Research

# Parabolic problems with data measure

Mohammmed Kbiri Alaoui

Author Affiliations

Department of Mathematics, King Khalid University, P.O. Box 9004, Abha, Kingdom of Saudi Arabia

Boundary Value Problems 2011, 2011:46  doi:10.1186/1687-2770-2011-46

 Received: 4 April 2011 Accepted: 23 November 2011 Published: 23 November 2011

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

The article deals with the existence of solutions of some unilateral problems in the Orlicz-Sobolev spaces framework when the right-hand side is a Radon measure.

Mathematics Subject Classification: 35K86.

##### Keywords:
unilateral problem; radon measure; Orlicz-Sobolev spaces

### 1 Introduction

We deal with boundary value problems

where

T > 0 and Ω is a bounded domain of RN, with the segment property. a : Ω × R × RN RN is a Carathéodory function (that is, measurable with respect to x in Ω for every (t, s, ξ) in R × R × RN , and continuous with respect to (s, ξ) in R × RN for almost every x in Ω) such that for all ξ, ξ* ∈ RN , ξ ξ*,

(1.1)

(1.2)

(1.3)

where c (x,t) belongs to , P is an N-function such that P M and ki (i = 1,2,3,4) belongs to R+ and α to .

(1.4)

(1.5)

There have obviously been many previous studies on nonlinear differential equations with nonsmooth coefficients and measures as data. The special case was cited in the references (see [1,2]).

It is noteworthy that the articles mentioned above differ in significant way, in the terms of the structure of the equations and data. In [1], when f L1(0,T;L1(Ω)) and u0 L1(Ω). The authors have shown the existence of solutions u of the corresponding equation of the problem for every q such that which is more restrictive than the one given in the elliptic case .

In this article, we are interested with an obstacle parabolic problem with measure as data. We give an improved regularity result of the study of Boccardo et al. [1].

In [1], the authors have shown the existence of a weak solutions for the corresponding equation, the function a(x, t, s, ξ) was assumed to satisfy a polynomial growth condition with respect to u and ∇u. When trying to relax this restriction on the function a(., s, ξ), we are led to replace the space Lp(0, T; W1,p()) by an inhomogeneous Sobolev space W1,xLM built from an Orlicz space LM instead of Lp, where the N-function M which defines LM is related to the actual growth of the Carathéodory's function.

For simplicity, one can suppose that there exist α > 0, β > 0 such that

### 2 Preliminaries

Let M : R+ R+ be an N-function, i.e. M is continuous, convex, with M(t) > 0 for as t → 0 and as t → ∞. Equivalently, M admits the representation: where a : R+ R+ is non-decreasing, right continuous, with a(0) = 0, a(t) > 0 for t > 0 and a(t) → ∞ as t → ∞. The N-function conjugate to M is defined by , where is given by (see [3,4]).

The N-function M is said to satisfy the Δ2 condition if, for some k > 0:

(2.1)

when this inequality holds only for t t0 > 0, M is said to satisfy the Δ2 condition near infinity.

Let P and Q be two N-functions. P Q means that P grows essentially less rapidly than Q; i.e., for each ε > 0,

Let Ω be an open subset of RN. The Orlicz class (resp. the Orlicz space Lm(Ω)) is defined as the set of (equivalence classes of) real-valued measurable functions u on Ω such that

Note that LM(Ω) is a Banach space under the norm and is a convex subset of LM(Ω). The closure in LM(Ω) of the set of bounded measurable functions with compact support in is denoted by EM(Ω). The equality EM(Ω) = LM(Ω) holds if and only if M satisfies the Δ2 condition, for all t or for t large according to whether Ω has infinite measure or not.

The dual of EM(Ω) can be identified with by means of the pairing , and the dual norm on is equivalent to . The space LM(Ω) is reflexive if and only if M and satisfy the Δ2 condition, for all t or for t large, according to whether Ω has infinite measure or not.

We now turn to the Orlicz-Sobolev space. W1LM(Ω) (resp. W1EM(Ω)) is the space of all functions u such that u and its distributional derivatives up to order 1 lie in LM(Ω) (resp. EM(Ω)). This is a Banach space under the norm . Thus, W1LM(Ω) and W1EM(Ω) can be identified with subspaces of the product of N + 1 copies of LM(Ω). Denoting this product by ΠLM, we will use the weak topologies and . The space is defined as the (norm) closure of the Schwartz space in W1EM(Ω) and the space as the closure of in W1LM(Ω). We say that un converges to u for the modular convergence in W1LM(Ω) if for some for all |α| ≤ 1. This implies convergence for . If M satisfies the Δ2 condition on R+(near infinity only when Ω has finite measure), then modular convergence coincides with norm convergence.

Let (resp. ) denote the space of distributions on Ω which can be written as sums of derivatives of order ≤ 1 of functions in (resp. ). It is a Banach space under the usual quotient norm.

If the open set Ω has the segment property, then the space is dense in for the modular convergence and for the topology (cf. [5,6]). Consequently, the action of a distribution in on an element of is well defined.

For k > 0, s R, we define the truncation at height k,Tk(s) = [k - (k - |s|)+]sign(s).

The following abstract lemmas will be applied to the truncation operators.

Lemma 2.1 [7]Let F : R R be uniformly lipschitzian, with F(0) = 0. Let M be an N-function and let (resp.).

Then (resp. ). Moreover, if the set of discontinuity points of F' is finite, then

Let Ω be a bounded open subset of RN, T > 0 and set Q = Ω × ]0, T[. Let m ≥ 1 be an integer and let M be an N-function. For each α INN , denote by the distributional derivative on Q of order α with respect to the variable x RN. The inhomogeneous Orlicz-Sobolev spaces are defined as follows .

The last space is a subspace of the first one, and both are Banach spaces under the norm . We can easily show that they form a complementary system when Ω satisfies the segment property. These spaces are considered as subspaces of the product space ΠLm(Q) which have as many copies as there are α-order derivatives, |α| ≤ m. We shall also consider the weak topologies and . If u Wm, xLM(Q), then the function : t u(t) = u(t,.) is defined on [0, T] with values in WmLM(Ω). If, further, u Wm,xEM(Q) then the concerned function is a WmEM(Ω)-valued and is strongly measurable. Furthermore, the following imbedding holds: Wm,xEM(Q) ⊂ L1(0,T; WmEM(Ω)). The space Wm,xLM(Q) is not in general separable, if u Wm,xLM(Q), we cannot conclude that the function u(t) is measurable on [0,T]. However, the scalar function t ↦ ||u(t)||M, is in L1(0,T). The space is defined as the (norm) closure in Wm,xEM(Q) of . We can easily show as in [6] that when Ω has the segment property, then each element u of the closure of with respect of the weak * topology is limit, in Wm,xLM(Q), of some subsequence for the modular convergence; i.e., there exists λ > 0 such that for all |α| ≤ m,

this implies that (ui) converges to u in Wm,xLM(Q) for the weak topology . Consequently, , and this space will be denoted by .

Furthermore, . Poincaré's inequality also holds in , i.e., there is a constant C > 0 such that for all one has . Thus both sides of the last inequality are equivalent norms on . We have then the following complementary system:

F being the dual space of . It is also, except for an isomorphism, the quotient of by the polar set , and will be denoted by , and it is shown that . This space will be equipped with the usual quotient norm where the infimum is taken on all possible decompositions . The space F0 is then given by and is denoted by .

We can easily check, using Lemma 4.4 of [6], that each uniformly lipschitzian mapping F, with F(0) = 0, acts in inhomogeneous Orlicz-Sobolev spaces of order 1: W1,xLM(Q) and .

### 3 Main results

First, we give the following results which will be used in our main result.

#### 3.1 Useful results

Hereafter, we denote by the real number defined by is the measure of the unit ball of RN, and for a fixed t ∈ [0, T], we denote μ(θ) = meas{(x,t) : |u(x, t)| > θ}.

Lemma 3.1 [8]Let , and let fixed t ∈ [0, T], then we have

and where is defined by

Lemma 3.2 Under the hypotheses (1.1)-(1.3), if f, u0 are regular functions and f, u0 ≥ 0, then there exists at least one positive weak solution of the problem

such that

Proof

Let u be a continuous function, we say that u satisfies (*) if: there exists a continuous and increasing function β such that ||u(t) - u(s)||2 β(||u0||2)|t - s|, where u0(x) = u(x, 0).

Let .

Let us consider the set , where C is a closed convex of . It is easy to see that is a closed convex (since all its elements satisfy (*) ).

We claim that the problem

has a weak solution which is unique in the sense defined in [9].

Indeed, let us consider the approximate problem

where the functional Φ is defined by Φ : X R ∪ {+ ∞} such that

The existence of such un X was ensured by Kacur et al. [10].

Following the same proof as in [9], we can prove the existence of a solution u of the problem (E') as limit of un (for more details see [9]).

Lemma 3.3 Let such that and .

Then, there exists a smooth function (vj) such that

vj v for the modular convergence in ,

for the modular convergence in .

For the proof, we use the same technique as in [11] in the parabolic case.

#### 3.2 Existence result

Let M be a fixed N-function, we define K as the set of N-function D satisfying the following conditions:

i) M(D-1(s)) is a convex function,

ii) ,

iii) There exists an N-function H such that and near infinity.

Theorem 3.1 Under the hypotheses (1.1)-(1.5), The problem (P) has at least one solution u in the following sense:

for all φ D(RN+1) which are zero in a neighborhood of (0, T) × ∂ Ω and {T} × Ω.

Remark 3.1 (1) If ψ = - ∞ in the problem (P), then the above theorem gives the same regularity as in the elliptic case.

(2) An improved regularity is reached for all N-function satisfying the conditions (i)-(ii)-(iii).

For example, , for all .

In the sequel and throughout the article, we will omit for simplicity the dependence on x and t in the function a(x, t, s, ξ) and denote ϵ(n, j, μ, s, m) all quantities (possibly different) such that

and this will be in the order in which the parameters we use will tend to infinity, that is, first n, then j, μ, s, and finally m. Similarly, we will write only ϵ(n), or ϵ(n, j),... to mean that the limits are made only on the specified parameters.

#### 3.2.1 A sequence of approximating problems

Let (fn) be a sequence in D(Q) which is bounded in L1(Q) and converge to μ in Mb(Q).

Let be a sequence in D(Ω) which is bounded in L1(Ω) and converge to u0 in Mb(Ω).

We define the following problems approximating the original (P):

Lemma 3.4 Under the hypotheses (1.1)-(1.3), there exists at least one solution un of the problem (Pn) such that a.e. in Q.

For the proof see Lemma 3.2.

#### 3.2.2 A priori estimates

Lemma 3.5 There exists a subsequence of (un) (also denoted (un)), there exists a measurable function u such that:

Proof:

Recall that un ≥ 0 since fn ≥ 0.

Let h > 0 and consider the following test function v = Th(un - Tk(un)) in (Pn), we obtain

We have

So,

Now, let us fix k > ||ψ||, we deduce the fact that: .

Let h to tend to zero, one has

(3.1)

Let us use as test function in (Pn),v = Tk(un), then as above, we obtain

(3.2)

Then (Tk(un)n) is bounded in , and then there exist some such that

Tk(un) ⇀ ωk, weakly in for , strongly in EM(Q) and a.e in Q.

Let consider the C2 function defined by

Multiplying the approximating equation by , we get in the distributions sense. We deduce then that ηk(un) being bounded in and in . By Corollary 1 of [12], ηk(un) is compact in L1 (Q).

Following the same way as in [2], we obtain for every k > 0,

(3.3)

Using now the estimation (3.1) and Fatou's lemma to obtain

Let fixed a t ∈ [0, T]. We argue now as for the elliptic case, the problem becomes:

We denote gn := nTn((un - ψ)-).

Let φ be a truncation defined by

(3.4)

for all θ, h > 0.

Using v = φ (un) as a test function in the approximate elliptic problem , we obtain by using the same technique as in [8]

(3.5)

here and below C denote positive constants not depending on n.

By using Lemma 3.1, we obtain (supposing -μ'(θ) > 0 which does not affect the proof) and following the same way as in [8], we have for D K,

(3.6)

Let denote , then

Using Lemma 3.1, denoting one has

Remark also that and using Lemma 3.2, we have .

Combining the inequalities (3.5) and (3.6) we obtain,

(3.7)

and since the function is absolutely continuous, we get

Then, the sequence (un) is bounded in and we deduce that for all N-function D K.

#### 3.3 Almost everywhere convergence of the gradients

Lemma 3.6 The subsequence (un) obtained in Lemma 3.5 satisfies:

Proof:

Let m > 0, k > 0 such that m > k. Let ρm be a truncation defined by

where vj D(Q) such that vj ψ and vj Tk(u) with the modular convergence in (for the existence of such function see [11] since ).

ωμ is the mollifier function defined in Landes [13], the function ωμ,j have the following properties:

Set v = (Tk(un) - ωμ,j) ρm(un) as test function, we have

(1)

(2)

Let us recall that for , there exists a smooth function u(see [14]) such that

Remark also that,

and it is easy to see that .

Concerning I2,

As in I1, we obtain ,

and

thus by using the fact that .

So, .

Set Φ(s) = s2/2, Φ ≥ 0,then

So,

and easily we deduce, .

Finally we conclude that: .

We are interested now with the terms of (1)-(4).

recall that ρm(un) = 1 on {|un| ≤ k}.

Let .

By using the inequality (1.3), we can deduce the existence of some measurable function hk such that

since

and strongly in .

Then,

Following the same way as in J2, one has

Concerning the terms J4 :

Letting n → ∞, then

Taking now the limits j → ∞ and after μ → ∞ in the last equality, we obtain

Then,

Since (un) is bounded in and using (iii), we have (a(., un,∇un)) is bounded in LH(Q), then

so,

Since u ψ, then Tk(u) ≥ Tk(ψ) and there exist a smooth function vj Tk(ψ) such that vj Tk(u) for the modular convergence in .

Taking into account now the estimation of (1), (2), (4)and (5), we obtain

(3.8)

On the other hand,

each term of the last right hand side is of the form ϵ(n, j, s), which gives

Following the same technique used by Porretta [2], we have for all r < s :

Thus, as in the elliptic case (see [7]), there exists a subsequence also denoted by un such that

We deduce then that,

Lemma 3.7 For all k > 0,

Proof:

We have proved that

≤ ϵ (n, j, μ, s, m) (see (3.8)).

We can also deduce that

Then

then,

Letting n → ∞, we deduce

Using the same argument as above, we obtain

and Vitali's theorem and (1.1) gives

#### 3.3.1 The convergence of the problems (Pn) and the completion of the proof of Theorem 3.1

The passage to the limit is an easy task by using the last steps, then

then,

for all φ D(RN+1) which are zero in a neighborhood of (0,T) × ∂Ω and {T} × Ω.

### 4 Conclusion

In this article, we have proved the existence of solutions of some class of unilateral problems in the Orlicz-Sobolev spaces when the right-hand side is a Radon measure.

### Competing interests

The authors declare that they have no competing interests.

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