# Symmetry of solutions to parabolic Monge-Ampère equations

Limei Dai

Author Affiliations

School of Mathematics and Information Science, Weifang University, Weifang, Shandong, 261061, China

Boundary Value Problems 2013, 2013:185  doi:10.1186/1687-2770-2013-185

 Received: 3 April 2013 Accepted: 5 August 2013 Published: 20 August 2013

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

In this paper, we study the parabolic Monge-Ampère equation

Using the method of moving planes, we show that any parabolically convex solution is symmetric with respect to some hyperplane. We also give a counterexample in and an example in a cylinder to illustrate the results.

MSC: 35K96, 35B06.

##### Keywords:
parabolic Monge-Ampère equations; symmetry; method of moving planes

### 1 Introduction

The Monge-Ampère equation has been of much importance in geometry, optics, stochastic theory, mass transfer problem, mathematical economics and mathematical finance theory. In optics, the reflector antenna system satisfies a partial differential equation of Monge-Ampère type. In [1,2], Wang showed that the reflector antenna design problem was equivalent to an optimal transfer problem. An optimal transportation problem can be interpreted as providing a weak or generalized solution to the Monge-Ampère mapping problem [3]. More applications of the Monge-Ampère equation and the optimal transportation can be found in [3,4]. In the meantime, the Monge-Ampère equation turned out to be the prototype for a class of questions arising in differential geometry.

For the study of elliptic Monge-Ampère equations, we can refer to the classical papers [5-7] and the study of parabolic Monge-Ampère equations; see the references [8-11]etc. The parabolic Monge-Ampère equation was first introduced by Krylov [12] together with the other parabolic versions of elliptic Monge-Ampère equations; see [8] for a complete description and related results. It is also relevant in the study of deformation of surfaces by Gauss-Kronecker curvature [13,14] and in a maximum principle for parabolic equations [15]. Tso [15] pointed out that the parabolic equation is the most appropriate parabolic version of the elliptic Monge-Ampère equation in the proof of Aleksandrov-Bakelman maximum principle of second-order parabolic equations. In this paper, we study the symmetry of solutions to the parabolic Monge-Ampère equation

(1.1)

(1.2)

(1.3)

where is the Hessian matrix of u in x, , Ω is a bounded and convex open subset in , denotes the side of Q, denotes the bottom of Q, and denotes the parabolic boundary of Q, f and are given functions.

There is vast literature on symmetry and monotonicity of positive solutions of elliptic equations. In 1979, Gidas et al.[16] first studied the symmetry of elliptic equations, and they proved that if or Ω is a smooth bounded domain in , convex in and symmetric with respect to the hyperplane , then any positive solution of the Dirichlet problem

satisfies the following symmetry and monotonicity properties:

(1.4)

(1.5)

The basic technique they applied is the method of moving planes first introduced by Alexandrov [17] and then developed by Serrin [18]. Later the symmetry results of elliptic equations have been generalized and extended by many authors. Especially, Li [19] considered fully nonlinear elliptic equations on smooth domains, and Berestycki and Nirenberg [20] found a way to deal with general equations with nonsmooth domains using the maximum principles on domains with small measure. Recently, Zhang and Wang [21] investigated the symmetry of the elliptic Monge-Ampère equation and they got the following results.

Let Ω be a bounded convex domain in with smooth boundary and symmetric with respect to the hyperplane , then each solution of the Dirichlet problem

has the above symmetry and monotonicity properties (1.4) and (1.5). Extensions in various directions including degenerate problems [22] or elliptic systems of equations [23] were studied by many authors.

For the symmetry results of parabolic equations on bounded and unbounded domains, the reader can be referred to [16,24,25] and the references therein. In particular, when , , Gidas et al.[16] studied parabolic equations and , and they proved that parabolic equations possessed the same symmetry as the above elliptic equations. When , Hess and Poláčik [25] first studied the asymptotic symmetry results for classical, bounded, positive solutions of the problem

(1.6)

(1.7)

The symmetry of general positive solutions of parabolic equations was investigated in [24,26,27] and the references therein. A typical theorem of is as follows.

Let Ω be convex and symmetric in . If u is a bounded positive solution of (1.6) and (1.7) with satisfying

then u has the symmetry and monotonicity properties for each :

The result of is as follows.

Assume that u is a bounded positive solution of (1.6) and (1.7) with such that for some sequence ,

Then u is asymptotically symmetric in the sense that

In this paper, using the method of moving planes, we obtain the same symmetry of solutions to problem (1.1), (1.2) and (1.3) as elliptic equations.

### 2 Maximum principles

In this section, we prove some maximum principles. Let Ω be a bounded domain in , let , , be continuous functions in . Suppose that , is bounded and there exist positive constants and such that

Here and in the sequel, we always denote

We use the standard notation to denote the class of functions u such that the derivatives are continuous in Q for .

Theorem 2.1Letbe a bounded continuous function on, and let the positive functionsatisfy

(2.1)

Suppose thatsatisfies

(2.2)

(2.3)

If

(2.4)

theninQ.

Proof We argue by contradiction. Suppose there exists such that . Let

Then . Set , then and . Since attains its minimum at , we have , . In addition, we have . A direct calculation gives

Taking into account , we have at ,

This is a contradiction and thus completes the proof of Theorem 2.1. □

Theorem 2.1 is also valid in unbounded domains if u is nonnegative at infinity. Thus we have the following corollary.

Corollary 2.2Suppose that Ω is unbounded, . Besides the conditions of Theorem 2.1, we assume

(2.5)

TheninQ.

Proof Still consider in the proof of Theorem 2.1. Condition (2.5) shows that the minimum of cannot be achieved at infinity. The rest of the proof is the same as the proof of Theorem 2.1. □

If Ω is a narrow region with width l,

then we have the following narrow region principle.

Corollary 2.3 (Narrow region principle)

Suppose thatsatisfies (2.2) and (2.3). Let the widthlof Ω be sufficiently small. If on, , then we havein Q. If Ω is unbounded, and, then the conclusion is also true.

Proof Let ,

Then φ is positive and

Choose . In virtue of the boundedness of , when l is sufficiently small, we have , and thus

From Theorem 2.1, we have . □

### 3 Main results

In this section, we prove that the solutions of (1.1), (1.2) and (1.3) are symmetric by the method of moving planes.

Definition 3.1 A function is called parabolically convex if it is continuous, convex in x and decreasing in t.

Suppose that the following conditions hold.

(A) is bounded in .

(B) and

(3.1)

where , ().

Theorem 3.1Let Ω be a strictly convex domain inand symmetric with respect to the plane, . Assume that conditions (A) and (B) hold andis any parabolically convex solution of (1.1), (1.2) and (1.3). Then, where, and when, .

Proof Let in , , that is,

Then

where . Therefore,

(3.2)

We rewrite (3.2) in the form

(3.3)

On the other hand, from (1.1), we have

(3.4)

According to (3.3) and (3.4), we have

Therefore

As a result, we have

(3.5)

where

is the inverse matrix of . Then , is bounded and by the a priori estimate [9] we know there exist positive constants and such that

Let

then from (3.5),

(3.6)

Clearly,

(3.7)

Because the image of about the plane lies in Ω, according to the maximum principle of parabolic Monge-Ampère equations,

Thus

(3.8)

On the other hand, from (3.1),

(3.9)

From Corollary 2.3, when the width of is sufficiently small, , .

Now we start to move the plane to its right limit. Define

We claim that

Otherwise, we will show that the plane can be further moved to the right by a small distance, and this would contradict with the definition of Λ.

In fact, if , then the image of under the reflection about lies inside Ω. According to the strong maximum principle of parabolic Monge-Ampère equations, for , . Therefore, for , we have . On the other hand, by the definition of Λ, we have for , . So, from the strong maximum principle [28] of linear parabolic equations and (3.6), we have for ,

(3.10)

Let be the maximum width of narrow regions so that we can apply the narrow region principle. Choose a small positive constant δ such that , . We consider the function on the narrow region

Then satisfies

(3.11)

Now we prove the boundary condition

(3.12)

Similar to boundary conditions (3.7), (3.8) and (3.9), boundary condition (3.12) is satisfied for , and for . In order to prove (3.12) is satisfied for , we apply the continuity argument. By (3.10) and the fact that is inside , there exists a positive constant such that

Because is continuous in λ, then for small δ, we still have

Therefore boundary condition (3.12) holds for small δ. From Corollary 2.3, we have

(3.13)

Combining (3.10) and the fact that is continuous for λ, we know that for when δ is small. Then from (3.13), we know that

This contradicts with the definition of Λ, and so .

As a result, for , which means that as ,

Since Ω is symmetric about the plane , then for , also satisfies (1.1). Thus we can move the plane from the right towards the left and get the reverse inequality. Therefore

(3.14)

Equation (3.14) means that u is symmetric about the plane . Theorem 3.1 is proved. □

If we put the axis in any direction, from Theorem 3.1, we have the following.

Corollary 3.2If Ω is a ball, , then any parabolically convex solutionof (1.1), (1.2) and (1.3) is radially symmetric about the origin.

Remark 3.1 Solutions of (1.1) in may not be radially symmetric. For example,

(3.15)

has a non-radially symmetric solution. In fact, we know that () satisfies in , and , . Define

then u is a solution of (3.15) but not radially symmetric.

We conclude this paper with a brief examination of Theorem 3.1. Let be the unit ball in , and let radially symmetric function , satisfy

(3.16)

(3.17)

Example 3.1 Let satisfy (3.16) and (3.17). Then any solution of

(3.18)

(3.19)

(3.20)

is of the form

(3.21)

where .

Proof According to Corollary 3.2, the solution is symmetric. Let

Then

Therefore (3.18) is

(3.22)

We seek the solution of the form

Then

That is,

(3.23)

Therefore

(3.24)

By (3.20), we know that

(3.25)

From (3.24) and (3.25), we have

As a result,

From the maximum principle, we know that the solution of (3.18)-(3.20) is unique. Thus any solution of (3.18), (3.19) and (3.20) is of the form of (3.21). □

### Competing interests

The author declares that they have no competing interests.

### Acknowledgements

The research was supported by NNSFC (11201343), Shandong Province Young and Middle-Aged Scientists Research Awards Fund (BS2011SF025), Shandong Province Science and Technology Development Project (2011YD16002).

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