Open Access Research

Global attractor of the extended Fisher-Kolmogorov equation in Hk spaces

Hong Luo

Author Affiliations

College of Mathematics, Sichuan University, Chengdu, Sichuan 610041, PR China

College of Mathematics and Software Science, Sichuan Normal University, Chengdu, Sichuan 610066, PR China

Boundary Value Problems 2011, 2011:39  doi:10.1186/1687-2770-2011-39


The electronic version of this article is the complete one and can be found online at: http://www.boundaryvalueproblems.com/content/2011/1/39


Received:31 May 2011
Accepted:25 October 2011
Published:25 October 2011

© 2011 Luo; 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

The long-time behavior of solution to extended Fisher-Kolmogorov equation is considered in this article. Using an iteration procedure, regularity estimates for the linear semigroups and a classical existence theorem of global attractor, we prove that the extended Fisher-Kolmogorov equation possesses a global attractor in Sobolev space Hk for all k > 0, which attracts any bounded subset of Hk(Ω) in the Hk-norm.

2000 Mathematics Subject Classification: 35B40; 35B41; 35K25; 35K30.

Keywords:
semigroup of operator; global attractor; extended Fisher-Kolmogorov equation; regularity

1 Introduction

This article is concerned with the following initial-boundary problem of extended Fisher-Kolmogorov equation involving an unknown function u = u(x, t):

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M1','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M1">View MathML</a>

(1.1)

where β > 0 is given, Δ is the Laplacian operator, and Ω denotes an open bounded set of Rn(n = 1, 2, 3) with smooth boundary ∂Ω.

The extended Fisher-Kolmogorov equation proposed by Dee and Saarloos [1-3] in 1987-1988, which serves as a model in studies of pattern formation in many physical, chemical, or biological systems, also arises in the theory of phase transitions near Lifshitz points. The extended Fisher-Kolmogorov equation (1.1) have extensively been studied during the last decades. In 1995-1998, Peletier and Troy [4-7] studied spatial patterns, the existence of kinds and stationary solutions of the extended Fisher-Kolmogorov equation (1.1) in their articles. Van der Berg and Kwapisz [8,9] proved uniqueness of solutions for the extended Fisher-Kolmogorov equation in 1998-2000. Tersian and Chaparova [10], Smets and Van den Berg [11], and Li [12] catch Periodic and homoclinic solution of Equation (1.1).

The global asymptotical behaviors of solutions and existence of global attractors are important for the study of the dynamical properties of general nonlinear dissipative dynamical systems. So, many authors are interested in the existence of global attractors such as Hale, Temam, among others [13-23].

In this article, we shall use the regularity estimates for the linear semigroups, combining with the classical existence theorem of global attractors, to prove that the extended Fisher-Kolmogorov equation possesses, in any kth differentiable function spaces Hk(Ω), a global attractor, which attracts any bounded set of Hk(Ω) in Hk-norm. The basic idea is an iteration procedure which is from recent books and articles [20-23].

2 Preliminaries

Let X and X1 be two Banach spaces, X1 X a compact and dense inclusion. Consider the abstract nonlinear evolution equation defined on X, given by

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M2','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M2">View MathML</a>

(2.1)

where u(t) is an unknown function, L: X1 X a linear operator, and G: X1 X a nonlinear operator.

A family of operators S(t): X X(t ≥ 0) is called a semigroup generated by (2.1) if it satisfies the following properties:

(1) S(t): X X is a continuous map for any t ≥ 0,

(2) S(0) = id: X X is the identity,

(3) S(t + s) = S(t) · S(s), ∀t, s ≥ 0. Then, the solution of (2.1) can be expressed as

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M3','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M3">View MathML</a>

Next, we introduce the concepts and definitions of invariant sets, global attractors, and ω-limit sets for the semigroup S(t).

Definition 2.1 Let S(t) be a semigroup defined on X. A set Σ ⊂ X is called an invariant set of S(t) if S(t)Σ = Σ, ∀t ≥ 0. An invariant set Σ is an attractor of S(t) if Σ is compact, and there exists a neighborhood U X of Σ such that for any u0 U,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M4','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M4">View MathML</a>

In this case, we say that Σ attracts U. Especially, if Σ attracts any bounded set of X, Σ is called a global attractor of S(t) in X.

For a set D X, we define the ω-limit set of D as follows:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M5','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M5">View MathML</a>

where the closure is taken in the X-norm. Lemma 2.1 is the classical existence theorem of global attractor by Temam [17].

Lemma 2.1 Let S(t): X X be the semigroup generated by (2.1). Assume the following conditions hold:

(1) S(t) has a bounded absorbing set B X, i.e., for any bounded set A X there exists a time tA ≥ 0 such that S(t)u0 B, ∀u0 A and t > tA;

(2) S(t) is uniformly compact, i.e., for any bounded set U X and some T > 0 sufficiently large, the set <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M6','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M6">View MathML</a> is compact in X.

Then the ω-limit set <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M7','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M7">View MathML</a> of B is a global attractor of (2.1), and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M8','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M8">View MathML</a> is connected providing B is connected.

Note that we used to assume that the linear operator L in (2.1) is a sectorial operator which generates an analytic semigroup etL. It is known that there exists a constant λ ≥ 0 such that L - λI generates the fractional power operators <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M9','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M9">View MathML</a> and fractional order spaces Xα for α R1, where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M10','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M10">View MathML</a>. Without loss of generality, we assume that L generates the fractional power operators <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M9','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M9">View MathML</a> and fractional order spaces Xα as follows:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M11','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M11">View MathML</a>

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M12','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M12">View MathML</a> is the domain of <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M9','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M9">View MathML</a>. By the semigroup theory of linear operators [24], we know that Xβ Xα is a compact inclusion for any β > α.

Thus, Lemma 2.1 can equivalently be expressed in Lemma 2.2 [20-23].

Lemma 2.2 Let u(t, u0) = S(t)u0(u0 X, t ≥ 0) be a solution of (2.1) and S(t) be the semigroup generated by (2.1). Let Xα be the fractional order space generated by L. Assume:

(1) for some α ≥ 0, there is a bounded set B Xα such that for any u0 Xα there exists <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M13','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M13">View MathML</a> with

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M14','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M14">View MathML</a>

(2) there is a β > α, for any bounded set U Xβ there are T > 0 and C > 0 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M15','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M15">View MathML</a>

Then, Equation (2.1) has a global attractor <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M16','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M16">View MathML</a> which attracts any bounded set of Xα in the Xα-norm.

For Equation (2.1) with variational characteristic, we have the following existence theorem of global attractor [20,22].

Lemma 2.3 Let L: X1 X be a sectorial operator, Xα = D((-L)α) and G: Xα X(0 < α < 1) be a compact mapping. If

(1) there is a functional F: Xα R such that DF = L + G and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M17','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M17">View MathML</a>,

(2) <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M18','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M18">View MathML</a>,

then

(1) Equation (2.1) has a global solution

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M19','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M19">View MathML</a>

(2) Equation (2.1) has a global attractor <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M20','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M20">View MathML</a> which attracts any bounded set of X, where DF is a derivative operator of F, and β1, β2, C1, C2 are positive constants.

For sectorial operators, we also have the following properties which can be found in [24].

Lemma 2.4 Let L: X1 X be a sectorial operator which generates an analytic semigroup T(t) = etL. If all eigenvalues λ of L satisfy Reλ < -λ0 for some real number λ0 > 0, then for <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M21','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M21">View MathML</a> we have

(1) T(t): X Xα is bounded for all α R1 and t > 0,

(2) <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M22','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M22">View MathML</a>,

(3) for each t > 0, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M23','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M23">View MathML</a> is bounded, and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M24','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M24">View MathML</a>

where δ > 0 and Cα > 0 are constants only depending on α,

(4) the Xα-norm can be defined by

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M25','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M25">View MathML</a>

(2.2)

(5) if <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M26','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M26">View MathML</a> is symmetric, for any α, β R1 we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M27','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M27">View MathML</a>

3 Main results

Let H and H1 be the spaces defined as follows:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M28','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M28">View MathML</a>

(3.1)

We define the operators L: H1 H and G: H1 H by

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M29','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M29">View MathML</a>

(3.2)

Thus, the extended Fisher-Kolmogorov equation (1.1) can be written into the abstract form (2.1). It is well known that the linear operator L: H1 H given by (3.2) is a sectorial operator and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M30','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M30">View MathML</a>. The space D(-L) = H1 is the same as (3.1), <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M31','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M31">View MathML</a> is given by <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M31','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M31">View MathML</a> = closure of H1 in H2(Ω) and Hk = H2k(Ω) ∩ H1 for k ≥ 1.

Before the main result in this article is given, we show the following theorem, which provides the existence of global attractors of the extended Fisher-Kolmogorov equation (1.1) in H.

Theorem 3.1 The extended Fisher-Kolmogorov equation (1.1) has a global attractor in H and a global solution

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M32','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M32">View MathML</a>

Proof. Clearly, L = -βΔ2 + Δ: H1 H is a sectorial operator, and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M33','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M33">View MathML</a> is a compact mapping.

We define functional <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M34','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M34">View MathML</a>, as

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M35','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M35">View MathML</a>

which satisfies DI(u) = Lu + G(u).

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M36','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M36">View MathML</a>

(3.3)

which implies condition (1) of Lemma 2.3.

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M37','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M37">View MathML</a>

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M38','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M38">View MathML</a>

(3.4)

which implies condition (2) of Lemma 2.3.

This theorem follows from (3.3), (3.4), and Lemma 2.3.

The main result in this article is given by the following theorem, which provides the existence of global attractors of the extended Fisher-Kolmogorov equation (1.1) in any kth-order space Hk.

Theorem 3.2 For any α ≥ 0 the extended Fisher-Kolmogorov equation (1.1) has a global attractor <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M8','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M8">View MathML</a> in Hα, and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M8','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M8">View MathML</a> attracts any bounded set of Hα in the Hα-norm.

Proof. From Theorem 3.1, we know that the solution of system (1.1) is a global weak solution for any φ H. Hence, the solution u(t, φ) of system (1.1) can be written as

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M39','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M39">View MathML</a>

(3.5)

Next, according to Lemma 2.2, we prove Theorem 3.2 in the following five steps.

Step 1. We prove that for any bounded set <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M40','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M40">View MathML</a> there is a constant C > 0 such that the solution u(t, φ) of system (1.1) is uniformly bounded by the constant C for any φ U and t ≥ 0. To do that, we firstly check that system (1.1) has a global Lyapunov function as follows:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M41','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M41">View MathML</a>

(3.6)

In fact, if u(t, ·) is a strong solution of system (1.1), we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M42','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M42">View MathML</a>

(3.7)

By (3.2) and (3.6), we get

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M43','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M43">View MathML</a>

(3.8)

Hence, it follows from (3.7) and (3.8) that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M44','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M44">View MathML</a>

(3.9)

which implies that (3.6) is a Lyapunov function.

Integrating (3.9) from 0 to t gives

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M45','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M45">View MathML</a>

(3.10)

Using (3.6), we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M46','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M46">View MathML</a>

Combining with (3.10) yields

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M47','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M47">View MathML</a>

which implies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M48','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M48">View MathML</a>

(3.11)

where C1, C2, and C are positive constants, and C only depends on φ.

Step 2. We prove that for any bounded set <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M49','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M49">View MathML</a> there exists C > 0 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M50','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M50">View MathML</a>

(3.12)

By <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M51','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M51">View MathML</a>, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M52','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M52">View MathML</a>

which implies that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M53','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M53">View MathML</a> is bounded.

Hence, it follows from (2.2) and (3.5) that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M54','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M54">View MathML</a>

where β = α(0 < β < 1). Hence, (3.12) holds.

Step 3. We prove that for any bounded set <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M55','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M55">View MathML</a> there exists C > 0 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M56','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M56">View MathML</a>

(3.13)

In fact, by the embedding theorems of fractional order spaces [24]:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M57','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M57">View MathML</a>

we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M58','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M58">View MathML</a>

which implies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M59','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M59">View MathML</a>

(3.14)

Therefore, it follows from (3.12) and (3.14) that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M60','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M60">View MathML</a>

(3.15)

Then, using same method as that in Step 2, we get from (3.15) that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M61','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M61">View MathML</a>

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M62','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M62">View MathML</a>. Hence, (3.13) holds.

Step 4. We prove that for any bounded set U Hα(α ≥ 0) there exists C > 0 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M63','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M63">View MathML</a>

(3.16)

In fact, by the embedding theorems of fractional order spaces [24]:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M64','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M64">View MathML</a>

we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M65','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M65">View MathML</a>

which implies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M66','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M66">View MathML</a>

(3.17)

Therefore, it follows from (3.13) and (3.17) that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M67','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M67">View MathML</a>

(3.18)

Then, we get from (3.18) that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M68','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M68">View MathML</a>

where β = α - 1(0 < β < 1). Hence, (3.16) holds.

By doing the same procedures as Steps 1-4, we can prove that (3.16) holds for all α ≥ 0.

Step 5. We show that for any α ≥ 0, system (1.1) has a bounded absorbing set in Hα. We first consider the case of <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M69','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M69">View MathML</a>.

From Theorem 3.1 we have known that the extended Fisher-Kolmogorov equation possesses a global attractor in H space, and the global attractor of this equation consists of equilibria with their stable and unstable manifolds. Thus, each trajectory has to converge to a critical point. From (3.9) and (3.16), we deduce that for any <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M70','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M70">View MathML</a> the solution u(t, φ) of system (1.1) converges to a critical point of F. Hence, we only need to prove the following two properties:

(1) <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M71','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M71">View MathML</a>,

(2) the set <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M72','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M72">View MathML</a> is bounded.

Property (1) is obviously true, we now prove (2) in the following. It is easy to check if DF(u) = 0, u is a solution of the following equation

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M73','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M73">View MathML</a>

(3.19)

Taking the scalar product of (3.19) with u, then we derive that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M74','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M74">View MathML</a>

Using Hölder inequality and the above inequality, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M75','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M75">View MathML</a>

where C > 0 is a constant. Thus, property (2) is proved.

Now, we show that system (1.1) has a bounded absorbing set in Hα for any <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M76','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M76">View MathML</a>, i.e., for any bounded set U Hα there are T > 0 and a constant C > 0 independent of φ such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M77','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M77">View MathML</a>

(3.20)

From the above discussion, we know that (3.20) holds as <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M69','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M69">View MathML</a>. By (3.5) we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M78','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M78">View MathML</a>

(3.21)

Let <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M79','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M79">View MathML</a> be the bounded absorbing set of system (1.1), and T0 > 0 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M80','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M80">View MathML</a>

(3.22)

It is well known that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M81','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M81">View MathML</a>

where λ1 > 0 is the first eigenvalue of the equation

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M82','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M82">View MathML</a>

Hence, for any given T > 0 and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M83','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M83">View MathML</a>. We have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M84','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M84">View MathML</a>

(3.23)

From (3.21),(3.22) and Lemma 2.4, for any <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M85','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M85">View MathML</a> we get that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M86','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M86">View MathML</a>

(3.24)

where C > 0 is a constant independent of φ.

Then, we infer from (3.23) and (3.24) that (3.20) holds for all <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M87','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M87">View MathML</a>. By the iteration method, we have that (3.20) holds for all <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M76','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/39/mathml/M76">View MathML</a>.

Finally, this theorem follows from (3.16), (3.20) and Lemma 2.2. The proof is completed.

Competing interests

The author declares that they have no competing interests.

Acknowledgements

The author is very grateful to the anonymous referees whose careful reading of the manuscript and valuable comments enhanced presentation of the manuscript. Foundation item: the National Natural Science Foundation of China (No. 11071177).

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