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Abstract and Applied Analysis

Volume 2013 (2013), Article ID 643819, 5 pages

http://dx.doi.org/10.1155/2013/643819

## Blow-Up in a Slow Diffusive -Laplace Equation with the Neumann Boundary Conditions

^{1}Department of Mathematics, Dalian Nationalities University, Dalian 116600, China^{2}School of Science, East China Institute of Technology, Nanchang 330013, China^{3}School of Science, Dalian Jiaotong University, Dalian 116028, China

Received 4 April 2013; Accepted 4 June 2013

Academic Editor: Daniel C. Biles

Copyright © 2013 Chengyuan Qu and Bo Liang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We study a slow diffusive -Laplace equation in a bounded domain with the Neumann boundary conditions. A natural energy is associated to the equation. It is shown that the solution blows up in finite time with the nonpositive initial energy, based on an energy technique. Furthermore, under some assumptions of initial data, we prove that the solutions with bounded initial energy also blow up.

#### 1. Introduction

In this paper, we consider a slow diffusive -Laplace equation: with , where is a bounded smooth domain , , , and , , and denote . It is easy to check that ; that is, the mass ofis conserved.

The problem (1) with can be used to model phenomena in population dynamics and biological sciences where the total mass of a chemical or an organism is conserved [1, 2]. If , the problem (1) is the degenerate parabolic equation and appears to be relevant in the theory of non-Newtonian fluids (see [3]). Here, we are mainly interested in the case , namely, the degenerate one. When , (1) becomes the heat equation which has been deeply studied in [4, 5]. When , (1) is singular, which can be handled similar to that of [6].

As an important feature of many evolutionary equations, the properties of blow-up solution have been the subject of intensive study during the last decades. Among those investigations in this area, it was Fujita [7] who first established the so-called theory of critical blow-up exponents for the heat equation with reaction sources in 1966, which can be, of course, regarded as the elegant description for either blow-up or global existence of solutions. From then on, there has been increasing interest in the study of critical Fujita exponents for different kinds of evolutionary equations; see [8, 9] for a survey of the literature. In recent years, special attention has been paid to the blow-up property to nonlinear degenerate or singular diffusion equations with different nonlinear sources, including the inner sources, boundary flux, or multiple sources; see, for example, [3, 10, 11].

In some situations, we have to deal with changing sign solutions. For instance, the changing sign solutions were considered in [1] for the nonlocal and quadratic equation with the Neumann boundary condition. The study in [5] for a natural generalization of (2), proposed with a global existence result (for small initial data) and a new blow-up criterion (based on the partial maximum principle and a Gamma-convergence argument). The authors also conjectured that the solutions blow up when , which was then proved with a positive answer [4]. The changing sign solutions to the reaction-diffusion equation were discussed in [2], with such as . The blow-up of solutions was obtained even under the source with . The semilinear parabolic equation [12] with a homogeneous Neumann’s boundary condition is studied. A blow-up result for the changing sign solution with positive initial energy is established. In [6], a fast diffusive -Laplace equation with the nonlocal source was considered. The authors showed that a critical blow-up criterion was determined for the changing sign weak solutions, depending on the size of and the sign of the natural energy associated. The relationship between the finite time blow-up and the nonpositivity of initial energy was discussed, based on an energy technique.

Notice that (1) is degenerate if at points where ; therefore, there is no classical solution in general. For this, a weak solution for problem (1) is defined as follows.

*Definition 1. *A function with is called a weak solution of (1) if
holds for all .

The local existence of the weak solutions can be obtained via the standard procedure of regularized approximations [10]. Throughout the paper, we always assume that the weak solution is appropriately smooth for convenience of arguments, instead of considering the corresponding regularized problems.

This paper is organized as follows. In Section 2, we show that the solutions to (1) blow up with nonpositive initial energy. In Section 3, under some assumptions of initial data, we prove that the solutions with bounded initial energy also blow up in finite time.

#### 2. Nonpositive Initial Energy Case

The technique used here is the same as in [4]; define the energy functional by and denote

Theorem 2. *Assume that , , and , , and let the initial energy
**
be nonpositive. Then, there exists with , such that
*

We need three lemmas for the functionals , , and , respectively.

Lemma 3. *The energy is a nonincreasing function and
*

*Proof. *A direct computation using (1) and by parts yields
Integrate from to to get (12).

Lemma 4. *Assume that , , and . Then, satisfies the following inequality:
*

*Proof. *An easy computation using (1) and the fact and by parts shows that
The last equality implies
because of (12) of Lemma 3 and the assumption .

Lemma 5. *Assume that , , and . Then, satisfies
*

*Proof. *Note the definition of and , and a simple calculation shows that
Furthermore,
Therefore,

*Proof of Theorem 2. *Assume for contradiction that the solution exists for all . We claim that
for any . Otherwise, there exists such that
and hence for a.e. . Therefore, noticing by Lemma 3, we have from (15) that
for a.e. . Using the Poincaré inequality with , we have for a.e. . This contradicts .

Integrating (14) from to , we have
which implies that
Thus, there exists such that for all
Thus, combining (17), we further have
for all . Now, we consider the function . Combining with the above inequality and a simple calculation shows that
for all . However, since
we also have
which is a contradiction.

#### 3. Bounded Initial Energy Case

Define with the norm . Let be the optimal constant of the embedding inequality where . Set

Theorem 6. *Assume that , . Let the initial data satisfying and . Then, there exists with , such that
*

First, we prove the following two Lemmas, similar to the idea in [13].

Lemma 7. *Assume that is a solution of the system (1). If and . Then, there exists a positive constant , such that
*

*Proof. *Let and by (32), we have
For convenience, we define
It is easy to find that increases if and decreases if . Moreover, as and . Due to , there exists such that . Let ; thus . Then by (37) and (38), we have , which implies that . For contradiction to establish (35), we assume that there exists such that
It follows from (37) and (38) that
which is in contradiction with Lemma 3. Hence, (35) is established.

Next to prove (36),
which implies that
Therefore, (36) is concluded.

Define Then, we have the following.

Lemma 8. *Assume that is a solution of the system (1). If and . Then for all ,
*

*Proof. *By Lemma 3, we know that . Thus,
According to (35) of Lemma 7, a simple computation shows that
which guarantees the conclusion of the lemma.

At the end, let us finish the proof of Theorem 6.

* Proof of Theorem 6. *According to (15), we have
By using (33) and (36), we obtain
Combining (47) and (48), we get
Since , blows up at a finite time. The proof of Theorem 6 is complete.

*Remark 9 (behavior of the energy). *Similar to Theorem 1.3 of [5], it is easy to be proved. Let , , and let be a weak solution of (1). If there exists a constant and a time , such that the solution exists on and satisfies on , then is bounded on . Thus, the above result and Theorem 6 reveal that even though the initial energy could be chosen as positive, the energy needs to become negative at a certain time and then goes to . Otherwise, has a lower bound on ; thus is bounded on . It is in contradiction with Theorem 6.

#### Acknowledgments

This study was supported by the National Natural Science Foundation of China (Grants nos. 11226179, 11201045) and the Doctor Startup Fund of Dalian Nationalities University (Grant no. 0701-110030).

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