A Two-Level Additive Schwarz Preconditioning Algorithm for the Weak Galerkin Method for the Second-Order Elliptic Equation

This paper proposes a two-level additive Schwarz preconditioning algorithm for the weak Galerkin approximation of the secondorder elliptic equation. In the algorithm, a P 1 conforming finite element space is defined on the coarse mesh, and a stable intergrid transfer operator is proposed to exchange the information between the spaces on the coarse mesh and the fine mesh. With the framework of the Schwarz method, it is proved that the condition number of the preconditioned system only depends on the rate of the coarse mesh size and the overlapping size. Some numerical experiments are carried out to verify the theoretical results.


Introduction
The weak Galerkin (WG) finite element method (FEM) is a new efficient numerical method for solving partial differential equations.It was first introduced for the second-order elliptic problem in [1].The central idea of the WG method is using weak derivatives in place of strong derivatives that define the weak formulation for the underlying partial differential equations.Due to the weak derivative, the degrees of freedom can be discontinuous from the element interior to the element boundary, which makes the weak Galerkin method have more choices of the finite elements than the standard FEMs.The WG FEM also has some other properties, such as high-order accuracy and multiphysics capability.The idea of the weak derivative has been generalized to some other problems, such as the biharmonic equation [2] and the twophase subsurface flow problems [3].Chen et al. introduced a posteriori error estimates for the WG method for the secondorder elliptic problems in [4].
Although many works focus on developing WG methods for different problems, there are few works on efficiently solving the discretized systems, of which the condition number is (ℎ −2 ), where ℎ denotes the mesh size.Recently, Li and Xie [5] proposed a multigrid algorithm for the WG method using  1 conforming element on the coarser meshes.In [6], Chen et al. also constructed a fast auxiliary space multigrid preconditioner for the WG method, as well as a corresponding reduced system involving only the degrees of freedom on edges/faces.It is proved that the condition number of the preconditioned system is independent of the mesh size.
Due to its high parallelizability and scalability, the domain decomposition method is one of the most efficient algorithms for the numerical solutions of partial differential equations (see, e.g., [7]).In this paper, we study an overlapping domain decomposition algorithm and propose a two-level additive Schwarz preconditioner for the WG method for the secondorder elliptic equation.The preconditioner is consisted of overlapping local subproblems on a fine mesh and a global subproblem on a coarse mesh.The coarse subproblem is defined by the  1 conforming element method, and an intergrid transfer operator, which has stability and approximability, is introduced to transmit information between the coarse mesh and the fine mesh.Under the Schwarz framework, we prove that the condition number of the preconditioned system is independent of the fine mesh size ℎ and is of ((1 + /) 2 ), where  denotes the coarse mesh size and  stands for the overlapping size.
The rest of this paper is organized as follows.In Section 2, we recall the WG method and give some notations.In Section 3, we propose an intergrid transfer operator and a two-level additive Schwarz preconditioner.Moreover, we prove the stability and approximation of the intergrid transfer operator.Then we estimate the upper bound of the maximum eigenvalue and the lower bound of minimum eigenvalue, respectively.Section 4 is devoted to numerical experiments, which are carried out to confirm our theoretical results.

The Weak Galerkin Method for the Second-Order Elliptic Equation
For the sake of simplicity, we consider the model problem as follows: where Ω ⊂ R 2 is bounded polygonal domain and  ∈  2 (Ω).
Assume that the matrix A is symmetric positive definite (SPD); namely, there exist positive constants ,  such that The variational formulations of ( 1) is to find  ∈  1 0 (Ω) such that where (, V) = (A∇, ∇V).It follows from Lax-Milgram's theorem that problem (3) has a unique solution.
Let T ℎ be a quasiuniform triangulation of the domain Ω with the mesh size ℎ.On each  ∈ T ℎ , the space () denotes the collections of weak functions, each of which is consisted of an interior part and a boundary part; that is, For any weak function V ∈ (), the weak gradient of V, denoted by ∇  V ∈ (div, ), satisfies where n is the unit outer normal of .It is easy to see that ∇  is the classical gradient if it actions on a function  ∈  1 ().
For each element  ∈ T ℎ , we denote by  0 and  the interior and the boundary of .The notation F  stands for the set of edges of .We also use F ℎ and F  ℎ to denote the edges of T ℎ in Ω and on Ω.Then the discrete WG space is defined as where  =  or  =  + 1 and   ( 0 ) denotes the set of polynomials on  0 with degree no more than  ≥ 0. To define a discrete weak gradient, we use   () to denote the RT element space [  ()] 2 + P () if  =  and the BDM element space [ +1 ()] 2 if  =  + 1, where P () is the homogeneous polynomial with the degree .
For each V ∈  ℎ , the discrete weak gradient ∇  V ∈   () is defined as where V 0 and V  denote the values of V in the interior and on the boundary of .Then the discrete problem for (3) is to seek where to indicate a special inner product defined as where (⋅, ⋅)  and (⋅, ⋅)  denote the  2 inner product on  and .Accordingly, a norm and seminorm are introduced for any where It can be proved that  ℎ is symmetric and positive definite and the condition number is of (ℎ −2 ) (see, e.g., [8]), which brings difficulty to solve the discrete problem when the mesh size is small.In the next section, we will present a preconditioner to overcome this difficulty.On each subdomain Ω  , the notation T ℎ, stands for the triangulation inherited from T ℎ .The corresponding weak Galerkin subspace on T ℎ, is defined as

A Two-Level Additive Schwarz
We introduce an operator   :   →   by It is easy to see that   is symmetric and positive definite.
Since   is a subset of  ℎ , we use   to denote a natural injection from   to  ℎ .
To define a coarse subproblem, we define a coarse triangulation T  with mesh size  such that each element in T ℎ is a subdivision of the one in T  .The notation   ⊂  1 0 (Ω) stands for  1 conforming finite element space associated with T  .We also introduce an operator   :   →   satisfying Note that the coarse space   is a subspace of  ℎ if the degree of the piecewise polynomials in  ℎ is greater than 0, and we can choose a natural injection as the intergird transfer operator   from   to  ℎ .For the piecewise constant space case, the intergrid transfer operator   :   →  ℎ is defined as where  0  and    satisfy for any  ∈ T ℎ and  ∈ F ℎ .We also need the transpose of the intergrid transfer operators    from  ℎ to the subspace   for  = , 1, 2, . . ., , defined as Then our two-level additive Schwarz preconditioner  ℎ is stated as 3.2.Analysis.Our analysis is based on the standard Schwarz framework (see, e.g., [7,9]).
Lemma 1 ([9, Theorem 7.1.20]).The eigenvalues of  ℎ  ℎ are positive, and one has the following characterizations of the maximum and minimum eigenvalues: where the sum is taken over  = , 1, . . ., .
Lemma 2. For the intergrid transfer operator   , it holds for any Proof.We only need consider the case that the functions in  ℎ are the constants in the interior and on the boundary of each element.It follows from the triangle inequality, the trace inequality, and the Poincaré-Friedrichs inequality that Similarly, for the lower-order term, we have which completes the proof.

Lemma 3.
There exists an operator   from  ℎ to   such that for any V ℎ in  ℎ it holds that Proof.Let  ℎ be the  1 conforming finite element space defined on T ℎ .Then, for any V ℎ in  ℎ , we can construct a Ṽℎ ∈  ℎ satisfying ([6, Lemma 3.5]) Define   V ℎ =   Ṽℎ , where   is the  2 projection operator from  ℎ to   .Using the standard properties of   (see, e.g., [7]) and the inequality (24), we have The proof is completed.
Lemma 4. Given any V ℎ ∈  ℎ , there exists a decomposition where V  ∈   and V  ∈   , such that , where  0 ℎ and   ℎ are the piecewise  2 projection to the interior polynomial space   ( 0 ) and the edge polynomial space   (), respectively.It is easy to check that On the other hand, we have for V  that By the inverse inequality, the stability of  2 projection, the Poincaré inequality, and the scaling argument, we deduce that which, together with the triangle inequality and the assumption of   , yields For the last term, the triangle inequality gives We estimate I, II, and III, respectively, as follows.
From the trace theorem, the scaling argument, and the stability and approximation of  0 ℎ , we have Combining the above five inequalities, we find which leads to Using ( 27) and (38), we achieve This ends the proof.
Lemma 5. Let V  ∈   and V  ∈   , 1 ≤  ≤ .For any Proof.The inequality can be obtained directly by using the triangle inequality and the assumption on the finite cover of subdomains.
As an immediate consequence, we have the following theorem.Theorem 6.There exists a positive constant , independent of , ℎ, , and , such that Remark 7. According the theorem, the two-level additive preconditioner is optimal if / is bounded above by a constant.In particular, when  = (), we have

Numerical Experiments
In this section, we give some numerical results to demonstrate the efficiency of our preconditioner.For convenience, we consider a simple two-dimensional Poisson equation with homogeneous boundary as follows: −Δ =  in Ω,  = 0 on Ω.
In Table 1, we list the iteration numbers of the preconditioning conjugate gradient (PCG) method and the conjugate gradient (CG) method with different meshes.From the second and third rows of the table, we see that the iteration numbers of the PCG method are almost the same if the overlapping factors / are fixed.If the overlap becomes small, that is, the rate / is increasing, one needs a little more steps to achieve the tolerance.This indicates that the condition number of the preconditioned system is almost a constant which is independent of the mesh size ℎ and only depends on the rate of the coarse mesh size and the overlap.
Finally, we will do some experiments to show the efficiency of our algorithm for problems with checkboard distributed coefficient, which equals 1 and 10  ( = 1, 3, 6) on adjacent subdomains.We assume that the coefficient is constant on each coarser element, and the coarser mesh size and the overlap size are fixed ( = 1/4,  = 1/8).The iteration numbers, with different mesh sizes ℎ and jumps in the coefficient, are reported in Table 2, from which, we conclude that our preconditioner also works well for problems with discontinuous coefficients at least in two dimensions.
Preconditioner.To introduce our preconditioner, we first divide the domain Ω by  overlapping subdomains Ω 1 , Ω 2 , . . ., Ω  such that each point in Ω belongs to no more than   subdomains.We assume that the boundary of each subdomain does not cut through any elements in the triangulation T ℎ , and there exist nonnegative 0 ℎ (   0 ) −    0      Denote    0 = (1/||) ∫     0 .The trace theorem, the  2 stability of   ℎ , and the Poincaré inequality imply that II =         0 −   ℎ (   0 )

Table 1 :
The iteration numbers of the PCG and CG.

Table 2 :
The iteration numbers with different mesh sizes ℎ and coefficient jumps (10  ).