This paper investigates the prescribed-time event-triggered cluster practical consensus problem for a class of nonlinear multi-agent systems with external disturbances. To begin, to reach the prescribed-time cluster practical consensus, a new time-varying function is introduced and a novel distributed continuous algorithm is designed. Based on the Lyapunov stability theory and inequality techniques, some sufficient conditions are given, ensuring the prescribed-time cluster practical consensus. Moreover, to avoid different clusters' final states overlapping, a virtual leader is considered for each cluster. In this case, an event-triggered distributed protocol is further established and some related conditions are given for achieving prescribed-time cluster practical consensus. Additionally, it is proven that the Zeno behavior can be avioded by choosing parameters appropriately. Finally, some numerical examples are presented to show the effectiveness of the theoretical results.
Citation: Wangming Lu, Zhiyong Yu, Zhanheng Chen, Haijun Jiang. Prescribed-time cluster practical consensus for nonlinear multi-agent systems based on event-triggered mechanism[J]. Mathematical Biosciences and Engineering, 2024, 21(3): 4440-4462. doi: 10.3934/mbe.2024196
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This paper investigates the prescribed-time event-triggered cluster practical consensus problem for a class of nonlinear multi-agent systems with external disturbances. To begin, to reach the prescribed-time cluster practical consensus, a new time-varying function is introduced and a novel distributed continuous algorithm is designed. Based on the Lyapunov stability theory and inequality techniques, some sufficient conditions are given, ensuring the prescribed-time cluster practical consensus. Moreover, to avoid different clusters' final states overlapping, a virtual leader is considered for each cluster. In this case, an event-triggered distributed protocol is further established and some related conditions are given for achieving prescribed-time cluster practical consensus. Additionally, it is proven that the Zeno behavior can be avioded by choosing parameters appropriately. Finally, some numerical examples are presented to show the effectiveness of the theoretical results.
In this paper, we are interested in the existence and some asymptotic properties of ground state homoclinic orbits for the following first-order Hamiltonian system
˙z=JHz(t,z), | (1.1) |
with the Hamiltonian function
H(t,z)=12Lz⋅z+A(ϵt)G(|z|). | (1.2) |
Here, z=(u,v)∈RN×RN=R2N, ϵ>0 is a parameter, L is a symmetric 2N×2N matrix-valued function, and
J=(0−II0). |
is the usual symplectic matrix with I being the identity matrix in RN. As usual, we refer to a solution z of system (1.1) as a homoclinic orbit if z(t)≢0 and z(t)→0 as |t|→∞.
It is widely known that Hamiltonian systems are very important dynamical systems, which have many applications in several natural science areas such as relativistic mechanics, celestial mechanics, gas dynamics, chemical kinetics, optimization and control theory and so on. The complicated dynamical behavior of Hamiltonian systems has the attracted attention of many mathematicians and physicists ever since Newton wrote down the differential equations describing planetary motions and derived Kepler's ellipses as solutions. For more details on applications of Hamiltonian systems, one can refer to [1] and the monograph [2] of Mawhin and Willem. We also refer the readers to see [3] for the Stokes-Dirac structures of the port-Hamiltonian systems.
In the past few decades, Hamiltonian systems have attracted a considerable amount of interest due to many powerful applications in different fields; the literature related to these systems is extensive and encompasses several interesting lines of research on the topic of nonlinear analysis, including the existence, nonexistence, multiplicity and finer qualitative properties of homoclinic orbits. Here we cannot provide a complete and fully detailed list of references, but rather we limit ourselves to mentioning the works which are closely related to the content of the present paper.
A major breakthrough was the pioneering paper of Rabinowitz [4] from 1978 who, for the first time, obtained periodic solutions of system (1.1) by using variational methods. After the celebrated work of Rabinowitz [4], based on the dual action and mountain pass argument, Coti-Zelati et al. [5] obtained the existence and multiplicity of homoclinic orbits under the condition of strictly convexity. Later on, this result was further detailed by [6,7] in which the authors established the existence result for infinitely many homoclinic orbits. Without the convexity condition, Hofer and Wysocki [8] independently investigated the existence of homoclinic orbits by combining the Fredholm operator theory and the linking argument. Tanaka [9] employed a suitable subharmonic approach to obtain one homoclinic orbit by relaxing the convexity condition. In [10], Rashkovskiy studied the quantization process of Hamiltonian and non-Hamiltonian systems.
The main unusual feature of the first-order Hamiltonian system is that the associated energy functional is strongly indefinite. Generally speaking, for the strongly indefinite functionals refined variational methods like the Nehari manifold method and mountain pass theorem still do not apply. Some general critical point theories like the generalized linking theorem and other weaker versions for strongly indefinite functionals were subsequently developed by Kryszewski and Szulkin in [11] and Bartsch and Ding in [12]. Since then, based on the critical point theorems from [11,12] for strongly indefinite functionals, many scholars have gradually begun to investigate the existence and multiplicity of homoclinic orbits for non-autonomous Hamiltonian systems under some different conditions. More precisely, under the conditions that H depends periodically on t and has super-quadratic growth in z, Arioli and Szulkin [13], Chen and Ma [14], and Ding and Willem [15] obtained the existence result. Ding [16], Ding and Girardi [17], and Zhang et al. [18] studied the multiplicity result for homoclinic orbits. Concerning the asymptotic quadratic growth case, we refer to the work done by Szulkin and Zou [19] and Sun et al. [20]. Here we would like to emphasize that the periodicity condition is used to resolve the issue stemming from the lack of compactness since system (1.1) is set on the whole space R.
On the other hand, without the condition of periodicity, the non-periodic problem is quite different due to the lack of compactness of Sobolev embeddings. In an early paper [21], Ding and Li utilized the coercive property of L to establish a variational framework with compactness, and they proved the existence of homoclinic orbits for the super-quadratic growth case. Also under the framework of compactness, Zhang and Liu [22] studied the sub-quadratic growth case. Regarding the asymptotically quadratic case, based on the infinite-dimensional linking argument, Ding and Jeanjean [23] established a multiplicity result for homoclinic orbits. Moreover, they imposed a control on the size of G with respect to the behavior of L to recover sufficient compactness. For the existence and exponential decay of homoclinic orbits for system (1.1) with nonperiodic super-quadratic and lack of compactness, we refer the reader to [24]. We also mention the recent paper by Zhang et al. [25] in which the existence and decay of ground state homoclinic orbits for system (1.1) with asymptotic periodicity are explored. For other results related to the Hamiltonian systems with strongly variational structure, we refer the reader to [26,27,28,29,30,31,32] and the references therein.
It is worth pointing out that, in all of the works mentioned above, the authors were concerned mainly with the study of the existence and multiplicity of homoclinic orbits, and there are no papers considering the asymptotic properties of homoclinic orbits. Inspired by this fact and the work done by Alves and Germano [33] in which the authors investigated the existence and concentration of ground state solutions for the Schrödinger equation; in the present paper we aim to further study the existence and some asymptotic properties of ground-state homoclinic orbits for system (1.1) with Hamiltonian function (1.2). This is a very interesting issue that has motivated the present work.
To continue the discussion, we introduce the following notation
S=−(Jddt+L), |
then, system (1.1) takes the following form
Sz=A(ϵt)g(|z|)z,t∈R. | (1.3) |
Before stating our results, we suppose the following conditions hold for L, A and G.
(L) L is a constant symmetric 2N×2N matrix such that σ(JL)∩iR=∅, where σ denotes the spectrum of operator JL.
(A) A∈C(R,R) and 0<inft∈RA(t)≤A∞:=lim|t|→∞A(t)<A(0)=maxt∈RA(t);
(g1) Gz(|z|)=g(|z|)z, g∈C(R+,R+), and there exist p>2 and c0>0 such that
|g(s)|≤c0(1+|s|p−2)for alls∈R+; |
(g2) g(s)=o(1) as s→0, and G(s)/s2→+∞ as s→+∞;
(g3) g(s) is strictly increasing in s on (0,+∞).
Next we state the main result of this paper as follows.
Theorem 1.1. Assume that conditions (L), (A), and (g1)-(g3) hold. Then we have the following results:
(a) there exists ϵ0>0 such that system (1.1) has a ground state homoclinic orbit zϵ for each ϵ∈(0,ϵ0);
(b) |zϵ| attains its maximum at tϵ, then,
limϵ→0A(ϵtϵ)=A(0), |
moreover, zϵ(t+tϵ)→z as ϵ→0, where z is a ground state homoclinic orbit of the limit system
Sz=A(0)g(|z|)z,t∈R; |
(c) additionally, if A,g∈C1, and g′(s)s=o(1) as s→0, then there exist constants c,C>0 such that
|z(t)|≤Cexp(−c|t−tϵ|)for allt∈R. |
We would like to emphasize that since our problem is carried out in the whole space, then the strongly indefiniteness of energy functionals and the lack of compactness are two major difficulties that we encounter in order to guarantee the existence of homoclinic orbits. More precisely, one reason is that strongly indefinite functionals are unbounded from below and from above so that the classical methods from the calculus of variations do not apply. The other reason is that the lack of compactness leads to the energy functionals not satisfying the necessary compactness property.
Let us now outline the methods involved to prove Theorem 1.1. Indeed, based on the above reasons, first, we will take advantage of the method of the generalized Nehari manifold developed by Szulkin-Weth [34] to handle system (1.1), this is because such a strategy helps to overcome the difficulty caused by strongly indefinite features. Second, we must verify that the energy functional possesses the necessary compactness property at some energy level. This target will be accomplished by applying the energy comparison argument to establish some precise comparison relationships for the ground-state energy value between the original problem and certain auxiliary problems. Finally, combining the compactness analysis technique, Kato's inequality, and the sub-solution estimate, we can obtain the concentration property and decay of homoclinic orbits. Then Theorem 1.1 follows naturally.
This paper is organized as follows. In Section 2, we establish the functional analytic setting associated with system (1.1). In Section 3, we present some technical results, and obtain the existence result for ground-state homoclinic orbits for the autonomous system. Section 4 is devoted to proofs of Theorem 1.1.
Throughout the present paper, we will use the following notations:
● ‖⋅‖s denotes the norm of the Lebesgue space Ls(R), 1≤s≤+∞;
● (⋅,⋅)2 denotes the usual inner product of the space L2(R);
● c, ci, Ci represent various different positive constants.
In what follows, we will establish the variational framework to work for system (1.1).
Recall that S=−(Jddt+L) is a self-adjoint operator on the space L2:=L2(R,R2N) with the domain D(S)=H1(R,R2N); according to the discussion in [13], we can know that, under the condition (L), there exists a>0 such that (−a,a)∩σ(S)=∅ (see also [16,19]). Therefore, the space L2 has the following orthogonal decomposition
L2=L−⊕L+,z=z−+z+ |
corresponding to the spectrum decomposition of S such that S is positive definite (resp. negative definite) in L+ (resp. L−).
We use |S| to denote the absolute value of S, and |S|1/2 denotes the square root of |S|. Let E=D(|S|1/2) be the domain of the self-adjoint operator |S|1/2, which is a Hilbert space equipped with the following inner product
(z,w)=(|S|1/2z,|S|1/2w)2, for z,w∈E, |
and the norm ‖z‖2=(z,z). Evidently, E possesses the following decomposition
E=E−⊕E+,whereE±=E∩L±, |
which is orthogonal with respect to the two inner products (⋅,⋅)2 and (⋅,⋅). Moreover, by using the polar decomposition of S we can obtain that
Sz−=−|S|z−, Sz+=|S|z+for allz=z++z−∈E. |
Furthermore, from [16] we have the embedding theorem, that is, E embeds continuously into Lq for each q≥2 and compactly into Lqloc for all q≥1. Hence, there exists a constant γq>0 such that for all z∈E
‖z‖q≤γq‖z‖for allq≥2. | (2.1) |
From the assumptions (g1) and (g2), we can deduce that for any ϵ>0, there exists a positive constant Cϵ such that
|g(s)|≤ϵ+Cϵ|s|p−2and|G(s)|≤ϵ|s|2+Cϵ|s|pfor eachs∈R+. | (2.2) |
Next, we define the corresponding energy functional of system (1.3) on E by
Iϵ(z)=12∫RSz⋅zdt−∫RA(ϵt)G(|z|)dt |
Applying the polar decomposition of S, then the energy functional Iϵ has another representation as follows
Iϵ(z)=12(‖z+‖2−‖z−‖2)−∫RA(ϵt)G(|z|)dt. |
Evidently, according to condition (L), we can see that Iϵ is strongly indefinite. Furthermore, from conditions (g1) and (g2) we can infer that Iϵ∈C1(E,R), and we have
⟨I′ϵ(z),ψ⟩=(z+,ψ+)−(z−,ψ−)−∫RA(ϵt)g(|z|)zψdt,∀ψ∈E. |
Making use of a standard argument we can check that critical points of Iϵ are homoclinic orbits of system (1.1).
We shall make use of the techniques of the limit problem to prove the main results; in this section we introduce some related results for the autonomous system.
For any constant μ>0, in what follows we consider the autonomous system given by
Sz=μg(|z|)z,t∈R. | (3.1) |
Similarly, following the above comments, we define the energy functional Iμ corresponding to system (3.1) as follows
Iμ(z)=12(‖z+‖2−‖z−‖2)−μ∫RG(|z|)dt. |
Evidently, we have
⟨I′μ(z),ψ⟩=(z+,ψ+)−(z−,ψ−)−μ∫Rg(|z|)zψdt,∀ψ∈E. |
In order to obtain the ground state homoclinic orbits of system (3.1), we will use the method of the generalized Nehari manifold developed by Szulkin and Weth [34]. To do this, we first introduce the following generalized Nehari manifold
Nμ={z∈E∖E−:⟨I′μ(z),z⟩=0and⟨I′μ(z),v⟩=0,∀v∈E−}, |
and we define the ground state energy value cμ of Iμ on Nμ
cμ=infz∈NμIμ(z). |
Furthermore, for every z∈E∖E−, we also need to define the subspace
E(z)=E−⊕Rz=E−⊕Rz+, |
and the convex subset
ˆE(z)=E−⊕[0,+∞)z=E−⊕[0,+∞)z+. |
We note that E(z) and ˆE(z) do not depend on μ, but depend on the operator S.
The main result in this section is the following theorem:
Theorem 3.1. Assume that condition (L) holds and let (g1)-(g3) be satisfied, then, problem (3.1) has at least a ground state homoclinic orbit z∈Nμ such that Iμ(z)=cμ>0.
In this subsection, we are going to prove some technical results which will be used in the proof of Theorem 3.1. The following result involves the translation that will be used frequently in this paper, the proof can be found in [33, Lemma 2.1].
Lemma 3.1. For all u=u++u−∈E and y∈R, if v(t):=u(t+y), then v∈E with v+(t)=u+(t+y) and v−(t)=u−(t+y).
We give an important estimate, which plays a crucial role in the later proof.
Lemma 3.2. Let z∈Nμ, then, for each v∈H:={sz+w:s≥−1,w∈E−} and v≠0, we have the following energy estimate
Iμ(z+v)<Iμ(z). |
Hence z is a unique global maximum of Iμ|ˆE(z).
Proof. We follow the similar ideas explored in [34, Proposition 2.3.] to prove this lemma. Observe that, for any z∈Nμ, we directly obtain
0=⟨I′μ(z),φ⟩=(Az,φ)2−μ∫Rg(|z|)zφdtfor allφ∈E(z). |
Let v=sz+w∈H, then, z+v=(1+s)z+w∈ˆE(z). By an elemental computation, we can get
Iμ(z+v)−Iμ(z)=12[(A(z+v),(z+v))2−(Az,z)2]+μ∫R[G(|z|)−G(|z+v|)]dt=12[(A((1+s)z+w),(1+s)z+w)2−(Az,z)2]+μ∫R[G(|z|)−G(|z+v|)]dt=−‖w‖22+(Az,s(s2+1)z+(1+s)w)2+μ∫R[G(|z|)−G(|z+v|)]dt=−‖w‖22+μ∫R[g(|z|)z⋅(s(s2+1)z(t)+(1+s)w(t))+G(|z|)−G(|z+v|)]dt=−‖w‖22+μ∫R˜g(s,z,v)dt, |
where
˜g(s,z,v)=g(|z|)z⋅(s(s2+1)z(t)+(1+s)w(t))+G(|z|)−G(|z+v|). |
Using (g2) and (g3) and combining the arguments used in [35] (see also [36]), we can conclude that ˜g(s,z,v)<0. Therefore, we have
Iμ(z+v)<Iμ(z). |
Evidently, we know that z is a unique global maximum of Iμ|ˆE(z).
Lemma 3.3. Assume that (g1) and (g2) are satisfied, then, we have the following two conclusions:
(i) there exists ρ>0 such that cμ=infNμIμ≥infSρIμ>0, where
Sρ:={z∈E+:‖z‖=ρ}; |
(ii) for any z∈Nμ, then ‖z+‖2≥max{‖z−‖2,2cμ}>0.
Proof. (ⅰ) Let z∈E+, we can deduce from (2.1) and (2.2) that
Iμ(z)=12‖z‖2−μ∫RG(|z|)dt≥(12−ϵμγ22)‖z‖2−μγppCϵ‖z‖p. |
Evidently, we can see that there is ρ>0, for ‖z‖=ρ small enough such that infSρIμ>0.
On the other hand, for each z∈Nμ, there exists a positive constant s such that s‖z‖=ρ, then sz∈ˆE(z)∩Sρ. From Lemma 3.2, one can derive that
Iμ(z)=maxv∈ˆE(z)Iμ(v)≥Iμ(sz). |
Therefore, we have
infNμIμ≥infSρIμ>0, |
which shows that the conclusion (ⅰ) holds.
(ⅱ) First we note that, from (g3), it follows that
12g(s)s2≥G(s)>0 for all s∈R+. |
For each z∈Nμ, combining this with the definition of cμ, we have
0<cμ≤12‖z+‖2−12‖z−‖2−μ∫RG(|z|)dt≤12‖z+‖2−12‖z−‖2. |
Hence, we can derive that ‖z+‖2≥max{‖z−‖2,2cμ}>0. The proof is finished.
Lemma 3.4. Assume that Ω⊂E+∖{0} is a compact subset, thus, there exists R>0 such that Iμ<0 on E(z)∖BR(0) for all z∈Ω.
Proof. The proof follows as in [34, Lemma 2.5], here, we omit the details.
Following the result in [34] (see [34, Lemma 2.6]), we can establish the uniqueness of maximum point of Iμ on the set ˆE(z).
Lemma 3.5. For any z∈E∖E−, then the set Nμ∩ˆE(z) consists of precisely one point ˜mμ(z)≠0, which is the unique global maximum of Iμ|ˆE(z).
Proof. On account of Lemma 3.2, it is sufficient to prove that Nμ∩ˆE(z)≠∅. Since ˆE(z)=ˆE(z+), without loss of generality, we can suppose that z∈E+ and ‖z‖=1. By Lemma 3.3-(ⅰ), we obtain that Iμ(sz)>0 for s∈(0,+∞) small enough. Lemma 3.4 yields that Iμ(sz)<0 for sz∈ˆE(z)∖BR(0). Consequently, we can deduce that 0<supIμ(ˆE(z))<∞. Because ˆE(z) is convex and the functional Iμ is weakly supper semi-continuous on ˆE(z), we conclude that there exists ˆz∈ˆE(z) such that Iμ(ˆz)=supIμ(ˆE(z)). This shows that ˆz is a critical point of Iμ|ˆE(z); therefore,
⟨I′μ(ˆz),ˆz⟩=⟨I′μ(ˆz),φ⟩=0for allφ∈ˆE(z), |
hence, ˆz∈Nμ. So, ˆz∈Nμ∩ˆE(z). The proof is finished.
Combining Lemma 3.2 with Lemma 3.5, we obtain the following conclusion.
Lemma 3.6. For each z∈E∖E−, then, there is a unique pair (s∗,φ∗) with s∗∈(0,+∞) and φ∗∈E− such that s∗z+φ∗∈Nμ∩ˆE(z) and
Iμ(s∗z+φ∗)=maxw∈ˆE(z)Iμ(w). |
Moreover, if z∈Nμ, then we have that s∗=1 and φ∗=z−.
Lemma 3.7. Iμ is coercive on Nμ, that is, Iμ(z)→+∞ as ‖z‖→+∞, z∈Nμ.
Proof. Seeking for a contradiction, assume that there exists {zn}⊂Nμ such that
Iμ(zn)≤ˆcfor someˆc∈[cμ,+∞)as‖zn‖→+∞. |
Setting wn:=zn/‖zn‖, we obtain that ‖z+n‖≥‖z−n‖ from Lemma 3.3(ⅱ), then, for every n∈N, we get that ‖w+n‖2≥‖w−n‖2 and ‖w+n‖2≥12. There exist {yn}⊂Z, r>0 and δ>0 such that
∫Br(yn)|w+n|2dt≥δ,∀n∈N. | (3.2) |
If this is not true, then according to Lions' concentration-compactness principle, we can conclude that w+n→0 in Lq(R) for q>2. Combining (2.1) and (2.2), we know that, for every s>0,
μ∫RG(|sw+n|)dt≤ϵμγ22s2‖w+n‖2+μCϵγppsp‖w+n‖p→0, |
then we get
ˆc≥Iμ(sw+n)=12s2‖w+n‖2−μ∫RG(|sw+n|)dt≥s24−μ∫RG(|sw+n|)dt→s24. |
This yields a contradiction if s>√4ˆc; hence we prove that (3.2) holds.
Let us define ˜zn(t):=zn(t+yn), and ˜wn(t):=wn(t+yn), then, ˜w+n⇀˜w+, and (3.2) yields that ˜w+≠0. Since ˜zn(t)=˜wn(t)‖˜zn‖, it follows that ˜zn(t)→+∞ almost everywhere in R as ‖˜zn‖=‖zn‖→+∞. Applying the Fatou's lemma, we can derive that
∫RG(|zn|)‖zn‖2dt=∫RG(|˜zn|)‖˜zn‖2dt=∫RG(|˜zn|)|˜zn|2|˜wn|2dt≥∫[˜zn≠0]G(|˜zn|)|˜zn|2|˜wn|2dt→+∞, |
where [˜zn≠0] is the Lebesgue measure of the set {t∈R:˜zn(t)≠0}. Therefore
0≤Iμ(zn)‖zn‖2=12‖w+n‖2−12‖w−n‖2−μ∫RG(|zn|)‖zn‖2dt≤12−μ∫RG(|˜zn|)|˜zn|2|˜wn|2dt→−∞, |
we get a contradiction. The proof is finished.
We want to utilize the method of the generalized Nehari manifold to prove the main result. To do this, we set S+:={z∈E+:‖z‖=1} in E+, and we define the following mapping
˜mμ:E+∖{0}→Nμandmμ=˜mμ|S+, |
and the inverse of mμ is
m−1μ:Nμ→S+,m−1μ(z)=z+/‖z+‖. |
Following from the proof of [34, Lemma 2.8], we can see that ˜mμ is continuous and mμ is a homeomorphism.
We now consider the reduced functionals
˜Φμ(z)=Iμ(˜mμ(z))andΦμ=˜Φμ|S+. |
which is continuous since ˜mμ is continuous.
The following results establish some crucial properties involving the reduced functionals ˜Φμ and Φμ, which play important roles in our arguments. And their proofs follow the proofs of [34, Proposition 2.9, Corollary 2.10].
Lemma 3.8. The following conclusions are true:
(a) ˜Φμ∈C1(E+∖{0},R) and for z,v∈E+ and z≠0,
⟨˜Φ′μ(z),v⟩=‖˜mμ(z)+‖‖z‖⟨I′μ(˜mμ(z)),v⟩. |
(b) Φμ∈C1(S+,R) and for each z∈S+ and v∈Tz(S+)={u∈E+:(z,u)=0},
⟨Φ′μ(z),v⟩=‖˜mμ(z)+‖⟨I′μ(˜mμ(z)),v⟩. |
(c) {zn} is a (PS)-sequence for Φμ if and only if {˜mμ(zn)} is a (PS)-sequence for Iμ.
(d) We have
infS+Φμ=infNμIμ=cμ. |
Moreover, z∈S+ is a critical point of Φμ if and only if ˜mμ(z)∈Nμ is a critical point of Iμ and the corresponding critical values coincide.
Based on the above preliminaries, in this subsection we give the complete proof of Theorem 3.1, and further study the monotonicity and continuity of the ground-state energy cμ.
Proof of Theorem 3.1: According to Lemma 3.3, it is easy to see that cμ>0. We note that, if z∈Nμ with Iμ(z)=cμ, then m−1μ(z)∈S+ is a minimizer of Φμ; hence, it is a critical point of Φμ. Consequently, Lemma 3.8 yields that z is a critical point of Iμ. We have to prove that there exists a minimizer ˜z∈Nμ such that Iμ(˜z)=cμ. Actually, Ekeland's variational principle yields that there exists a sequence {vn}⊂S+ such that Φμ(vn)→cμ and Φ′μ(vn)→0 as n→∞. For all n∈N, setting zn=˜mμ(vn)∈Nμ, then Iμ(zn)→cμ and I′μ(zn)→0 by Lemma 3.8. By virtue of Lemma 3.7, we can see that {zn} is bounded in E. Moreover, it satisfies
lim_n→∞supy∈R∫B1(y)|zn|2dt>0. |
If this is not true, then Lions' concentration-compactness principle implies that zn→0 in Lq(R) for any q>2. Therefore, from (2.1) and (2.2), we can derive that
∫R[12g(|zn|)|zn|2−G(|zn|)]dt=on(1). |
Then, we get
cμ+on(1)=Iμ(zn)−12⟨I′μ(zn),zn⟩=μ∫R[12g(|zn|)|zn|2−G(|zn|)]dt=on(1). |
Since cμ>0, obviously we get a contradiction. Thus, there exist {yn}⊂Z and δ>0 such that
∫B2(yn)|zn|2dt≥δ. |
Let us define ˜zn(t)=zn(t+yn), then, we have
∫B2(0)|˜zn|2dt≥δ. | (3.3) |
Observe that Iμ is the invariant under translation since (3.1) is autonomous, then, we have ‖˜zn‖=‖zn‖ and
Iμ(˜zn)→cμandI′μ(˜zn)→0. | (3.4) |
Passing to a subsequence, we may suppose that ˜zn⇀˜z in E, ˜zn→˜z in Lqloc(R) for q>2, and ˜zn(t)→˜z(t) almost everywhere on R. According to (3.3) and (3.4), then we can derive that ˜z≠0 and I′μ(˜z)=0. This implies that ˜z∈Nμ and Iμ(˜z)≥cμ. On the other hand, applying Fatou's lemma we can obtain
cμ=limn→∞(Iμ(˜zn)−12⟨I′μ(˜zn),˜zn⟩)=limn→∞μ∫R(12g(|zn|)|zn|2−G(|zn|))dt≥μ∫Rlimn→∞(12g(|zn|)|zn|2−G(|zn|))dt=Iμ(˜z)−12⟨I′μ(˜z),˜z⟩=Iμ(˜z), |
that is, Iμ(˜z)≤cμ. Consequently, Iμ(˜z)=cμ and ˜z is a critical point of Iμ, which implies that ˜z is a ground-state homoclinic orbit of problem (3.1). So, we have completed the proof of Theorem 3.1.
As a byproduct of the Theorem 3.1, we show the monotonicity and continuity of cμ.
Lemma 3.9. The function μ↦cμ is decreasing and continuous on (0,+∞).
Proof. In what follows, let zμ1 and zμ2 be as ground state homoclinic orbits of Iμ1 and Iμ2, respectively. Assume that μ1>μ2. First of all, we want to verify that the function μ↦cμ is decreasing. On account of Lemma 3.6 we can find that there exist s1>0 and φ1∈E− such that
Iμ1(s1zμ2+φ1)=maxz∈ˆE(zμ2)Iμ1(z), |
then we have
cμ1≤Iμ1(s1zμ2+φ1)=Iμ2(s1zμ2+φ1)+(μ2−μ1)∫RG(|s1zμ2+φ1|)dt≤Iμ2(zμ2)+(μ2−μ1)∫RG(|s1zμ2+φ1|)dt=cμ2+(μ2−μ1)∫RG(|s1zμ2+φ1|)dt. |
Combining the fact that
∫RG(|s1zμ2+φ1|)dt≥0, |
with the inequality μ2−μ1<0, we can infer that
cμ1≤cμ2. |
We finish the proof by demonstrating that the function μ↦cμ is decreasing on (0,+∞).
In order to claim the continuity of cμ, we divide the proof into two steps:
Step 1: Let {μn} be a sequence such that μ1≤μ2≤⋯≤μn≤⋯≤μ and μn→μ.
Claim 1: cμn→cμ as n→∞.
Indeed, let zμ be the ground state homoclinic orbit of system (3.1). On account of Lemma 3.6, we can see that there exist sn>0 and φn∈E− such that
Iμn(snzμ+φn)=maxz∈ˆE(zμ)Iμn(z)for alln∈N. |
Note that, using (g3) and computing directly, we get
Iμ1(z)−Iμn(z)=(μn−μ1)∫RG(|z|)dt≥0, |
so, for all n∈N and z∈E, we have that Iμ1(z)≥Iμn(z). Then by Lemma 3.4, it holds that there exists R>0 such that
Iμn(z)≤Iμ1(z)≤0,∀z∈ˆE(zμ)∖BR(0). | (3.5) |
According to Lemma 3.3 and the monotonicity of cμ, we can obtain
Iμn(snzμ+φn)=maxz∈ˆE(zμ)Iμn(z)≥cμn≥cμ>0, |
consequently, it follows that
Iμn(snzμ+φn)>0. | (3.6) |
From (3.5) and (3.6), one can check that ‖snzμ+φn‖≤R; this shows that the sequence {snzμ+φn} is bounded in E. Hence, it is easy to see that
∫RG(|snzμ+φn|)dtis also bounded, |
then we get
cμn≤Iμn(snzμ+φn)=Iμ(snzμ+φn)+(μ−μn)∫RG(|snzμ+φn|)dt≤Iμ(zμ)+(μ−μn)∫RG(|snzμ+φn|)dt=cμ+on(1). |
On the other hand, since cμ≤cμn for all n∈N, we can infer that
cμn→cμ as n→∞. |
Step 2: Let {μn} be a sequence such that μ1≥μ2≥⋯≥μn≥⋯≥μ and μn→μ.
Claim 2: cμn→cμ as n→∞.
In fact, let zn be the ground state homoclinic orbit of the system (3.1) with μ=μn, then, there exist sn>0 and φn∈E− such that
Iμ(snzn+φn)=maxz∈ˆE(zn)Iμ(z). |
We can easily obtain that the sequence {zn} is bounded by Lemma 3.7. Moreover, we can find that there exist δ>0, r>0 and {yn}⊂Z such that for each n∈N, we have
∫Br(yn)|z+n|2dt≥δ. | (3.7) |
Otherwise, using Lions' concentration-compactness principle we can deduce that z+n→0 in Lq(R) for all q>2. Combining (2.1) with (2.2), it holds that
∫Rg(|zn|)znz+ndt→0. |
Therefore, we have
0=⟨I′μn(zn),z+n⟩=‖z+n‖2−μn∫Rg(|zn|)znz+ndt=‖z+n‖2+on(1), |
this shows that ‖z+n‖2→0, which is a contradiction to the inequality ‖z+n‖2≥2cμn>0 from Lemma 3.3. So, (3.7) holds.
Setting ˜zn(t):=zn(t+yn), one can check that {˜zn} is bounded in E; passing to a subsequence, ˜z+n⇀˜z+≠0 in E. Set V:={˜z+n}⊂E+∖{0}; hence, V is bounded and the sequence does not weakly converge to zero in E. Then by Lemma 3.4, there exists R>0 such that for every z∈V, we obtain
Iμ(w)<0,forw∈E(z)∖BR(0). | (3.8) |
Define ˜φn(t):=φn(t+yn), we have
Iμ(sn˜zn+˜φn)=Iμ(snzn+φn)=maxz∈ˆE(zn)Iμ(z)≥cμ>0,∀n∈N. | (3.9) |
In view of (3.8) and (3.9), we can conclude that ‖sn˜zn+˜φn‖≤R for all n∈N, then, ‖snzn+φn‖≤R, which implies that the sequence {snzn+φn} is bounded in E, and ∫RG(|snzn+φn|)dt is also bounded. Thus, we obtain
cμ≤Iμ(snzn+φn)=Iμn(snzn+φn)+(μn−μ)∫RG(|snzn+φn|)dt≤Iμn(zn)+(μn−μ)∫RG(|snzn+φn|)dt=cμn+on(1). |
Combining this with the fact that cμ≥cμn for all n∈N, then we have
cμn→cμ as n→∞. |
We have finished the proof of the lemma.
In this subsection we will give the proof involving the existence result for ground state homoclinic orbits for system (1.1). As before, we define the associated generalized Nehari manifold
Nϵ:={z∈E∖E−:⟨I′ϵ(z),z⟩=0and⟨I′ϵ(z),φ⟩=0,∀φ∈E−} |
and the ground state energy value
cϵ=infNϵIϵ. |
Applying the same arguments explored in the Section 3, we can show that for every z∈E∖E− the set Nϵ∩ˆE(z) is a singleton set, and the element of this set is the unique global maximum of Iϵ|ˆE(z), that is, there exists a unique pair t>0 and φ∈E− such that
Iϵ(tz+φ)=maxw∈ˆE(z)Iϵ(w). |
Therefore, the following mapping is well-defined:
˜mϵ:E+∖{0}→Nϵandmϵ=˜mϵ|S+, |
and the inverse of mϵ is
m−1ϵ:Nϵ→S+,m−1ϵ(z)=z+/‖z+‖. |
Accordingly, the reduced functional ˜Φϵ:E+∖{0}→R and the restriction Φϵ:S+→R can respectively be defined by
˜Φϵ(z)=Iϵ(˜mϵ(z))andΦϵ=˜Φϵ|S+. |
Moreover, from the above discussions in Section 3, we can check that all related conclusions in Section 3 hold for Iϵ, cϵ, Nϵ, ˜mϵ, mϵ, ˜Φϵ and Φϵ, respectively.
Meanwhile, concerning the limit problem given by
Sz=A(0)g(|z|)z,t∈R, | (4.1) |
for the sake of simplicity, we will use the notations I0, c0 and N0 to denote IA(0), cA(0) and NA(0), respectively.
Next, we will state the relationship of the ground state energy value between system (1.3) and limit system (4.1), and this is very significant in our following arguments.
Lemma 4.1. The limit limϵ→0cϵ=c0 holds.
Proof. Let be ϵn→0 as n→∞. Evidently, using Lemma 3.9 we obtain that c0≤cϵn for all n∈N; thus, c0≤lim infn→∞cϵn.
On the other hand, Theorem 3.1 shows that the limit system (4.1) has a ground state homoclinic orbit z0. Then, according to Lemma 3.6, we can find that there are sn∈(0,+∞) and φn∈E− such that snz+0+φn∈Nϵn, and
Iϵn(snz+0+φn)≥cϵn≥c0>0,∀n∈N. |
As in the previous section, we can see that {snz+0+φn} is bounded in E. Thus, without loss of generality, we assume that sn→s0 and φn⇀φ in E−. Therefore, we can deduce from the weakly lower semi-continuity of the norm and Fatou's Lemma that
c0=lim infn→∞cϵn≤lim supn→∞cϵn≤lim supn→∞Iϵn(snz+0+φn)≤lim supn→∞[12s2n‖z+0‖2−12‖φn‖2−∫RA(ϵnt)G(|snz+0+φn|)dt]≤12s20‖z+0‖2−12‖φ‖2−A(0)∫RG(|s0z+0+φ|)dt=I0(s0z+0+φ)≤I0(z0)=c0. |
Obviously, we can get
limn→∞cϵn=c0, |
finishing the proof.
In view of the above discussion, we obtain that I0(s0z+0+φ)=I0(z0)=c0; then, both s0z+0+φ and z0 are the elements of N0∩ˆE(z0). But, according to Lemma 3.6, there is only one element in N0∩ˆE(z0), so we can conclude that s0z+0+φ=z0 and sn→s0=1, where φn⇀φ=z−0.
As a byproduct of the Lemma 4.1, we can directly obtain the following result.
Lemma 4.2. Assume that condition (A) holds, then, there is ϵ0>0 such that cϵ<cA∞ for ϵ∈(0,ϵ0).
Proof. From condition (A) we can see that A(0)>A∞. Then from Lemma 3.9 we have that c0<cA∞. Observe that, Lemma 4.1 yields that there exists ϵ0>0 small enough such that cϵ<cA∞ for all ϵ∈(0,ϵ0). Therefore, we get that cϵ<cA∞ for ϵ∈(0,ϵ0).
Using similar arguments as for the proof of Lemma 3.7, one can easily check the following lemma.
Lemma 4.3. The energy functional Iϵ is coercive on Nϵ for each ϵ≥0.
Next we give the proof involving the existence result for ground state homoclinic orbits for system (1.1).
Lemma 4.4. Assume that conditions (L), (A) and (g1)-(g3) are satisfied, then, system (1.1) has a ground-state homoclinic orbit for each ϵ∈(0,ϵ0).
Proof. Following the proof of Theorem 3.1 and using Lemma 3.8, we must prove that there exists z∈Nϵ such that Iϵ(z)=cϵ. Indeed, applying Ekeland's variational principle, there exists {un}⊂S+ such that Φϵ(un)→cϵ and Φ′ϵ(un)→0. Put zn=˜mϵ(un)∈Nϵ for all n∈N. Then from Lemma 3.8 we have that Iϵ(zn)→cϵ and I′ϵ(zn)→0. Furthermore, in view of Lemma 4.3, we can prove that {zn} is bounded. Then, up to a subsequence, we can suppose that zn⇀z in E. Evidently, I′ϵ(z)=0.
In what follows we need to show that z≠0 and Iϵ(z)=cϵ. Combining the fact that zn∈Nϵ with Lemma 3.3, we have
on(1)=⟨I′ϵ(zn),z+n⟩=‖z+n‖2−∫RA(ϵt)g(|zn|)znz+ndt≥2cϵ−∫RA(ϵt)g(|zn|)znz+ndt, |
which yields that
∫RA(ϵt)g(|zn|)znz+ndt≥2cϵ>0. |
As in the previous section, we can check that there exists a sequence {yn}⊂Z, r>0 and δ>0 such that
∫Br(yn)|z+n|2dt≥δ,∀n∈N. | (4.2) |
Now, we need to prove that the sequence {yn} is bounded in R. Arguing by contradiction we can suppose that {yn} is unbounded and |yn|→+∞ as n→∞. Setting wn(t):=zn(t+yn), then, wn⇀w in E; we can obtain w≠0 from (4.2). By choosing the test function ψ∈C∞0(R), we get
on(1)=⟨I′ϵ(zn),ψ(t−yn)⟩=(z+n,ψ+(t−yn))−(z−n,ψ−(t−yn))−∫RA(ϵt)g(|zn|)znψ(t−yn)dt=(w+n,ψ+)−(w−n,ψ−)−∫RA(ϵt+ϵyn)g(|wn|)wnψ(t)dt. | (4.3) |
Letting n→+∞, then we obtain
(w+,ψ+)−(w−,ψ−)−∫RA∞g(|w|)wψ(t)dt=⟨I′∞(w),ψ⟩=0. | (4.4) |
This shows that w is a nontrivial solution of system (3.1) with μ=A∞ and w∈NA∞.
Employing the Fatou's lemma we can derive that
cA∞≤IA∞(w)=IA∞(w)−12⟨I′A∞(w),w⟩=∫RA∞[12g(|w|)|w|2−G(|w|)]dt≤lim infn→∞∫RA(ϵt+ϵyn)[12g(|wn|)|wn|2−G(|wn|)]dt=lim infn→∞∫RA(ϵt)[12g(|zn|)|zn|2−G(|zn|)]dt=lim infn→∞[Iϵ(zn)−12⟨I′ϵ(zn),zn⟩]=cϵ. |
Therefore, it follows that
cA∞≤cϵ,∀ϵ>0. |
However, Lemma 4.2 yields that cϵ<cA∞ when ϵ<ϵ0, which leads to a contradiction. So, we can conclude that {yn} is bounded. Then for all n∈N, there exists r0>0 such that Br(yn)⊂Br0(0), it holds that
∫Br0(0)|zn|2dt≥∫Br(yn)|zn|2dt≥δ. |
Therefore, we obtain that zn⇀z in E with z≠0. By repeating the steps in (4.3) and (4.4), we know that z∈Nϵ is a nontrivial solution for system (1.1), thus, cϵ≤Iϵ(z).
On the other hand, according to Fatou's lemma, we infer that
cϵ=lim infn→∞[Iϵ(zn)−12⟨I′ϵ(zn),zn⟩]=lim infn→∞∫RA(ϵt)[12g(|zn|)|zn|2−G(|zn|)]dt≥∫RA(ϵt)[12g(|z|)|z|2−G(|z|)]dt=Iϵ(z)−12⟨I′ϵ(z),z⟩=Iϵ(z). |
Thus, cϵ=Iϵ(z). Evidently, it is easy to see that z is a ground state homoclinic orbit of system (1.1). We complete the proof.
Let
Kϵ:={z∈E∖{0}:I′ϵ(z)=0} |
be the set of all nontrivial critical points of Iϵ. In order to describe some important properties of ground state homoclinic orbits, next, we get the following regularity result by taking advantage of the bootstrap argument (see [37] for the iterative steps), this result can also be found in [24, Lemma 2.3].
Lemma 4.5. If z∈Kϵ with |Iϵ(z)|≤C1 and ‖z‖2≤C2; then, z∈W1,q(R,R2N) for any q>2, and ‖z‖W1,q≤Cq, where Cq depends only on C1,C2 and q.
Below, we use L to denote the set of all ground state homoclinic orbits of system (1.1). Let z∈L, then, Iϵ(z)=cμ; applying a standard argument we can show that L is bounded in E; therefore, ‖z‖2≤ˆc for all z∈L and some ˆc>0. Hence, making use of Lemma 4.5, we see that, for each q>2, there exists Cq such that
‖z‖W1,q≤Cq,∀z∈L. | (4.5) |
Moreover, combining the Sobolev embedding theorem, we can show that there exists C∞>0 such that
‖z‖∞≤C∞,∀z∈L. | (4.6) |
We now shall prove the concentration behavior of the maximum points of the ground state homoclinic orbit. Let zϵ be a ground state homoclinic orbit of system (1.1), which can be obtained by Lemma 4.4. Our aim is to show that if tϵ is a maximum point of |zϵ|, then,
limϵ→0A(ϵtϵ)=A(0). |
In other words, we must show that if ϵn→0, up to a subsequence, ϵntϵn→t0 for some t0∈A, where
A={t∈R:A(t)=A(0)} |
denotes the set of the maximum points of A(t).
Let {ϵn}⊂(0,ϵ0) with ϵn→0 as n→∞ and zϵn∈L; we write zn:=zϵn. Then, we have
Iϵn(zn)=cϵnandI′ϵn(zn)=0 |
Evidently, in view of Lemma 4.3, we can easily check that {zn} is bounded in E.
Lemma 4.6. There exist a sequence {yn}⊂Z and two constants r>0, δ>0 such that
∫Br(yn)|zn|2dt≥δ. |
Proof. Arguing by contradiction, we suppose that for any r1>0,
limn→∞supy∈R∫Br1(y)|zn|2dt=0. |
Then, according to Lions' concentration-compactness principle, we conclude that zn→0 in Lq(R) for all q>2. Furthermore, by (2.1) and (2.2), we obtain
∫RA(ϵnt)[12g(|zn|)|zn|2−G(|zn|)]dt→0. |
Therefore, it follows that
cϵn=Iϵn(zn)−12⟨I′ϵn(zn),zn⟩=∫RA(ϵnt)[12g(|zn|)|zn|2−G(|zn|)]dt→0. |
Evidently, this is impossible because cϵn>0 (see Lemma 3.3). We complete the proof.
Lemma 4.7. The sequence {ϵnyn} is bounded, and limn→∞ϵnyn=x0∈A.
Proof. Setting vn(t):=zn(t+yn), up to a subsequence, it is easy to see that vn⇀v in E with v≠0 from Lemma 4.6. In what follows, we want to prove that the sequence {ϵnyn} is bounded. If this is not true, we can suppose that there is a subsequence {ϵnyn} such that |ϵnyn|→+∞ as n→+∞. Since zn is the ground state homoclinic orbit of system (1.1), vn solves the following system
Svn=A(ϵnt+ϵnyn)g(|vn|)vn, | (4.7) |
and the energy
ˆIϵn(vn)=12(‖v+n‖2−‖v−n‖2)−∫RA(ϵnt+ϵnyn)G(|vn|)dt=12(‖z+n‖2−‖z−n‖2)−∫RA(ϵnt)G(|zn|)dt=∫RA(ϵnt)[12g(|zn|)|zn|2−G(|zn|)]dt=Iϵn(zn)=cϵn. |
Furthermore, for every ϕ∈E, we have
(v+n,ϕ+)−(v−n,ϕ−)−∫RA(ϵnt+ϵnyn)g(|vn|)vnϕdt=0. |
Since A(ϵnt+ϵnyn)→A∞, given that vn⇀v and ϕ∈C∞0(R), we get
(v+,ϕ+)−(v−,ϕ−)−∫RA∞g(|v|)vϕdt=0. |
Thereby, v is a nontrivial homoclinic orbit of system (3.1) with μ=A∞ and v∈NA∞. In view of Lemma 4.1 and Fatou's lemma, we can conclude that
cA∞≤IA∞(v)=IA∞(v)−12⟨I′A∞(v),v⟩=A∞∫R[12g(|v|)|v|2−G(|v|)]dt≤lim infn→∞∫RA(ϵnt+ϵnyn)[12g(|vn|)|vn|2−G(|vn|)]dt=lim infn→∞∫RA(ϵt)[12g(|zn|)|zn|2−G(|zn|)]dt=lim infn→∞[Iϵn(zn)−12⟨I′ϵn(zn),zn⟩]=lim infn→∞Iϵn(zn)=limn→∞cϵn=c0. | (4.8) |
However, according to Lemma 4.2 we know that c0<cA∞. Evidently, this is a contradiction. Therefore, {ϵnyn} is bounded in R, and passing to a subsequence, we can assume that ϵnyn→x0. According to the above argument, for ∀ψ∈E, we get
(v+,ψ+)−(v−,ψ−)−∫RA(x0)g(|v|)vψdt=0, |
Obviously, we can see that v is a ground state homoclinic orbit of the following system
Sv=A(x0)g(|v|)v,t∈R, | (4.9) |
and v∈NA(x0). Following to the proof of (4.8), we can get tht cA(x0)≤c0, then, using Lemma 3.9, it follows that A(x0)≥A(0); together with condition (A), we can obtain that A(x0)=A(0). Hence, we show that limn→∞ϵnyn=x0 and x0∈A. The proof is completed.
According to Lemma 4.7, we see that v is a ground state homoclinic orbit of system (4.7), then, I0(v)=c0 and I′0(v)=0. Using Lemma 4.1 and Fatou's lemma, we directly obtain
c0≤∫RA(0)[12g(|v|)|v|2−G(|v|)]dt≤lim infn→∞∫RA(ϵnt+ϵnyn)[12g(|vn|)|vn|2−G(|vn|)]dt=lim infn→∞ˆIϵn(vn)≤lim supn→∞Iϵn(zn)≤c0. |
Hence, we have
limn→∞ˆIϵn(vn)=limn→∞cϵn=c0=I0(v). | (4.10) |
Lemma 4.8. We have the convergence conclusion: vn→v in E as n→∞.
Proof. Let η:[0,+∞)→[0,1] be a smooth function satisfying that η(s)=1 if s≤1, and η(s)=0 if s≥2. Define ˜vn(t)=η(2|t|/n)v(t), then, for q∈[2,+∞), one has
‖v−˜vn‖→0and‖v−˜vn‖q→0asn→∞. | (4.11) |
Setting θn=vn−˜vn, it is not difficult to verify that along a subsequence
limn→∞|∫RA(ϵnt+ϵnyn)[G(|vn|)−G(|θn|)−G(|˜vn|)]dt|=0 | (4.12) |
and
limn→∞|∫RA(ϵnt+ϵnyn)[g(|vn|)vn−g(|θn|)θn−g(|˜vn|)˜vn]φdt|=0 | (4.13) |
uniformly in φ∈E with ‖φ‖≤1. Using the fact that A(ϵnt+ϵnyn)→A0 as n→∞ uniformly on any bounded set of t, and combining the decay of v and (4.11) we can easily check the following result
∫RA(ϵnt+ϵnyn)G(|˜vn|)dt→∫RA0G(|v|)dt. | (4.14) |
Consequently, using (4.10), (4.11), (4.12) and (4.14) we infer that
ˆIϵn(θn)=ˆIϵn(vn)−I0(v)+∫RA(ϵnt+ϵnyn)[G(|vn|)−G(|θn|)−G(|˜vn|)]dt+on(1)=on(1)asn→∞, |
which implies that ˆIϵn(θn)→0. Similarly, we also obtain
⟨ˆI′ϵn(θn),φ⟩=∫RA(ϵnt+ϵnyn)[g(|vn|)vn−g(|θn|)θn−g(|˜vn|)˜vn]φdt+on(1)=on(1)uniformly in‖φ‖≤1asn→∞, |
which implies that \widehat{I}'_{\epsilon_{n}}(\theta_{n})\rightarrow 0 . Therefore
\begin{equation} \nonumber o_{n}(1) = \widehat{I}'_{\epsilon_{n}}(\theta_{n})-\frac{1}{2}\langle\widehat{I}'_{\epsilon_{n}}(\theta_{n}), \theta_{n}\rangle = \int_{\mathbb{R}}A(\epsilon_{n}t+\epsilon_{n}y_{n})\left[\frac{1}{2}g(|\theta_{n}|)|\theta_{n}|^{2}-G(|\theta_{n}|)\right]\mathrm{d}t, \end{equation} |
from which together with (g_{3}) , we can infer that
\begin{equation} \nonumber \int_{\mathbb{R}}A(\epsilon_{n}t+\epsilon_{n}y_{n})g(|\theta_{n}|)|\theta_{n}|^{2}\mathrm{d}t\rightarrow 0. \end{equation} |
Notice that \{\|v_{n}\|_{\infty}\} is bounded, thus,
\begin{equation} \nonumber \int_{\mathbb{R}}A(\epsilon_{n}t+\epsilon_{n}y_{n})g(|\theta_{n}|)|\theta_{n}^{+}-\theta_{n}^{-}|^{2}\mathrm{d}t\leq C. \end{equation} |
As a consequence, we obtain
\begin{equation} \nonumber \begin{aligned} \|\theta_{n}\|^{2}& = \langle\widehat{I}'_{\epsilon_{n}}(\theta_{n}), \theta_{n}^{+}-\theta_{n}^{-}\rangle +\int_{\mathbb{R}}A(\epsilon_{n}t+\epsilon_{n}y_{n})g(|\theta_{n}|)\theta_{n}(\theta_{n}^{+}-\theta_{n}^{-})\mathrm{d}t\\ = &o_{n}(1)+\int_{\mathbb{R}}A^{\frac{1}{2}}(\epsilon_{n}t+\epsilon_{n}y_{n})g^{\frac{1}{2}}(|\theta_{n}|)|\theta_{n}| A^{\frac{1}{2}}(\epsilon_{n}t+\epsilon_{n}y_{n})g^{\frac{1}{2}}(|\theta_{n}|)|\theta_{n}^{+}-\theta_{n}^{-}|\mathrm{d}t\\ \leq& o_{n}(1)+\left(\int_{\mathbb{R}}A(\epsilon_{n}t+\epsilon_{n}y_{n})g(|\theta_{n}|)|\theta_{n}|^{2}\mathrm{d}t\right)^{\frac{1}{2}}\\ &\left(\int_{\mathbb{R}}A(\epsilon_{n}t+\epsilon_{n}y_{n})g(|\theta_{n}|)|\theta_{n}^{+}-\theta_{n}^{-}|^{2}\mathrm{d}t\right)^{\frac{1}{2}}\\ \leq& o_{n}(1)+C\left(\int_{\mathbb{R}}A(\epsilon_{n}t+\epsilon_{n}y_{n})g(|\theta_{n}|)|\theta_{n}|^{2}\mathrm{d}t\right)^{\frac{1}{2}}\\ = &o_{n}(1), \end{aligned} \end{equation} |
that is, \|\theta_{n}\|\rightarrow 0 , which together with (4.11) leads to v_{n}\rightarrow v in E as n\rightarrow \infty .
Lemma 4.9. We have that v_{n}(t)\to0 uniformly in n\in \mathbb{N} as t\to\infty . Moreover, there exist c, C > 0 such that for all t\in\mathbb{R} , it holds that
\begin{equation} \nonumber |v_{n}(t)|\leq C \exp(-c|t|). \end{equation} |
Proof. Firstly, we observe that if z is a homoclinic orbit of system (1.1), then it satisfies the following relation
\begin{equation} \nonumber \frac{\mathrm{d}}{\mathrm{d}t}z = \mathscr{J}\bigg(Lz+A(\epsilon t)g(|z|)z\bigg). \end{equation} |
Computing directly, we obtain
\begin{equation} \nonumber \frac{\mathrm{d}^{2}}{\mathrm{d}t^{2}}z = (\mathscr{J}L)^{2}z+Q(t, z) \end{equation} |
with
\begin{equation} \begin{aligned} Q(t, z) = &\mathscr{J}\bigg[\bigg(\epsilon A'(\epsilon t)+L\mathscr{J}A(\epsilon t)\bigg)g(|z|)z+\bigg(g'_{z}(|z|)|z|+g(|z|)\bigg)A(\epsilon t)\mathscr{J}Lz\\ & \ +\bigg(g'_{z}(|z|)|z|+g(|z|)\bigg)A^{2}(\epsilon t)Lg(|z|)z\bigg]. \end{aligned} \end{equation} | (4.15) |
Setting
\begin{eqnarray*} \hbox{sgn}z \left\{ \begin{array}{ll} \frac{z}{|z|}&\hbox{if}\; z\neq 0, \\ 0, &\hbox{if}\; \; z = 0.\\ \end{array} \right. \end{eqnarray*} |
Applying Kato's inequality and (4.15), and using the real positivity of (\mathscr{J}L)^{2} , we can find some \rho > 0 such that
\begin{equation} \frac{\mathrm{d}^{2}}{\mathrm{d}t^{2}}|z|\geq \frac{\mathrm{d}^{2}}{\mathrm{d}t^{2}}z (\hbox{sgn}z) = (\mathscr{J}L)^{2}z\frac{z}{|z|}+Q(t, z)\frac{z}{|z|}\geq \rho|z|-|Q(t, z)|. \end{equation} | (4.16) |
Hence, using (2.1), (4.6), (4.15) and (4.16) we conclude that there exists \kappa > 0 such that
\begin{equation} \nonumber \frac{\mathrm{d}^{2}}{\mathrm{d}t^{2}}|z|\geq -\kappa|z|\; \; \; \hbox{for all}\; \; t\in \mathbb{R}. \end{equation} |
Then by the sub-solution estimate [38], there is a \widehat{c}_{0} independent of t ; we have the following estimate
\begin{equation} |z(t)|\leq \widehat{c}_{0}\int_{B_{1}(t)}|z(s)|\mathrm{d}s. \end{equation} | (4.17) |
Now we claim that v_{n}(t)\to0 uniformly in n\in \mathbb{N} as t\to\infty . Indeed, if it is not true, then using (4.17) we can find that there exist c_{0} > 0 and t_{n}\in \mathbb{R} with |t_{n}|\rightarrow \infty such that
\begin{equation} \nonumber c_{0}\leq |v_{n}(t_{n})|\leq \widehat{c}_{0}\int_{B_{1}(t_{n})}|v_{n}(t)|\mathrm{d}t, \end{equation} |
this is because v_n satisfies \widehat{I}'_{\epsilon_{n}}(v_{n}) = 0 , then, the above processes still hold for v_{n} . From Lemma 4.8, it follows that v_n\to v in E . Therefore, we get
\begin{equation} \nonumber \begin{aligned} c_{0}&\leq |v_{n}(t_{n})|\leq \widehat{c}_{0}\int_{B_{1}(t_{n})}|v_{n}(t)|\mathrm{d}t\leq \widehat{c}_{0}\int_{B_{1}(t_{n})}|v_{n}-v|\mathrm{d}t+\widehat{c}_{0}\int_{B_{1}(t_{n})}|v|\mathrm{d}t\\ &\leq \overline{c}\bigg(\int_{\mathbb{R}}|v_{n}-v|^{2}\mathrm{d}t\bigg)^{\frac{1}{2}}+\widehat{c}_{0}\int_{B_{1}(t_{n})}|v|\mathrm{d}t\rightarrow 0, \end{aligned} \end{equation} |
which yields a contradiction. So, the claim holds.
Note that g(s) = o(1) and g'_{s}(s)s = o(1) as s\rightarrow 0 ; then, we can find suitable constants 0 < \delta < \frac{\rho}{2} and R > 0 such that
\begin{equation} \nonumber |Q(t, v_{n})|\leq \frac{\rho}{2}|v_{n}|, \; \; \; \forall|t|\geq R. \end{equation} |
Combining the above relation and (4.16), we get
\begin{equation} \nonumber \frac{\mathrm{d}^{2}}{\mathrm{d}t^{2}}|v_{n}|\geq \delta |v_{n}|, \; \; \; \forall|t|\geq R. \end{equation} |
Let \Lambda(t) be a fundamental solution of the following equation
\begin{equation} \nonumber -\frac{\mathrm{d}^{2}}{\mathrm{d}t^{2}}\Lambda+\delta \Lambda = 0. \end{equation} |
From the uniform boundedness, we may choose \Lambda(t) such that |v_{n}(t)|\leq\delta\Lambda(t) holds on |t| = R for all n\in\mathbb{N} . Let u_{n} = |v_{n}|-\delta\Lambda ; thus, we obtain
\begin{equation} \nonumber \frac{\mathrm{d}^{2}}{\mathrm{d}t^{2}}u_{n} = \frac{\mathrm{d}^{2}}{\mathrm{d}t^{2}}|v_{n}|-\delta \frac{\mathrm{d}^{2}}{\mathrm{d}t^{2}}\Lambda\geq\delta(|v_{n}|-\delta\Lambda) = \delta u_{n}, \; \; \; \hbox{for all}\; \; |t|\geq R. \end{equation} |
The maximum principle yields that u_{n}(t)\leq 0 for |t|\geq R , i.e., |v_{n}(t)|\leq \delta \Lambda(t) for |t|\geq R . As we know that there exists c_{1} > 0 such that
\begin{equation} \nonumber \Lambda(t)\leq c_{1}\exp(-\sqrt{\delta}|t|)\; \; \; \; \; \hbox{for all}\; \; \; |t|\geq 1. \end{equation} |
Therefore, there are constants C, c > 0 ; we obtain
\begin{equation} \nonumber |v_{n}(t)|\leq C\exp(-c|t|)\; \; \; \hbox{for all}\; \; t\in\mathbb{R}. \end{equation} |
We complete the proof.
Lemma 4.10. There exists \nu > 0 such that \|v_{n}\|_{\infty}\geq \nu for all n\in\mathbb{N} .
Proof. According to Lemma 4.6, we can see that there exist r > 0 and \delta > 0 such that
\begin{equation} \nonumber \int_{B_{r}(0)}|v_{n}|^{2}\mathrm{d}t\geq\delta. \end{equation} |
Suppose by contradiction that \|v_{n}\|_{\infty}\to0 as n\to\infty , then, it holds that
\begin{equation} \nonumber 0 < \delta\leq\int_{B_{r}(0)}|v_{n}|^{2}\mathrm{d}t\leq |B_{r}|\|v_{n}\|_{\infty}^{2}\to0\; \hbox{as}\; n\to\infty, \end{equation} |
which is absurd. This ends the proof.
Finally, based on the above facts, next we give the completed proof of Theorem 1.1.
Proof of Theorem 1.1 (completed). Suppose that q_{n} is a global maximum point of |v_{n}(t)| for each n\in \mathbb{N} , then,
\begin{equation} \nonumber |v_{n}(q_{n})| = \max\limits_{t\in \mathbb{R}}|v_{n}(t)|. \end{equation} |
Since v_{n}(t) = z_{n}(t+y_{n}) , we can see that p_{n} = q_{n}+y_{n} is a maximum point of |z_{n}(t)| . Lemma 4.10 shows that there exists \nu > 0 such that
\begin{equation} \nonumber |v_{n}(q_{n})|\geq \nu \; \; \hbox{for all}\; \; n\in \mathbb{N}, \end{equation} |
then we know that \{q_{n}\} is bounded. So, we conclude from Lemma 4.7 that
\begin{equation} \nonumber \epsilon_{n}p_{n} = \epsilon_{n}q_{n}+\epsilon_{n}y_{n}\rightarrow x_{0}\in \mathscr{A}. \end{equation} |
Consequently, it follows that
\begin{equation} \nonumber \lim\limits_{n\rightarrow \infty}A(\epsilon_{n}p_{n}) = A(x_{0}) = A(0). \end{equation} |
Furthermore, from Lemma 4.7 and Lemma 4.8, it is easy to see that z_{n}(t+p_n) converges to a ground state homoclinic orbit v of the following limit system
\begin{equation} \nonumber Sz = A(0)g(|z|)z, \; \; t\in \mathbb{R}. \end{equation} |
From Lemma 4.9 and the boundedness of \{q_{n}\} , we derive that
\begin{equation} \nonumber \begin{aligned} |z_{n}(t)| = &|v_{n}(t-y_n)|\leq C\exp\left(-c|t-y_{n}|\right) = C\exp\left(-c|t-p_{n}+q_{n}|\right)\\ \leq & C\exp\left(-c|t-p_{n}|+c|q_{n}|\right)\leq \widetilde{C}\exp\left(-\tilde{c}|t-p_{n}|\right) \end{aligned} \end{equation} |
for some \tilde{c}, \widetilde{C} > 0 and all t\in\mathbb{R} .
Finally, we observe that Lemma 4.2 shows that, there is \epsilon_{0} > 0 ; system (1.1) has a ground state homoclinic orbit z_{\epsilon} for each \epsilon \in(0, \epsilon_{0}) . So, the conclusion (a) of Theorem 1.1 holds. Moreover, according to the above discussions, we directly obtain the following conclusions:
(b) let t_{\epsilon} be the maximum point of |z_{\epsilon}(t)| , then,
\lim\limits_{\epsilon\to0}A(\epsilon t_{\epsilon}) = A(0); |
and z_{\epsilon}(t+t_{\epsilon})\to v in E , where v is a ground state homoclinic orbit of the limit system
\begin{equation} \nonumber Sz = A(0)g(|z|)z, \; \; t\in \mathbb{R}; \end{equation} |
(c) there are two positive constants \tilde{c} , \widetilde{C} such that
\begin{equation} \nonumber |z_{\epsilon}(t)|\leq \widetilde{C}\exp\left(-\tilde{c}|t-t_{\epsilon}|\right). \end{equation} |
We have finished the proof of all conclusions of Theorem 1.1.
The authors declare that no artificial intelligence tools were used in the creation of this article.
The research of Tianfang Wang was supported by the High Level Research Achievement Project-General Project of Natural Science of Baotou Teachers' College (BSYKJ2022-ZY09), the Youth Innovative Talent Project of Baotou City (2022). The research of Wen Zhang was supported by the Natural Science Foundation of Hunan Province (2022JJ30200), the Key project of Scientific Research Project of Department of Education of Hunan Province (22A0461), and Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province.
The authors declare that they have no competing interests.
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