the barycenters of , we have
(3.9) 
Then, when , using Schwarz inequality we deduce
(3.10) 
by (3.3) and the standard error estimates for projection, we deduce
Substituting the above two estimates into (3), we obtain
(3.11) 
and substituting (3.11) into (3) we conclude that
(3.6) holds.
When , from (3) we deduce that
(3.12) 
By using inverse estimate, (3.3) and the error estimate of projection, we derive
Substituting the above estimate and (3.4) into (3.12), we obtain
plugging the above inequality into (3) we also get (3.6).
Using the same argument as above, we can prove
(3.7).
The CR element approximation (2.11) of (2.2) does not satisfy the condition of Strang lemma, that is
is not uniformly elliptic.
To overcome this difficulty,
Inspired by the works in §5.7 in brenner (), next we use standard duality techniques to prove an
extension version of the wellknown Strang lemma.
First, we will use the standard duality argument to prove that is a quantity of higher order than .
Introduce the auxiliary problem: Find , such that
(3.13) 
Let be solution of (3.13), then from elliptic regularity estimates for homogeneous Neumann boundary value problem we know that there exists , such that
(3.14) 
Let , then
(3.15) 
Lemma 5
Proof
By Riesz representation theorem we have
(3.18) 
Let be CR nonconforming finite element interpolation function of , then according to the interpolation theory (see ciarlet ()) we have
(3.19) 
By computing, we deduce
and
combining the above two inequalities we get
Substituting the above equality into (3.18) we get
(3.20) 
Let be the Lagrange interpolation operator, , according definition of we deduce