Blow-Up Phenomena for a Class of Parabolic Systems with Time Dependent Coefficients ()

Lawrence E. Payne, Gérard A. Philippin

Département de Mathématiques et de Statistique, Université Laval, Québec City, Canada.

Department of Mathematics, Cornell University, Ithaca, USA.

**DOI: **10.4236/am.2012.34049
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Département de Mathématiques et de Statistique, Université Laval, Québec City, Canada.

Department of Mathematics, Cornell University, Ithaca, USA.

Blow-up phenomena for solutions of some nonlinear parabolic systems with time dependent coefficients are investigated. Both lower and upper bounds for the blow-up time are derived when blow-up occurs.

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Payne, L. and Philippin, G. (2012) Blow-Up Phenomena for a Class of Parabolic Systems with Time Dependent Coefficients. *Applied Mathematics*, **3**, 325-330. doi: 10.4236/am.2012.34049.

1. Introduction

It is well known that the solutions of parabolic problems may remain bounded for all time, or may blow-up in finite or infinite time. When blow-up occurs at time, the evaluation of is of great practical interest.

In a recent paper [1] Payne and Schaefer have investigated the blow-up phenomena of solutions in some parabolic systems of equations under homogeneous Dirichlet boundary conditions. The contribution of this note is to extend their investigations to a class of parabolic systems with time dependent coefficients. The case of a single parabolic equation was investigated recently in [2].

There is an abounding literature dealing with blow-up phenomena of solutions to parabolic partial differential equations. We refer the interested readers to [3-5]. A variety of physical, chemical, biological applications are discussed in [5,6]. Further references to the field are [1,7-19]. In this note we investigate the blow-up phenomena of the solution of the following parabolic system

(1.1)

where is a bounded domain in. The initial data as well as the data are assumed nonnegative, so that the solution of (1.1) will be nonnegative by the maximum principle. More specific assumptions on the data will be made later.

In Section 2 we derive conditions on the data of problem (1.1) sufficient to guarantee that blow-up will occur, and derive under these conditions some upper bound for. In Section 3 we derive some lower bounds for the blow-up time when blow-up occurs. However this section is limited to the case of in and in respectively, because our technique makes use of some Sobolev type inequalities available in and in only. For convenience we include the proof of one of these inequalities in Section 4.

2. Conditions for Blow-Up in Finite Time t^{*}

Let be the first eigenvalue and be the associated eigenfunction of the Dirichlet-Laplace operator defined as

(2.1)

(2.2)

Let the auxiliary function be defined in as

(2.3)

with

(2.4)

where is the solution of problem (1.1). We assume in this section that is a bounded domain of, and that

(2.5)

(2.6)

We then compute

(2.7)

Making use of Hölder’s inequality, we have

(2.8)

Combining (2.7) and (2.8), we obtain

(2.9)

A similar computation leads to

(2.10)

Adding (2.9) and (2.10), we obtain

(2.11)

where is defined in (2.6). We first investigate the particular case. Making use of Hölder’s inequality, we have

(2.12)

Inserted in (2.11), we obtain the first order differential inequality

(2.13)

Integrating (2.13) from 0 to, we obtain the inequality

(2.14)

Suppose that the data satisfy the condition

(2.15)

Then vanishes at some time, and must blow up at some time. We obtain

(2.16)

In the general case, we suppose without loss of generality that, and make use of the inequality

(2.17)

valid for arbitrary. Choosing, we obtain

(2.18)

with

(2.19)

Inserted in (2.12), we obtain the first order differential inequality

(2.20)

Suppose that the initial data are so large that . Then is increasing for t small. Since is increasing in from its negative minimum, it follows then that is increasing for. This shows that remains positive, so that blows up at time. Integrating (2.20) leads to the following upper bound for

(2.21)

These results are summarized in the following.

Theorem 1

1) Assume (2.5) with, (2.6), and (2.15). Then defined in (2.3) blows up at finite time bounded above by (2.16).

2) Assume (2.5) with, (2.6), and with defined in (2.20). Then blows up at finite time bounded above by (2.21).

To conclude this section, we note that if the condition (2.6) is replaced by

(2.22)

then we have to replace the initial data by in Theorem 1. Clearly we may use a lower bound for. For instance we may integrate the differential inequality

(2.23)

that follows from (2.11), leading to the lower bound

(2.24)

3. Lower Bounds for t^{*}

In this section we assume that the data satisfy the conditions

(3.1)

and that the data are nonnegative for all. Moreover the solution is assumed to blow up in the sense that as, where is defined as

(3.2)

with

(3.3)

(3.4)

Differentiating (3.3) and making use of (1.1), (3.1), we obtain

(3.5)

with

(3.6)

Making use of Schwarz and Hölder’s inequalities we have

(3.7)

In we make use of the following Sobolev type inequality

(3.8)

derived in the last section of the paper. Combining (3.7) and (3.8), we obtain

(3.9)

where we have used the arithmetic-geometric mean inequality. Making use of the inequality

(3.10)

we have

(3.11)

valid for arbitrary to be chosen later. Inserted in (3.9) and (3.5), we obtain

(3.12)

We now select

(3.13)

in order to have in (3.12), arriving at

(3.14)

with

(3.15)

A similar computation leads to

(3.16)

where is defined in (3.4). In, we replace (3.7) by

(3.17)

and make use of the Sobolev type inequality

(3.18)

derived by Talenti in [20] with. Inserted in (3.17), we obtain

(3.19)

with

(3.20)

Moreover we make use of (3.10) to write

(3.21)

with arbitrary to be chosen later. Combining (3.5), (3.19) and (3.21), we obtain

We now select such that the quantity in (3.22) vanishes. We are then led to the inequality

(3.23)

with

(3.24)

Finally we make use of (3.10) to write

(3.25)

and select to satisfy, leading to

(3.26)

Inserted in (3.23), we obtain

(3.27)

with

(3.28)

A similar computation leads to

(3.29)

If we suppose that

(3.30)

then there exists such that and we have

(3.31)

valid for, with

(3.32)

(3.33)

(3.34)

Integrating (3.31), we obtain in the two-dimensional case

(3.35)

from which we obtain a lower bound for of the form

(3.36)

where is the inverse function of. In the threedimensional case, we obtain

(3.37)

from which we obtain a lower bound for of the form

(3.38)

These results are summarized in the following

Theorem 2

Under the assumption (3.30), a lower bound for the blow-up time t^{*} of the solution of (1.1) is given by (3.36) in the two-dimensional case and by (3.38) in the three-dimensional case.

In the particular case in which and are constant, we have

(3.39)

in the two-dimensional case and

(3.40)

in the three-dimensional case.

Theorem 2 could easily be extended to systems of n parabolic equations of the form

(3.41)

4. Sobolev Type Inequality in

The Sobolev type inequality (3.8) in may be known, but for the convenience of the reader we present a proof here.

Lemma 1

Let be a nonnegative piecewise -function defined in a bounded domain that vanishes on the boundary. Let be any constant. Then we have the following Sobolev type inequality

(4.1)

valid for.

For the proof of (4.1), we follow the argument of Payne in [21]. We note that (4.1) is equivalent to

(4.2)

where is the convex hull of, and

It is therefore sufficient to establish (4.1) for convex. For the proof, let be an arbitrary point in Let be two pairs of boundary points associated to P with. Since vanishes on, we have for any constant

(4.3)

from which we obtain

(4.4)

Similarly we have

(4.5)

Multiplying (4.4) by (4.5) and integrating over leads to

(4.6)

which is the desired inequality (4.1). We note that we have used the Schwarz and the arithmetic-geometric mean inequalities in the two last steps of (4.6).

Conflicts of Interest

The authors declare no conflicts of interest.

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