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Attractors for parabolic equations related to Caffarelli-Kohn-Nirenberg inequalities

Nguyen Dinh Binh1* and Cung The Anh2

Author affiliations

1 Department of Applied Mathematics and Informatics, Hanoi University of Science and Technology, 1 Dai Co Viet, Hai Ba Trung, Hanoi, Vietnam

2 Department of Mathematics, Hanoi National University of Education, 136 Xuan Thuy, Cau Giay, Hanoi, Vietnam

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Citation and License

Boundary Value Problems 2012, 2012:35  doi:10.1186/1687-2770-2012-35

The electronic version of this article is the complete one and can be found online at: http://www.boundaryvalueproblems.com/content/2012/1/35


Received:21 February 2011
Accepted:28 March 2012
Published:28 March 2012

© 2012 Binh and Anh; licensee Springer.

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

Using the theory of uniform global attractors for multi-valued semiprocesses, we prove the existence of attractors for quasilinear parabolic equations related to Caffarelli-Kohn- Nirenberg inequalities, in which the conditions imposed on the nonlinearity provide the global existence of weak solutions but not uniqueness, in both autonomous and non-autonomous cases.

Mathematics Subject Classification 2010: 35B41, 35K65, 35D30.

Keywords:
Caffarelli-Kohn-Nirenberg inequalities; non-uniqueness; weak solution; multivalued semiflow; multi-valued semiprocess; compact attractor; compactness and monotonicity methods

1. Introduction

The understanding of the asymptotic behavior of dynamical systems is one of the most important problems of modern mathematical physics. One way to attack the problem for a dissipative dynamical system is to consider its attractor. The existence of the attractor has been derived for a large class of PDEs (see e.g., [1,2] and references therein) for both autonomous and non-autonomous equations. However, these researches may not be applied to a wide class of problems, in which solutions may not be unique. Good examples of such systems are differential inclusions, variational inequalities, control infinite dimensional systems and also some partial differential equations for which solutions may not be known unique as, for example, some certain semilinear wave equations with high power nonlinearities, the incompressible Navier-Stokes equation in three space dimension, the Ginzburg-Landau equation, etc. For the qualitative analysis of the above mentioned systems from the point of view of the theory of dynamical systems, it is necessary to develop a corresponding theory for multi-valued semigroups.

In the last years, there have been some theories for which one can treat multi-valued semi-flows and their asymptotic behavior, including the generalized semiflows theory of Ball [3], theory of trajectory attractors of Chepyzhov and Vishik [4] and theories of multi-valued semiflows and semiprocesses of Melnik and Valero [5-7]. Thanks to these theories, several results concerning attractors in the case of equations without uniqueness have been obtained recently for differential inclusion [5,6], parabolic equations [8-10], the phase-field equation [11], the wave equation [12], the three-dimensional Navier-Stokes equation [3,13], etc. Although the existence of attractors has been derived for many classes of partial differential equations without uniqueness, to the best of our knowledge, little seems to be known for singular/degenerate equations, expecially in the quasilinear case.

Let Ω be a bounded domain in ℝN(N ≥ 2) containing the origin with boundary Ω. In this paper we consider the following quasilinear parabolic equation

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M1','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M1">View MathML</a>

(1.1)

where τ ∈ ℝ, uτ L2(Ω) are given, the nonlinearity f, the external force g, and the numbers p, γ satisfy the following conditions:

(H1) f: ℝ × ℝ → ℝ is a continuous function satisfying

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M2','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M2">View MathML</a>

(1.2)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M3','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M3">View MathML</a>

(1.3)

for some q ≥ 2, where C1, C2, k1, k2 are positive constants;

(H2) <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M4','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M4">View MathML</a>, where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M5','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M5">View MathML</a> is the set of all translation compact functions in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M6','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M6">View MathML</a> whose definition is given in Definition 1.1 below.

(H3) <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M7','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M7">View MathML</a> and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M8','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M8">View MathML</a>.

Let us give some comments about assumptions (H1)-(H3). The nonlinearity f is assumed to have a polynomial growth and to satisfy a standard dissipative condition. A typical example of functions satisfying conditions (H1) is f (t, u) = |u|q-2u. arctan t, q ≥ 2. We refer the reader to [[1], Chapter 5, Propositions 3.3 and 3.5] for translation compact criterions in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M6','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M6">View MathML</a>. While (H3) is a technical condition ensuring that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M9','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M9">View MathML</a> is embedded compactly into L2(Ω), where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M9','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M9">View MathML</a> is the natural energy space related to problem (1.1), which is defined later in this section. This is essential for proving the existence of a weak solution to problem (1.1) using the compactness method.

Problem (1.1), which is related to some Caffarelli-Kohn-Nirenberg inequalities [14], contains some important classes of parabolic equations, such as the semilinear heat equations (when γ = 0, p = 2), semilinear singular/degenerate parabolic equations (when p = 2), the p-Laplacian equations (when γ = 0, p ≠ 2), etc. The existence and properties of solutions to problem type (1.1) have attracted interest in recent years [15-19]. However, to the best of our knowledge, little seems to be known for the long-time behavior of solutions to problem (1.1).

In this article we study the long-time behavior of solutions to problem (1.1) via the concept of uniform global attractors for multi-valued semiprocesses. Here there is no restrictions on the growth of the nonlinearity f and the conditions imposed on f provide the global existence of a weak solution to problem (1.1), but not uniqueness. Thus, when studying the long-time behavior of solutions, in order to handle nonuniqueness of solutions, we need use the theory of attractors for multi-valued semiprocesses. Following the general lines of the approach used in [8-10,20] for non-degenerate parabolic equations, we prove the existence of a global compact attractor in the autonomous case, and of a uniform global compact attractor in the non-autonomous case. Noting that when the nonlinearity f does not depend on time t, the existence of an attractor for problem (1.1) in the semilinear non-degenerate case, namely when γ = 0 and p = 2, was studied in [8,9]. Thus, our results extend some known results on the existence and long-time behavior of solutions of nondegenerate semilinear parabolic equations.

It is worth noticing that under some additional conditions on f, for example, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M10','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M10">View MathML</a> for all t > τ, u ∈ ℝ, or a weaker assumption

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M11','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M11">View MathML</a>

one can prove that the weak solution of problem (1.1) is unique. Then the multivalued semiprocess turns to be a single-valued one and the uniform compact global attractor is exactly the usual uniform attractor for the family of single-valued semiprocesses [1].

In the rest of this section, for convenience of the reader, we recall some results on function spaces related to Caffarelli-Kohn-Nirenberg inequalities and translation compact functions.

For 1 < p < ∞ and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M12','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M12">View MathML</a>, we define the weighted space

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M13','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M13">View MathML</a>

equipped with the norm

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M14','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M14">View MathML</a>

It is easy to check that the dual space <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M15','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M15">View MathML</a> of <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M16','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M16">View MathML</a> is the space <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M17','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M17">View MathML</a>, where p' is defined by <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M18','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M18">View MathML</a>. Moreover, we define the weighted Sobolev space <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M9','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M9">View MathML</a> as the closure of <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M19','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M19">View MathML</a> in the norm

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M20','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M20">View MathML</a>

(1.4)

As 1 < p < ∞, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M9','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M9">View MathML</a> is reflexive, and the dual space of <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M9','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M9">View MathML</a> will be denoted by <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M21','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M21">View MathML</a>.

We now state some results which we will use later. The first is the Caffarelli-Kohn-Nirenberg inequality.

Proposition 1.1. [14]Assume that 1 < p < N. Then there exists a positive constant CN,p,γ,q such that for every <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M22','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M22">View MathML</a>,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M23','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M23">View MathML</a>

(1.5)

where p, q, γ, δ are related by

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M24','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M24">View MathML</a>

(1.6)

and δq < N, γp < N.

The inequality (1.5) implies that the embedding

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M25','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M25">View MathML</a>

This implies, by duality,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M26','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M26">View MathML</a>

It is pointed out in [19] that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M27','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M27">View MathML</a>

(1.7)

for every p, q, γ, δ satisfying <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M28','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M28">View MathML</a> with γ δ γ + 1 and δq < N, γ p < N.

From assumption (H3), it is easy to check that there exists a positive number δ such that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M29','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M29">View MathML</a> compactly. Since the embedding <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M30','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M30">View MathML</a> is continuous, it is seen that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M31','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M31">View MathML</a> is an evolution triplet.

We now define the following "evolution" spaces which will be useful in what follows.

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M32','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M32">View MathML</a>

endowed with the norm

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M33','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M33">View MathML</a>

The dual space of <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M34','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M34">View MathML</a> is <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M35','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M35">View MathML</a>.

Putting

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M36','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M36">View MathML</a>

The following proposition, which is easily proved by using similar arguments as in [[21], Chapter 2], gives some important properties of the operator -Δp,γ.

Proposition 1.2. The operator p,γ maps <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M9','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M9">View MathML</a>into its dual <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M21','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M21">View MathML</a>. Moreover,

(1) -Δp,γ is hemicontinuous, i.e., for all <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M37','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M37">View MathML</a>, the map λ ↦ 〈-Δ p,γ(u + λv), wis continuous from to ℝ.

(2) -Δp,γ is monotone, i.e., 〈-Δp,γu + Δp,γv,u - v〉 ≥ 0, for all <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M38','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M38">View MathML</a>.

Definition 1.1. Assume that ℰ is a reflexive Banach space.

(1) A function <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M40','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M40">View MathML</a>is said to be translation bounded if

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M41','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M41">View MathML</a>

(2) A function <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M40','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M40">View MathML</a>is said to be translation compact if the closure of {φ(⋅ + h)|h ∈ ℝ} is compact in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M42','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M42">View MathML</a>.

Denote by <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M43','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M43">View MathML</a> and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M44','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M44">View MathML</a> the sets of all translation bounded functions and of all translation compact functions in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M42','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M42">View MathML</a>, respectively. It is well-known (see [4]) that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M45','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M45">View MathML</a>.

Let <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M46','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M46">View MathML</a> be the closure of the set {g(· + h)|h ∈ ℝ} in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M47','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M47">View MathML</a>. The following results were proved in [[1], Chapter 5, Proposition 3.4].

Lemma 1.3. (1) <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M46','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M46">View MathML</a>is compact.

(2) For all <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M48','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M48">View MathML</a>;

(3) The translation group {T(h)}, which is defined by T(h)σ(s) = σ(h + s), s, h ∈ ℝ, is continuous on <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M46','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M46">View MathML</a>;

(4) <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M49','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M49">View MathML</a>;

The rest of the article is organized as follows. In Section 2, we prove the global existence of a weak solution to problem (1.1) by using the monotonicity and compactness methods. In Section 3, the existence of global attractors for problem (1.1) is proved in both the autonomous and non-autonomous cases.

2. Existence of a weak solution

We denote

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M50','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M50">View MathML</a>

where p', q' are the conjugate indexes of p, q, respectively.

Definition 2.1. A function u(x, t) is called a weak solution of (1.1) on (τ, T) iff

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M51','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M51">View MathML</a>

and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M52','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M52">View MathML</a>

for all test functions φ V.

It is known (see [[1], Theorem 1.8, p. 33]) that if u V and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M53','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M53">View MathML</a>, then u C([τ, T];L2(Ω)). This makes the initial condition in problem (1.1) meaningful.

Theorem 2.1. For any τ, T ∈ ℝ, T > τ and uτ L2(Ω) given, problem (1.1) has at least one weak solution u on (τ, T). Moreover, the solution u can be extended to the whole interval (τ, +∞).

Proof. We split the proof into three steps.

Step 1: A Galerkin scheme. Consider the approximating solution un(t) in the form

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M54','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M54">View MathML</a>

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M55','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M55">View MathML</a> is a basis of <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M56','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M56">View MathML</a>, which is orthonormal in L2(Ω). We get un from solving the problem

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M57','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M57">View MathML</a>

Using the Peano theorem in the theory of ODEs, we get the local existence of un.

Step 2: A priori estimates. We have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M58','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M58">View MathML</a>

By assumption (H3), we can choose δ > 0 such that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M59','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M59">View MathML</a>, then <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M60','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M60">View MathML</a> and therefore there exists λ > 0 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M61','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M61">View MathML</a>

(2.1)

where the last inequality follows from the Young inequality. Using (1.3) and the Cauchy inequality, we get

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M62','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M62">View MathML</a>

Hence

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M63','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M63">View MathML</a>

(2.2)

We show that the local solution un can be extended to the interval [τ, ∞). Indeed, from (2.2) we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M64','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M64">View MathML</a>

By the Gronwall inequality, we obtain

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M65','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M65">View MathML</a>

(2.3)

where we have used the facts that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M66','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M66">View MathML</a> and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M67','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M67">View MathML</a>

We now establish some a priori estimates for un. Integrating (2.2) on [τ, T], τ < t T, and using the fact that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M66','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M66">View MathML</a>, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M68','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M68">View MathML</a>

(2.4)

The last inequality implies that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M69','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M69">View MathML</a>

(2.5)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M70','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M70">View MathML</a>

(2.6)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M71','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M71">View MathML</a>

(2.7)

Using hypothesis (1.2), we get

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M72','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M72">View MathML</a>

Hence, we can conclude that {f(t, un)} is bounded in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M73','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M73">View MathML</a> and thus,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M74','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M74">View MathML</a>

(2.8)

We have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M75','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M75">View MathML</a>

for all <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M76','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M76">View MathML</a>, where we have used the Hölder inequality. Because of the boundedness of {un} in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M34','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M34">View MathML</a>, we infer that {-Δp,γ un} is bounded in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M35','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M35">View MathML</a>.

Step 3: Passing limits. From the above estimates, there exists a subsequence {uμ} ⊂ {un} such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M77','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M77">View MathML</a>

(2.9)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M78','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M78">View MathML</a>

(2.10)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M79','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M79">View MathML</a>

(2.11)

up to a subsequence.

To prove that η(t) = f(t, u(t)), we argue similarly to [22,23] to deduce that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M80','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M80">View MathML</a>

(2.12)

for all T > τ. In particular, we obtain from (2.5) that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M81','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M81">View MathML</a>

(2.13)

Then, by Theorem 13.3 and Remark 13.1 in [24], we obtain that uμ u strongly in L2(τ, T; L2(Ω)), up to a subsequence. Hence, we can assume that uμ u a.e. in Qτ,T. Therefore, f(t, uμ) → f(t, u) a.e. in Qτ,T since f is continuous. By Lemma 1.3 in [[21], Chapter 1], one has

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M82','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M82">View MathML</a>

Thus, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M83','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M83">View MathML</a>

(2.14)

We now show that ψ = -Δp, γ u. Since -Δp, γ is monotone, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M84','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M84">View MathML</a>

Note that {un(T)} is bounded in L2(Ω), so by arguments as in [[21], pp. 159-160], we have that un(T) ⇀ u(T) in L2(Ω). Because

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M85','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M85">View MathML</a>

(2.15)

we obtain

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M86','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M86">View MathML</a>

(2.16)

where we have used the facts that un(τ) → uτ in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M87','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M87">View MathML</a>. On the other hand, by integrating by parts, from (2.14) we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M88','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M88">View MathML</a>

and therefore thanks to (2.15) and (2.16) one gets

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M89','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M89">View MathML</a>

We now use the hemicontinuity of the operator Δp,γ to show that ψ = -Δp,γ u. Taking v = u - λw, where λ > 0 and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M90','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M90">View MathML</a>, we obtain

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M91','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M91">View MathML</a>

hence

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M92','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M92">View MathML</a>

(2.17)

leting λ → 0 in (2.17), we conclude that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M93','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M93">View MathML</a>

So ψ = -Δp,γ u. Thus,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M94','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M94">View MathML</a>

We now show that u(τ) = uτ. Choosing some <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M95','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M95">View MathML</a> with φ(T) = 0, observe that φ V, by the Lebesgue dominated theorem, one can check that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M96','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M96">View MathML</a>

Doing the same in the Galerkin approximations yields

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M97','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M97">View MathML</a>

Passing to the limit as n → ∞, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M98','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M98">View MathML</a>

Therefore, u(τ) = uτ and u is a weak solution of (1.1) on (τ, T).

Finally, it is easy to check that the solution u satisfies the inequality similar to (2.3), and this implies that the solution u exists globally on the interval (τ, +∞).

3. Existence of global attractors

3.1. The autonomous case

Consider the case where f and g do not depend on the time t, and let us recall the definition of multi-valued semiflows.

Definition 3.1. [5]Let E be a Banach space. The mapping

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M99','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M99">View MathML</a>

is called a multi-valued semiflow if the following conditions are satisfied:

(1) <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M100','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M100">View MathML</a>for arbitrary w E;

(2) <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M101','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M101">View MathML</a>for all w E, t1, t2 ∈ ℝ+, where G (t, B) = ∪xB G (t, x), B E.

It is called a strict multi-valued semiflow if <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M102','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M102">View MathML</a>, for all w E, t1, t2 ∈ ℝ+.

We now consider problem (1.1) with τ = 0. By Theorem 2.1, we construct a multi-valued mapping as follows

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M103','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M103">View MathML</a>

Lemma 3.1. is a strict multi-valued semiflow in the sense of Definition 3.1.

Proof. Assume that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M105','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M105">View MathML</a>, then ξ = u(t1 + t2), where u(t) is a solution of (1.1). Denoting v (t) = u(t + t2), we see that v(.) is also in the set of solutions of (1.1) with respect to initial condition v(0) = u(t2). Therefore, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M106','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M106">View MathML</a>. It remains to show that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M107','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M107">View MathML</a>. If <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M108','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M108">View MathML</a> then ξ = v(t1), where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M109','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M109">View MathML</a>. One can suppose that v(0) = u(t2), where u(0) = u0. Set

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M110','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M110">View MathML</a>

Since u and v are two solutions of (1.1), we obtain that w is a solution of (1.1) with w(0) = u(0) = u0. In addition, since ξ = v(t1) = w(t1 + t2), we have <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M105','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M105">View MathML</a>.

Definition 3.2. [5]A set is said to be a global attractor of the multi-valued semiflow if the following conditions hold:

is an attracting, i.e., <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M112','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M112">View MathML</a>as t → ∞ for all bounded subsets B E,

is negatively semi-invariant: <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M113','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M113">View MathML</a>for arbitrary t ≥ 0,

If is an attracting of then <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M115','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M115">View MathML</a>,

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M116','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M116">View MathML</a>is the Hausdorff semi-distance.

The following theorem gives the sufficient conditions for the existence of a global attractor for the multi-valued semiflow .

Theorem 3.2. [5,7]Suppose that the strict multi-valued semiflow has the following properties:

(1) is pointwise dissipative, i.e., there exists K > 0 such that for <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M117','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M117">View MathML</a>one has u(t)∥E K if t t0 (∥u0E);

(2) <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M118','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M118">View MathML</a>is a closed map for any t ≥ 0, i.e., if ξn ξ, ηn η, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M119','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M119">View MathML</a>then <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M120','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M120">View MathML</a>;

(3) is asymptotically upper semicompact, i.e., if B is a bounded set in E such that for some <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M121','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M121">View MathML</a>is bounded, any sequence <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M122','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M122">View MathML</a>with tn → ∞ is precompact in E.

Then has a compact global attractor in E. Moreover, is invariant, i.e., <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M123','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M123">View MathML</a>for any t ≥ 0.

Lemma 3.3. <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M124','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M124">View MathML</a>is a compact mapping for each t* ∈ (0, T].

Proof. This lemma is a direct consequence of Lemma 3.8 in Section 3.2 below.

We now can prove the existence of a global attractor.

Theorem 3.4. Under conditions (H1)-(H3), where f andg are assumed to be independent of time t, the strict multi-valued semiflow generated by problem (1.1) has an invariant compact global attractor in L2(Ω).

Proof. We will check hypotheses (1)-(3) of Theorem 3.2. First, assume <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M125','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M125">View MathML</a>, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M126','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M126">View MathML</a>

Noting that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M127','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M127">View MathML</a>

we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M128','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M128">View MathML</a>

(3.1)

Therefore

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M129','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M129">View MathML</a>

Hence one can deduce that is pointwise dissipative.

We now check hypothesis (2) of Theorem 3.2. Assume that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M130','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M130">View MathML</a> in L2(Ω). Then there exists a sequence {un} such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M131','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M131">View MathML</a>

Using the same arguments as in the proof of Theorem 2.1, we have

un u in L2(Q0,T),

un(t) ⇀ u(t) in L2(Ω) for arbitrary t ∈ [0, T] (and then u(0) = η),

f(un)⇀ f(u) in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M132','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M132">View MathML</a>,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M133','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M133">View MathML</a> in V,

• -Δp,γ un ⇀ -Δp,γ u in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M134','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M134">View MathML</a>,

up to a subsequence. Hence, passing to the limit in the equality

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M135','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M135">View MathML</a>

we conclude that u(t) is a weak solution of (1.1) with the initial condition u(0) = η. Thus, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M136','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M136">View MathML</a>.

For hypothesis (3), one observes that for n large enough,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M137','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M137">View MathML</a>

where t* > 0 and B* is a bounded set in L2(Ω). Using Lemma 3.3, we conclude that, if <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M138','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M138">View MathML</a>, then {ξn} is precompact in L2(Ω).

3.2. The non-autonomous case

Let us recall some definitions and related results. The pair of functions (f(s,⋅),g(⋅,s)) = σ(s) is called a symbol of (1.1). We consider (1.1) with a family of symbols including the shifted forms σ(s + h) = (f(s + h,⋅), g (⋅, s + h)) and the limits of some sequence {σ(s + hn)}nN in an appropriate topological space Σ. The family of such symbols is said to be the hull of σ in Σ and is denoted by <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M139','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M139">View MathML</a>, i.e.,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M140','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M140">View MathML</a>

If the hull <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M139','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M139">View MathML</a> is a compact set in Σ, we say that σ is translation compact in Σ.

Denote ℝd = {(t, τ) ∈ ℝ2 | τ t}. Let X be a complete metric space, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M141','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M141">View MathML</a> and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M142','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M142">View MathML</a> be the set of all nonempty subsets and the set of all nonempty bounded subsets of the space X, respectively and let Σ be a subspace of Σ.

Denote

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M143','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M143">View MathML</a>

Then Z is a Banach space. We say that fn f in the space C(ℝ; Z) if

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M144','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M144">View MathML</a>

(3.2)

for all t ∈ ℝ, r > 0.

Let <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M145','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M145">View MathML</a>, and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M146','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M146">View MathML</a>

where the topology in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M147','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M147">View MathML</a> is equipped by the local weak convergence, i.e., gn g in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M147','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M147">View MathML</a> if

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M148','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M148">View MathML</a>

for all t ∈ ℝ, r > 0 and ϕ L2 (Qt,t+r). We define <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M149','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M149">View MathML</a>.

In order to deal with a uniform attractor with respect to the family of symbols, one usually requires the translation compact property. Let us recall some discussions on this requirement. It is known that hypothesis (H2) ensures that g is translation compact in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M147','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M147">View MathML</a> (see [4] for details). In addition, the following statement gives a sufficient condition for the translation compact property in C (ℝ; Z).

Proposition 3.5. [4]The function f C(ℝ; Z) is translation compact if and only if for all R > 0 one has

(1) |f(t, v)| ≤ C(R) for all t ∈ ℝ, v ∈ [-R, R],

(2) |f(t1, v1)-f(t2, v2)| ≤ α(|t1-t2| + |v1-v2|,R), ∀t1, t2 ∈ ℝ, v1, v2 ∈ [-R, R], here C(R) > 0 and α(.,.) is a function such that α(s, R) → 0 as s → 0+.

From now on, we suppose that f is translation compact. Together with the fact that g is translation compact in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M147','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M147">View MathML</a>, one sees that Σ is a compact set in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M147','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M147">View MathML</a>. Then it follows from [4] that T(h) : Σ → Σ is continuous and T(h)Σ ⊂ Σ for all h ∈ ℝ.

Definition 3.3. [6]The map <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M150','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M150">View MathML</a>is called an multi-valued semiprocess (MSP) if

(1)U (τ, τ,.) = Id (the identity map),

(2)U (t, τ, x) ⊂ U(t, s, U(s, τ, x)), for all x X, t, s, τ ∈ ℝ,τ s t.

It is called a strict multi-valued semiprocess if U(t, τ, x) = U(t, s, U(s, τ, x)).

We denote by <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M151','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M151">View MathML</a> the set of all global weak solutions (defined for all t τ) of the problem (1.1) with data (fσ, gσ) instead of (f, g) such that u(τ) = uτ. For each σ = (f, g) ∈ Σ, we consider the family of MSP {Uσ : σ ∈ Σ} defined by

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M152','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M152">View MathML</a>

Lemma 3.6. Uσ (t, τ, uτ) is a multi-valued semiprocess. Moreover,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M153','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M153">View MathML</a>

Proof. Given z Uσ(t, τ, uτ)) we have to prove that z Uσ(t, s, Uσ(s, τ, uτ)). Take <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M154','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M154">View MathML</a> such that y(τ) = uτ and y(t) = z. Clearly, y(s) ∈ Uσ(s, τ, uτ). Then if we define z(t) = y(t) for t s we have that z(s) = y(s) and obviously <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M155','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M155">View MathML</a>. Consequently, z(t) ∈ Uσ(t, s, Uσ(s, τ, uτ)).

Let z Uσ(t + s, τ + s, uτ). Then there exists <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M156','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M156">View MathML</a> such that z = u(t + s) and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M157','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M157">View MathML</a>, so that z = v(t) ∈ uτ,T (s)σ (uτ).

Conversely, if z Uτ,T(s)σ (uτ), then there is <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M158','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M158">View MathML</a> such that z = u(t) and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M159','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M159">View MathML</a> so that z = v(t + s) ∈ Uσ(t + s, τ + s, uτ).

Denote by

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M160','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M160">View MathML</a>

Definition 3.4. [6]A set is called a uniform global attractor for the family of multi-valued semiprocesses UΣ if:

(1) it is negatively semiinvariant, i.e., <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M161','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M161">View MathML</a>for all t τ;

(2) it is uniformly attracting, i.e., <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M162','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M162">View MathML</a>, as t → ∞ , for all <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M163','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M163">View MathML</a>and τ ∈ ℝ;

(3) for any closed uniformly attracting set Y, we have <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M164','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M164">View MathML</a>(minimality).

Theorem 3.7. [[6], Theorem 2] Suppose that the family of multi-valued semiprocesses UΣ satisfies the following conditions:

(1) On Σ is defined the continuous shift operator T(s)σ(t) = σ(t + s), s ∈ ℝ such that T(h)Σ ⊂ Σ, and for any (t, τ) ∈ ℝd, σ ∈ Σ, s ∈ ℝ, x X, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M165','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M165">View MathML</a>

(2) Uσ is uniformly asymtopically upper semicompact;

(3) Uσ is pointwise dissipative;

(4) The map (x, σ) ↦ Uσ(t, 0, x) has closed values and is w-upper semicontinuous.

Then the family of multi-valued semiprocesses UΣ has a uniform global compact attractor .

The following is the key point of this subsection.

Lemma 3.8. Let conditions (H1)-(H3) hold and let {un}n∈ℕ is a sequence of weak solutions of (1.1) with respect to the sequence of symbols {σn} ⊂ Σ such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M166','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M166">View MathML</a>

Then there exists a solution u of (1.1) with respect to the symbol σ such that u(τ) = uT and un(t*) → u(t*) in L2(Ω) for any t* > τ, up to a subsequence.

Proof. Let σn = (fn, gn). Since f satisfies (H1) for all t ∈ ℝ and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M167','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M167">View MathML</a>, one sees that fn also satisfies (H1). On the other hand, noting that {un(τ)} is bounded in L2(Ω) and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M168','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M168">View MathML</a>. Thus, repeating the arguments in the proof of Theorem 2.1, we obtain that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M169','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M169">View MathML</a>

In particular, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M170','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M170">View MathML</a>

(3.3)

up to a subsequence. Let <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M171','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M171">View MathML</a> in Σ, to show that u is a solution of (1.1) with respect to the symbol σ such that u(τ) = uT, we need to pass to the limits in the following relation

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M172','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M172">View MathML</a>

for all v V. Since <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M173','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M173">View MathML</a> in L2(τ,T; L2(Ω)), it remains to prove that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M174','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M174">View MathML</a> in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M175','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M175">View MathML</a>. We first show that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M176','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M176">View MathML</a> in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M175','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M175">View MathML</a>. Indeed,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M177','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M177">View MathML</a>

because <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M178','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M178">View MathML</a> in Z and {un} is bounded in Lq(Qτ,T). On the other hand, since <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M179','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M179">View MathML</a> is bounded in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M175','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M175">View MathML</a>, by using Lemma 1.3 in [[21], Chapter 1] and the continuity of <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M180','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M180">View MathML</a> as in the proof of Theorem 2.1, we can conclude that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M181','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M181">View MathML</a> weakly in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M175','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M175">View MathML</a>. Hence, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M182','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M182">View MathML</a>

We now have to show that un(t*) → u(t*) in L2(Ω) for any t* > τ. Taking into account of (3.3), we have to check that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M183','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M183">View MathML</a>.

Putting

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M184','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M184">View MathML</a>

It is easy to check that the functions Jn(t), J(t) are continuous and non-increasing on [τ, T]. We first show that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M185','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M185">View MathML</a>

(3.4)

Indeed,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M186','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M186">View MathML</a>

and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M187','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M187">View MathML</a>

as n → ∞ since un u strongly in L2(Qτ,t) and {gn} is bounded in L2(Qτ,t). In addition,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M188','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M188">View MathML</a>

as n → ∞ since gn g in L2(Qτ,t). Then (3.4) is proved due to the fact that un(t) → u(t) in L2(Ω) for a.e. t ∈ [τ, T].

We choose an increasing sequence {tm} ⊂ [τ, T], tm t* such that Jn(tm) ⇀ J(tm) as n → ∞. Then, by the continuity,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M189','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M189">View MathML</a>

So

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M190','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M190">View MathML</a>

for n n0(ε) and any ε > 0. Hence, lim sup Jn(t*) ≤ J(t*) and then lim sup ∥un(t*)∥ ≤ ∥u(t*)∥. From the weak convergence un(t*) ⇀ u(t*) we have then ∥un(t*)∥ → ∥u(t*)∥, so un(t*) → u(t*) strongly in L2(Ω) as n → ∞. This completes the proof.

Theorem 3.9. Let conditions (H1)-(H3) hold. Then the family of multi-valued semipro-cesses {Uσ (t, τ)} has a uniform global compact attractor .

Proof. We know that each symbol σn = (fn, gn) ∈ Σ satisfies the same conditions as in (H1)-(H2). Furthermore, since <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M191','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M191">View MathML</a>, we have <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M192','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M192">View MathML</a>. Hence if un is a weak solution of (1.1) with respect to the symbol σn, one has

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M193','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M193">View MathML</a>

(3.5)

The last inequality ensures the existence of a positive number R0 such that if un(τ) ∈ BR, the ball in L2(Ω) centered at 0 with radius R, then there exists T0 = T0(τ, R) such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M194','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M194">View MathML</a>

that is, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M195','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M195">View MathML</a>, for all t T0(τ, R). Thus, {Uσ(t, τ)} fulfills condition (3) in Theorem 3.7.

We now define the set <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M196','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M196">View MathML</a>. Lemma 3.8 implies that K is compact. Moreover, since <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M197','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M197">View MathML</a> is an absorbing set, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M198','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M198">View MathML</a>

for all <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M199','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M199">View MathML</a>, and t T0(τ, BR). It follows that any sequence {ξn} such that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M200','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M200">View MathML</a>, is precompact in L2(Ω). It is a consequence of Lemma 3.8 that the map Uσ has compact values for any σ ∈ Σ.

Finally, let us prove that the map (σ, x) ↦ Uσ(t, τ, x) is upper semicontinuous for each fixed t τ. Suppose that it is not true, that is, there exist <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M201','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M201">View MathML</a>, and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M202','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M202">View MathML</a> such that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M203','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M203">View MathML</a>. But Lemma 3.8 implies (up to a subsequence) that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M204','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/35/mathml/M204">View MathML</a>, which is a contracdition. Thus, the existence of the uniform global compact attractor follows then from Theorem 3.7.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

All authors read and approved the final manuscript.

Acknowledgements

This work was supported by Vietnam's National Foundation for Science and Technology Development (NAFOSTED), Project 101.01-2010.05.

The authors would like to thank the reviewers for valuable comments and suggestions.

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