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# The Boundary Value Problem of the Equations with Nonnegative Characteristic Form

*Boundary Value Problems*
**volume 2010**, Article number: 208085 (2010)

## Abstract

We study the generalized Keldys-Fichera boundary value problem for a class of higher order equations with nonnegative characteristic. By using the acute angle principle and the Hölder inequalities and Young inequalities we discuss the existence of the weak solution. Then by using the inverse Hölder inequalities, we obtain the regularity of the weak solution in the anisotropic Sobolev space.

## 1. Introduction

Keldys [1] studies the boundary problem for linear elliptic equations with degenerationg on the boundary. For the linear elliptic equations with nonnegative characteristic forms, Oleinik and Radkevich [2] had discussed the Keldys-Fichera boundary value problem. In 1989, Ma and Yu [3] studied the existence of weak solution for the Keldys-Fichera boundary value of the nonlinear degenerate elliptic equations of second-order. Chen [4] and Chen and Xuan [5], Li [6], and Wang [7] had investigated the existence and the regularity of degenerate elliptic equations by using different methods. In this paper, we study the generalized Keldys-Fichera boundary value problem which is a kind of new boundary conditions for a class of higher-order equations with nonnegative characteristic form. We discuss the existence and uniqueness of weak solution by using the acute angle principle, then study the regularity of solution by using inverse Hölder inequalities in the anisotropic Sobolev Space.

We firstly study the following linear partial differential operator

where is an open set, the coefficients of are bounded measurable, and the leading term coefficients satisfy

We investigate the generalized Keldys-Fichera boundary value conditions as follows:

with and , where .

The leading term coefficients are symmetric, that is, which can be made into a symmetric matrix . The odd order term coefficients can be made into a matrix is the outward normal at . and are the eigenvalues of matrices and , respectively. and are orthogonal matrix satisfying

The boundary sets are

At last, we study the existence and regularity of the following quasilinear differential operator with boundary conditions (1.3)–(1.5):

where and

## 2. Formulation of the Boundary Value Problem

For second-order equations with nonnegative characteristic form, Keldys [1] and Fichera presented a kind of boundary that is the Keldys-Fichera boundary value problem, with that the associated problem is of well-posedness. However, for higher-order ones, the discussion of well-posed boundary value problem has not been seen. Here we will give a kind of boundary value condition, which is consistent with Dirichlet problem if the equations are elliptic, and coincident with Keldys-Fichera boundary value problem when the equations are of second-order.

We consider the linear partial differential operator

where is an open set, the coefficients of are bounded measurable functions, and

Let be a series of functions with . If in certain order we put all multiple indexes with into a row , then can be made into a symmetric matrix . By this rule, we get a symmetric leading term matrix of (2.1), as follows:

Suppose that the matrix is semipositive, that is,

and the odd order part of (2.1) can be written as

where is the Kronecker symbol. Assume that for all , we have

We introduce another symmetric matrix

where is the outward normal at . Let the following matrices be orthogonal:

satisfying

where is the transposed matrix of are the eigenvalues of and are the eigenvalues of . Denote by

For multiple indices means that . Now let us consider the following boundary value problem,

for all and , where .

We can see that the item (2.13) of boundary value condition is determined by the leading term matrix (2.2), and (2.12) is defined by the odd term matrix (2.6). Moreover, if the operator is a not elliptic, then the operator

has to be elliptic.

In order to illustrate the boundary value conditions (2.11)–(2.13), in the following we give an example.

Example 2.1.

Given the differential equation

Here . Let and , then the leading and odd term matrices of (2.15) respectively are

and the orthogonal matrices are

We can see that , and as shown in Figure 1.The item (2.12) is

and the item (2.13) is

for all and . Since only , hence we have

however, and , therefore,

Thus the associated boundary value condition of (2.15) is as follows:

which implies that is free on

Remark 2.2.

In general the matrices and arranged are not unique, hence the boundary value conditions relating to the operator may not be unique.

Remark 2.3.

When all leading terms of are zero, (2.10) is an odd order one. In this case, only (2.11) and (2.12) remain.

Now we return to discuss the relations between the conditions (2.11)–(2.13) with Dirichlet and Keldys-Fichera boundary value conditions.

It is easy to verify that the problem (2.10)–(2.13) is the Dirichlet problem provided the operator being elliptic (see [11]). In this case, for all . Besides, (2.13) run over all and , moreover is nondegenerate for any . Solving the system of equations, we get .

When , namely, is of second-order, the condition (2.12) is the form

and (2.13) is

Noticing

thus the condition (2.13) is the form

It shows that when , (2.12) and (2.13) are coincide with Keldys-Fichera boundary value condition.

Next, we will give the definition of weak solutions of (2.10)–(2.13) (see [12]). Let

where is defined by

We denote by the completion of under the norm and by the completion of with the following norm

Definition 2.4.

is a weak solution of (2.10)–(2.13) if for any , the following equality holds:

We need to check the reasonableness of the boundary value problem (2.10)–(2.13) under the definition of weak solutions, that is, the solution in the classical sense are necessarily the solutions in weak sense, and conversely when a weak solution satisfies certain regularity conditions, it will surely satisfy the given boundary value conditions. Here, we assume that all coefficients of are sufficiently smooth.

Let be a classical solution of (2.10)–(2.13). Denote by

Thanks to integration by part, we have

Since , we have

Because satisfies (2.12),

From the three equalities above we obtain (2.30).

Let be a weak solution of (2.10)–(2.13). Then the boundary value conditions (2.11) and (2.13) can be reflected by the space . In fact, we can show that if , then satisfies

Evidently, when , we have

If we can verify that for any , (2.36) holds true, then we get

which means that (2.35) holds true. Since is dense in , for given, let and in . Then

Due to satisfying (2.36), hence satisfies (2.36). Thus (2.31) is verified.

Remark 2.5.

When (2.2) is a diagonal matrix, then (2.13) is the form

where In this case, the corresponding trace embedding theorem can be set, and the boundary value condition (2.13) is naturally satisfied. On the other hand, if the weak solution of (2.10)–(2.13) belongs to for some , then by the trace embedding theorems, the condition (2.13) also holds true.

It remains to verify the condition (2.12). Let satisfy (2.30). Since , hence we have

On the other hand, by (2.30), for any , we get

Because the coefficients of are sufficiently smooth, and is dense in , equality (2.41) also holds for any . Therefore, due to , we have

From (2.36), one drives

Furthermore,

From (2.30) and (2.42), one can see that

Noticing in , one deduces that satisfies (2.12) provided Finally, we discuss the well-posedness of the boundary value problem (2.10)–(2.13).

Let be a linear space, and be the completion of , respectively, with the norm . Suppose that is a reflexive Banach space and is a separable Banach space.

Definition 2.6.

A mapping is called to be weakly continuous, if for any in , one has

Lemma 2.7 (see [3]).

Suppose that is a weakly continuous, if there exists a bounded open set , such that

then the equation has a solution in .

Theorem 2.8 (existence theorem).

Let be an arbitrary open set, and . If there exist a constant and such that

where is the component of corresponding to , then the problem (2.10)–(2.13) has a weak solution in .

Proof.

Let be the inner product as in (2.31). It is easy to verify that defines a bounded linear operator . Hence is weakly continuous (see [3]). From (2.42), for we drive that

Hence we obtain

Thus by Hölder inequality (see [13]), we have

By Lemma 2.7, the theorem is proven.

Theorem 2.9 (uniqueness theorem).

Under the assumptions of Theorem 2.8 with in (2.48). If the problem (2.10)–(2.13) has a weak solution in , then such a solution is unique. Moreover, if in , , then the weak solution of (2.10)–(2.13) is unique.

Proof.

Let be a weak solution of (2.10)–(2.13). We can see that (2.30) holds for all . Hence is well defined. Let . Then from (2.49) it follows that , we obtain , which means that the solution of (2.10)–(2.13) in is unique. If all the odd terms of , then (2.30) holds for all , in the same fashion we known that the weak solution of (2.10)–(2.13) in is unique. The proof is complete.

Remark 2.10.

In next subsection, we can see that under certain assumptions, the weak solutions of degenerate elliptic equations are in .

## 3. Existence of Higher-Order Quasilinear Equations

Given the quasilinear differential operator

where and

Let , the odd order part of (3.1) be as that in (2.4), , and be the same as those in Section 2. The leading matrix is

and the eigenvalues are . We denote

We consider the following problem:

Denote the anisotropic Sobolev space by

whose norm is

when all for , then the space is denoted by . is termed the critical embedding exponent from to , if is the largest number of the exponent in where , and the embedding is continuous.

For example, when is bounded, the space with norm is an anisotropic Sobolev space, and the critical embedding exponents from to are .

Suppose that the following hold.

The coefficients of the leading term of satisfy one of the following two conditions:

()

()

There is a constant such that

There are functions with , such that

There is a constant such that

where or .

There is a constant such that

where is a critical embedding exponent from to . Let be defined by (2.27) and be the completion of under the norm

and be the completion of with the norm

where

is a weak solution of (3.3), if for any , we have

Theorem 3.1.

Under the conditions , if , then the problem (3.3) has a weak solution in .

Proof.

Denote by the left part of (3.12). It is easy to verify that the inner product defines a bounded mapping by the condition .

Let , by , one can deduce that

Noticing that , , , by Hölder and Young inequalities (see[13]), from (3.13) we can get

Ones can easily show that the mapping is weakly continuous. Here we omit the details of the proof. By Lemma 2.7, this theorem is proven.

In the following, we take an example to illustrate the application of Theorem 3.1.

Example 3.2.

We consider the boundary value problem of odd order equation as follows:

where is an unit ball in , see Figure 2

The odd term matrix is

It is easy to see that

The boundary value condition associated with (3.15) is

Applying Theorem 3.1, if , then the problem (3.15)–(3.18) has a weak solution .

## 4. -Solutions of Degenerate Elliptic Equations

We start with an abstract regularity result which is useful for the existence problem of -solutions of degenerate quasilinear elliptic equations of order . Let be the spaces defined in Definition 2.6, and be a reflective Banach space, at the same time .

Lemma 4.1.

Under the hypotheses of Lemma 2.7, there exists a sequence of in such that . Furthermore, if, we can derive that , is a constant, then the solution of belongs to .

In the following, we give some existence theorems of -solutions for the boundary value conditions (4.3)–(4.5) of higher-order degenerate elliptic equations.

First, we consider the quasilinear equations

where . Now, we consider the following problem

The boundary value condition associated with (4.1) is given by (4.3)–(4.5). Suppose that is bounded, and the following assumptions hold.

The condition (3.6) holds, and there is a continuous function on such that

where ,

is a measure zero set in , and there is a sequence of subdomains with cone property such that and

The positive definite condition is

where is a constant, or for .

The structure conditions are

where is a constant, , is the critical embedding exponent from to .

Let be defined by (2.27) and be the completion of with the norm

Definition 4.2.

is a weak solution of (4.2)–(4.5), if for any , the following equality holds:

Theorem 4.3.

Under the assumptions , if , then the problem and (4.2)–(4.5) has a weak solution . Moreover, if there is a real number , such that

then the weak solution

Proof.

According to Lemma 4.1, it suffices to prove that there is a constant such that for any ( is as that in Section 3) with , we have

From (4.10) we know

Due to and we have

Noticing that , and consequently we have

where the or , is the critical embedding exponent from to . By the reversed Hölder inequality (see [14])

Then we obtain

From (4.15) and (4.17), the estimates (4.12) follows. This completes the proof.

Next, we consider a quasilinear equation

where

Suppose that the following holds.

There is a real number such that

The structural conditions are

where C is a constant, , are the critical embedding exponents from to .

Theorem 4.4.

Let the conditions and be satisfied. If , then the problem (4.2)–(4.5) has a weak solution .

The proof of Theorem 4.4 is parallel to that of Theorem 4.3; here we omit the detail.

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## Acknowledgment

This project was supported by the National Natural Science Foundation of China (no. 10971148).

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### Keywords

- Weak Solution
- Quasilinear Equation
- Young Inequality
- Linear Partial Differential Operator
- Multiple Index