In this paper, we are concerned with the fast solvers for higher order finite element discretizations of H(div)-elliptic problem. We present the preconditioners for the first family and second family of higher order divergence conforming element equations, respectively. By combining the stable decompositions of two kinds of finite element spaces with the abstract theory of auxiliary space preconditioning, we prove that the corresponding condition numbers of our preconditioners are uniformly bounded on quasi-uniform grids.
preconditioner; higher order finite element; stable decomposition; H(div)-elliptic problem
Preconditioners for higher order finite element discretizations of H(div)-elliptic problem
Junxian Wang; Liuqiang Zhong; Shi Shu
School of Mathematical and Computational Sciences, Xiangtan University, Hunan 411105, China. E-mails: xianxian.student@sina.com / zhonglq@xtu.edu.cn / shushi@xtu.edu.cn
ABSTRACT
In this paper, we are concerned with the fast solvers for higher order finite element discretizations of H(div)-elliptic problem. We present the preconditioners for the first family and second family of higher order divergence conforming element equations, respectively. By combining the stable decompositions of two kinds of finite element spaces with the abstract theory of auxiliary space preconditioning, we prove that the corresponding condition numbers of our preconditioners are uniformly bounded on quasi-uniform grids.
Mathematical subject classification: Primary: 65F10; Secondary: 65N22.
Key words: preconditioner, higher order finite element, stable decomposition, H(div)-elliptic problem.
1 Introduction
Let Ω be a simply connected polyhedron in
3 with boundary Γ and unit outward normal ν. We define the Hilbert spaces H0(div; Ω) as followsH0 (div; Ω) = {u ∈ (L2 (Ω))3 | ∇ . u∈L2 (Ω), ν . u = 0 on Γ}
with the inner product
(u, ν)div = (u, ν) + (∇. u, ∇.ν),
where (·, ·) denotes the inner product in (L2(Ω))3 or L2(Ω).
In this paper, we consider the following variational problem: Find u∈ H0(div; Ω) such that
where ∈ H0(div; Ω)' is a given data and
with the constant τ > 0.
The bilinear form a(·, ·)induces the energy norm
Variational problem of the form (1) arises in numerous problems of practical import. Typical examples include the mixed method for second order elliptic problems, the least squares method of the form discussed in [3], and the sequential regularization method for the time dependent Navier-Stokes equation discussed in [6]. For a more detailed discussion of applications, we refer to [1].
To avoid the repeated use of generic but unspecified constants, following [9], we will use the following short notation: x y means x < Cy, x
y means x > cy, and x ≈ y means cx < y < Cy, where c and C are generic positiveconstants independent of the variables that appear in the inequalities and especially the mesh parameters.
Outline. The remainder of this article is organized as follows. In the next section, we introduce two kinds of higher order finite element equations, and present the corresponding frame of constructing preconditioner. We construct the preconditioners for two kinds of higher order divergence conforming element equations, and prove that their corresponding condition number is uniformly bounded in Section 3 and Section 4, respectively.
2 Finite element equations and framework of preconditioner
Let
h be a shape regular tetrahedron meshes of Ω, where h is the maximum diameter of the tetrahedra in
where
k denote the standard space of polynomials of total degree less than or equal to k, and
We consider the solution of systems of linear algebraic equations which arise from the finite element discretization of variational problems (1): Find (k > 1, l = 1, 2) such that
Their algebraic systems can be described as
Since is symmetric positive definite, we use precondition conjugate gradient (PCG) methods to solve algebraic systems (5). In this paper, we will construct the preconditioners for the cases of higher order finite equations, and present some estimates of the corresponding condition numbers.
For this purpose, we need to introduce some auxiliary spaces and corresponding operators.
Let V = with inner product a(·, ·) given by (2).
Let
1, ...,













here we tag dual spaces by ' and use angle brackets for duality pairings. For each
j, there exist continuous transfer operators Πj :

where
j :


Now, we present the following theorem of an estimate for the spectral condition number of the preconditioner given by (6).
Theorem 2.1. Assume that there exist constants cj, such that
and for ∀ u ∈ V, there exist j∈
j such that u =
and
then for the preconditioner B given by (6), we have the following estimate for the spectral condition number
Proof. We define the space
=
1 ×
2 × ... ×
j
with the inner product
and the following two operators
Π = (Π1,Π2, ..., Πj) :
= diag (
1,
2, ...,
j) :

= diag (
1,
2, ...,
j) :

Thus we can rewrite the definition of operator B given by (6):
B = Π Π*.
Using the definitions of inner product in , operators Π and
, and conditions (7)-(8), then there exists a constant
:=
, such that
and for ∀ u ∈ V, there exists ∈
, such that u = Π
and
||||
< c0||u||A
From Corollary 2.3 of [5], we immediately get an estimate for the spectral condition number of the preconditioned operator B
The desired estimates then follow by combining the above inequality and the following fact
□
The principal challenge confronted in the development of preconditioners by applying Theorem 2.1 is to construct some appropriate spaces and operators which satisfy (7) and (8). In the following two sections, we present the corresponding spaces and operators for two kinds of divergence conforming element spaces, respectively.
3 Preconditioner for finite element equations of first kind
We first introduce Sobolev functional space
H0(curl; Ω) = {u ∈ (L2(Ω))3|∇ × u ∈ (L2(Ω))3, ν × u = 0 on Γ}
with the norm
There exist two families of edge finite element spaces for the space H0(curl; Ω) (see [2, 4, 7]).
1. k order Nédélec element of first kind:
We also need to introduce the following space of piecewise k–degree discontinuous scalar elements on h:
The Sobolev spaces H0(div; Ω), H0(curl; Ω) and the corresponding finite element spaces possess the exceptional exact sequence properties (see [4, 7])
Assuming that u has the necessary smoothness, we can define two kinds of interpolants: and
, such that
u ∈
and
u ∈
(more details refer to [4, 7]). Especially, the interpolation
is not defined for a general function in H0(div; Ω). Here let us quote a slightly simplified version (see Theorem 5.25 of [7]).
Lemma 3.1. Suppose that there are constants δ > 0 such that u ∈ (H1/2+δ(K))3 for each K in h. Then
u is well-defined, and we have
with a constant only depending on the shape regularity of
h. The finite element spaces is equipped with bases
(k, 1) comprising locally supported functions. These bases are L2 stable in the sense that
with constant only depending on the shape-regularity of
h.Lemma 3.2. The interpolation operator is bounded on (
(Ω))3 and satisfies
with a constant only depending on the shape regularity of
h.Furthermore, all above operators possess the following commuting diagram property (see [7])
We may apply the quasi-interpolation operators for Lagrangian finite element space introduced in [8] to the components of vector fields separately. This gives rise to the projectors
h:(

and satisfies the local projection error esitmate
Now, we present the stable decomposition of , k > 2.
Lemma 3.3. For any ∈
, there exist
Span{b},
∈
, such that
and
where the constant
0 only depends on Ω and the shape regularity of
Proof. For any given ∈
, using the continuous Helmholtz decomposition, there exist Ψ∈ (
(Ω)) 3, p ∈ H0(curl; Ω) such that
and
with constants only depending on Ω.
Taking the div of both sides of (23) and using (14), we get
Owing to Lemma
Ψ is well defined. Furthermore, the commuting diagram property (18) implies
This confirms that the third term in the splitting
actually belongs to the kernel of div. By (12), then there esists q ∈ H0(curl; Ω) such that
Noting that
h Ψ∈

Substituting (25), (26) and (27) into (23), we have
Since ,
(Id –
h ) Ψ ,
hΨ ∈
, we obtain ∇ × (q + p) ∈
(div0) by using (28), then observing (13), there exists qh ∈
, such that
Let
It's easy to obtain ∈
by noting that
hΨ∈
⊂
and ∇ × qh ∈∇ ×
⊂
. Substituting (29), (30) and (31) into (28), we conclude
which completes the proof of (21).
Using (30), triangular inequality, Lemma 3.2, (20) and (24), we have
which leads to
It follows readily from inverse estimate and (16) that
Using inverse estimate again yields
By means of (33) and inverse estimate, we get
In view of (32), triangular inequality (34), (35) and (36), we have
which completes the proof of (22). □
We rely on the stable decomposition for V = in Lemma 3.3 and apply the abstract theory in Section 2 to define the preconditioner for finite element equations of first kind.
Let V = and choose two auxiliary spaces and the corresponding transfer operators as follows.
1.
=
, with inner product
1(·, ·) which is defined by
where
The transfer operator is Π1 = Id.
2.
2 =with inner product
2(·, ·) = a(·, ·) in the sense that
Making use of (6), the auxiliary space preconditioner for reads
where is the preconditioner of
,
1 is the preconditioners of
1.
Noting that
1 denotes the diagonal matrix of


where the constant
1 is independent of the mesh parameters.First, we prove that the above transfer operators satisfy the condition (7).
Due to the definitions of inner product and transfer operator in space
1, for any given

where the constant M bounds the number of basis functions whose support overlaps with a single element K.
For any given
2∈
Combining (39) with (40), we conclude that (7) holds with the constants c1 = M and c2 = 1.
Secondly, the above spaces and operators satisfy the condition (8) by using the Lemma 3.3.
Summing up, we obtain the following theorem by using Theorem 2.1.
Theorem 3.4. Forgiven by (37), and
1satisfies the condition of (38), then we have
with a constant only depending on the constants0,
1and the shape regularity of
h.
4 Preconditioner for finite element equations of second kind
Now, we present the another stable decomposition of with k > 2.
Lemma 4.1. For any ∈
, there are
∈
and φh ∈
such that
and
where the constant c0only depends on Ω and the shape regularity of h.
Proof. For any ∈
, we can interpolate
by Lemma 3.1. Thus, using (18), we have
In view of (14), we have
Making use of (45) and noting that = Id in (44), we get
namely
Noting that -
∈
, then by (46) and (13), there exists φh∈
, such that
where =
, which completes the proof of (42).
Using (47), (15) with δ = 1/2, and the inverse estimate, we obtain
Squaring and summing over all the elements, we get
In view of (3) and (48), we find
Making use of (47), triangular inequality and (48), we have
A direct manipulation of (47) gives that
A combination of (49), (50) and (51) concludes (43). □
In this case, let V = . We choose the following two auxiliary spaces and the corresponding transfer operator.
which concludes that
1 =. The corresponding transfer operator is Π1 = Id.
2.
2 =with inner product
The corresponding transfer operator is Π2 = curl.
Then by using (6), we obtain the auxiliary space preconditioner for as follows
where is the preconditioner of
, and
2 is the preconditioners of
2 given by (52).
Especially, we adopt the preconditioner
2 in [10], this choice satisfy
where the constant C1 is independent of the mesh parameters.
It is easy to prove that the above transfer operators satisfy the conditions (7). In fact, using the definitions of inner products and transfer operators in spaces
l (l = 1,2), we have
namely, the conditions (7) of Theorem 2.1 hold with the constants c1 = c2 = 1.
Applying Theorem 2.1 and using Lemma 4.1, we have the following Theorem.
Theorem 4.2. Forgiven by (53), and
2satisfies the condition of (54), then we have
with a constant only depending on the constants c0and C1and the shape regularity of h.
Combining Theorem 3.4 and Theorem 4.2, by using a Jacobi (or Gauss-Seidel) smoothing, we can translate the construction of preconditioner for into the one of
. Furthermore, by using the preconditioner of H(curl; Ω)-elliptic problem, we can translate the preconditioner for
into the one for
. Since Hiptmair and Xu [5] have constructed an efficient preconditioner
for
, we construct the efficient precondtioners for
(k = 1, l = 2 or k > 2, l = 1,2) and prove the corresponding spectral condition numbers are uniformly bounded and independent of mesh size h and the parameter τ by this recursive form.
5 Implementation of algorithm and numerical experiments
For simplicity, we only give the description of the preconditioning algorithm defined by (53) when k = 2.
Note that when k = 2, (53) turn to
In the following, we first discuss the description of algorithm about the preconditioner . For this purpose, we introduce the following operators
and
then, the algorithm about the operator can be described by (see [5] formore details)
Algorithm 5.1. For a given g ∈ , then ug =
g ∈
can be obtained as follows:
Step 1: Applying m 1 times symmetric Gauss_Seidel iterations in variational problem
with a zero initial guess to get1, where = g.
Step 2: Computing2∈
by
Step 3:Computing3∈
by
which can be obtained by
1. Applying m2times symmetric Gauss_Seidel iterations in (59) with a zero initial guess to get
4.
Step 4:Set ug = 1+(
)T

By [5], the preconditioner defined by Algorithm 5.1 satisfy
K () < C1
where the constant C1 is independent of the mesh size h and parameter τ.
Next, we give the description of algorithm for the operator curl
2curl*. Firstly, letn = dim (), dim (
),
and
then we introduce the transfer matrix(or operator)
By using , we can define the following matrix(or operator)
In view of (4.1) in [10], we can construct the preconditioner
2 for
where the constant C2 is independent of the mesh size h and parameter τ.
Noting that the operator
2 can be divided into three parts: the first part is to use the Jacobi (or Gauss-Seidel) smoothing for (52) in space




Summing up, we can obtain the following algorithm of the preconditioner .
Algorithm 5.2.For g ∈ , the solution ug =
g ∈
can be gotten as follows:
Step 1:Computing u1∈ by Algorithm 5.1.
Step 2:Applying m3times symmetric Gauss_Seidel iterations to get u2∈ V2,1by
( u2, v2) = (g, ∇ x v2), ∀ v2∈ V2,1.
Step 3: Set
ug = u1+ u2
For variational problem (4), we apply Algorithm 5.2 to the following two examples:
Example 5.1.The computational domain is Ω = [0,1] × [0, 1] × [0, 1] and the corresponding structured grids can be seen in Figure 1. For the convenience of computing the exact errors, we construct an exact solution u = (u1, u2, u3) as
Example 5.2. The computational domain is the spheres of radius 1 and the corresponding unstructured grids can be seen in Figure 2, the exact solution u = (u1, u2, u3) is
Now, we present some numerical experiments with m1 = m2 = m3 = 3.
Table 1 gives the L2 and H(div) error estimates for Example 5.1 when τ = 1, which shows that is the optimal convergence.
The condition number estimates and iteration counts for Example 5.1 and Example 5.2 are listed in Tables 2 - 5 for different values of the mesh size h and the scaling parameter τ. By these Tables, we find that the condition number and iteration counts are independent of the mesh size h and weakly dependent on the parameter τ.
Acknowledgements. The authors are partially supported by the National Natural Science Foundation of China (Grant No. 10771178), NSAF (Grant No. 10676031), the National Basic Research Program of China (973 Program) (Grant No. 2005CB321702). Especially, the first author is supported by Hunan Provincial Innovation Foundation For Postgraduate (Grant No. CX2009B121).
Received: 27/II/09.
Accepted: 30/VII/09.
#CAM-68/09.
References
- [1] D.N. Arnold, R.S. Falk and R Winther, Preconditioning in H(div) and applications, Math. Comp., 66 (1997), 957-984.
- [2] A. Bossavit, Computational Electromagnetism. Variational Formulation, Complementarity, Edge Elments San Diego: Academic Press (1998).
- [3] Z. Cai, R. Lazarov, T. Manteuffel and S. McCormick, First-order system least squares for second-order partial differential equations: Part I, SIAM J. Numer. Anal., 31 (1994), 1785-1799.
- [4] R. Hiptmair, Finite elements in computational electromagnetism, Acta Numer., 11 (2002), 237-339.
- [5] R. Hiptmair and J. Xu, Nodal auxiliary space preconditioning in H(curl) and H(div) spaces, SIAM J. Numer. Anal., 45 (2007), 2483-2509.
- [6] P. Lin, A sequential regularization method for time-dependent incompressible Navier-Stokes equations, SIAM J. Numer. Anal., 34 (1997), 1051-1071.
- [7] P. Monk, Finite Element Methods for Maxwell Equations Oxford University Press, Oxford (2003).
- [8] L. R. Scott and S. Zhang, Finite element interpolation of nonsmooth functions satisfying boundary conditions, Math. Comp., 54 (1990), 483-493.
- [9] J. Xu, Iterative methods by space decomposition and subspace correction, SIAM Review, 34 (1992), 581-613.
- [10] L. Zhong, S. Shu, D. Sun and L. Tan, Preconditioners for higher order edge finite element discretizations of Maxwell's equations, Sci. China Ser. A, 50 (2008), 1537-1548.