We prove the existence of positive ground states for the nonlinear Schrödinger system
MSC: 35J05, 35J50, 35J61.
Keywords:nonlinear Schrödinger system; Nehari manifold; lack of compactness; ground state
1 Introduction and statement of the main result
This paper was motivated by the following two-component system of nonlinear Schrödinger equations:
where , , and . The system (1.1) has applications in many physical problems, especially in nonlinear optics (see ). To obtain standing wave solutions of (1.1) of the form , with , the system (1.1) turns out to be
Following the work  by Lin and Wei about the existence of ground states for the problem (1.2), there are many results on the existence of ground states relevant to five parameters (, , , and β); see [3-9] and the references therein. Later in , assuming , Pomponio and Secchi established the existence of radially symmetric ground states for (1.2) with general nonlinearities ( and ).
On the other hand, some authors considered the existence of ground states for non-autonomous similar problems. We recall the results about non-autonomous case for two subcases. For periodic case, in  Szulkin and Weth referred that treating as periodic Schrödinger equations, it is possible to deduce that there are ground states for the following system using the method of Nehari manifold:
where G is periodic in x and satisfies some superlinear conditions in . For non-periodic case, we refer to [8,12-14] for instance. As we can observe, most of the previous results on ground states for the non-periodic system have used the condition that there exists a limit system (or the problem at infinity; for precise statement, refer to ). Moreover, the limit system is autonomous. Here we mainly deal with an asymptotically periodic Schrödinger system which has a periodic non-autonomous limit system, roughly speaking. In this paper, we are concerned with the existence of positive ground states for the nonlinear Schrödinger system in ()
(F1)-(F4) are similar to the conditions of the nonlinearities for the periodic system (1.3) as considered in . We divide the study of (NLS) into two cases as follows.
First, we consider the periodic case
We have the following result.
Theorem 1.1Let (V1), (V2) and (F1)-(F6) hold. Then the system (NLS) has a positive ground state.
Remark 1.1 It is observed that the system (NLS) with periodic a and b is a particular case of the problem (1.3) with
We have the following result.
In addition, we consider the following conditions:
We have the following result.
We will prove Theorems 1.1, 1.2 and 1.3 using the method of Nehari manifold. We first reduce the problem of seeking for ground states of (NLS) into that of looking for minimizers of the functional constrained on the Nehari manifold. Then we apply the concentration-compactness principle to solve the minimization problem. Since the Nehari manifold for (NLS) may not be smooth, in the same way as , we will make use of the differential structure of a unit sphere in to find a sequence (c is the infimum of the functional constrained on the Nehari manifold). When (NLS) is periodic, we will use the invariance of the functional under translation to recover the compactness of the sequence. When the system (NLS) is asymptotically periodic, the difficulty is to recover the compactness for the sequence. By comparing c with the infimum of the functional of the related periodic limit system constrained on the corresponding Nehari manifold, we will restore the compactness.
The paper is organized as follows. In Section 2 we give some preliminaries. In Section 3 we introduce the variational setting. In Section 4 we consider the periodic case and prove Theorem 1.1. Section 5 is devoted to studying the asymptotically periodic case and showing Theorems 1.2 and 1.3.
2 Notation and preliminaries
We use the following notation:
(F2) and (F3) yield that
So, one easily has that
3 Variational setting
This section is devoted to describing the variational framework for the study of ground states for (NLS).
It is easy to see that Φ is bounded neither from above nor from below. So, it is convenient to consider Φ on the Nehari manifold that contains all nontrivial critical points of Φ and on which Φ turns out to be bounded from below. The Nehari manifold M corresponding to Φ is defined by
Below we investigate the main properties of Φ on M.
Lemma 3.1Let (F2) and (F3) hold. Then Φ is bounded from below onMby 0.
Therefore, (2.3) and the Fatou lemma yield that
Using (2.1) we have
Lemma 3.3Let (V1) and (F1)-(F4) hold. Then
(i) Note that
Then when t is large enough, . Then has maximum points in . Moreover, from (F3) one easily deduces that the critical point of is unique in , and then it is the maximum point. We denote it by . Then for and for .
By (2.1) and (2.2), we get
(iii) We argue by contradiction. Suppose that there exist a compact set W and a sequence such that and . Since W is compact, there exists such that in H. Then Lemma 3.2 implies that . Contrary to Lemma 3.1 since . This ends the proof. □
As a consequence of Lemma 3.3(i), we can define the mapping by . By Lemma 3.3, [, Proposition 3.1(b)] yields the following result.
Lemma 3.4If (V1) and (F1)-(F4) are satisfied, then m is a homeomorphism betweenSandM, and M is a manifold.
If M is a manifold, we can make use of the differential structure of M to reduce the problem of finding a ground state for (NLS) into that of looking for a minimizer of and solve the minimizing problem. However, since , M may not be a manifold. Noting that M and S are homeomorphic, we will take advantage of the differential structure of S to seek for ground states for (NLS) as . Therefore, as in , we introduce the functional defined by , and we have the following conclusion.
Proposition 3.1Let (V1) and (F1)-(F4) hold. Then the following results hold:
Proof As in the proof of [, Corollary 3.3], we can show (i) and (ii). Now, we prove the conclusion (iii). Indeed, let such that . Then , where . By the conclusion (ii), we have . So, . Using the conclusion (ii) again, we deduce that . □
From the definition of a ground state, we translate the problem of looking for a ground state for (NLS) into that of seeking for a solution for (NLS) which is a minimizer of . By Proposition 3.1(iii), in order to look for a ground state for (NLS), we just need to seek for a minimizer of .
4 The periodic case
In this section, we consider the periodic case and prove Theorem 1.1. In , Szulkin and Weth considered the existence of ground states for periodic single Schrödinger equations. Treating as in , we find ground states for a periodic case for the system (NLS). In addition, under conditions (F5) and (F6), we deduce that there are positive ground states.
From the statement in Section 3, it suffices to solve the minimizing problem. By conclusions (i) and (ii) of Proposition 3.1, we first make use of the minimizing sequence of Ψ to obtain a sequence of Φ. Then we use the invariant of the functional under translation of the form , to recover the compactness for the sequence.
Proof of Theorem 1.1 Let be a minimizing sequence of Ψ. By the Ekeland variational principle [, Theorem 8.5], we may assume that . Using Proposition 3.1(i), we have that , where . Proposition 3.1(ii) implies that .
We claim that is bounded in H. Otherwise, suppose up to a subsequence. Set . Then we assume in H, in and a.e. on after passing to a subsequence. Moreover, the Sobolev embedding theorem implies that is bounded in , namely, is bounded. Taking a subsequence, we suppose .
(ii) If , then we can assume that in . From the Lions compactness lemma [, Lemma 1.21], it follows that there exist and such that
Since Φ and M are invariant by translation of the form , , translating if necessary, we may assume is bounded. Since in , then (4.1) implies . Then from Lemma 3.2, we deduce that . This is impossible since .
It remains to look for a positive ground state for (NLS). First, we can assume that is non-negative. In fact, note that and for all . Then . Let be such that . By (F6) we easily have that . Moreover, since . Then . So, is also a minimizer of Φ on M. Then is also a ground state of (NLS). Thus we can assume that is a non-negative ground state for (NLS). Now, we claim that , . Indeed, if , then from (F5) and , the first equation of (NLS) yields that . Then . This is impossible. So, . Similarly, . By (F5), applying the maximum principle to each equation of (NLS), we infer that , . The proof is complete. □
5 The asymptotically periodic case
In this section, we will consider the asymptotically periodic case and prove Theorems 1.2 and 1.3. As in the proof of Theorem 1.1, we first take advantage of the minimizing sequence of Ψ to find a sequence of Φ. In what follows, the important thing is to recover the compactness for the sequence. For this purpose, we need to estimate the functional levels of the problem (NLS) and those of a related periodic problem of (NLS) (roughly speaking, the limit system of (NLS) by (V3))
Hence, first we introduce some definitions and look for solutions for the problem (NLS)p. The functional of (NLS)p is defined by
The Nehari manifold of (NLS)p is
Proof As a corollary of Theorem 1.1, we infer that the problem (NLS)p has a positive ground state. Moreover, from the argument of Theorem 1.1, we find that the ground state of the problem (NLS)p we obtained is a minimizer of on . □
If , we are done. Otherwise, . Then by (5.3) and (5.4), we get and . Then is a ground state for (NLS). Note that is a solution of (NLS)p. From the first equations of (NLS) and (NLS)p, we infer that . Similarly, contrary to (5.2). The proof is now complete. □
Without loss of generality, we assume that
Now, we are ready to prove Theorems 1.2 and 1.3. The proof is partially inspired by , where the authors dealt with Schrödinger-Poisson equations.
We claim that is bounded in H. Otherwise, suppose up to a subsequence. Set . As in the proof of Theorem 1.1, taking a subsequence, we suppose and exclude the case that . So, , then we can assume that in . From the Lions compactness lemma, it follows that there exist and such that
Then by (2.1), we get
This is a contradiction.
First, we prove that
which is impossible. Consequently, (5.7) holds.
Combining (5.11) with (5.12), one easily has that
where the inequality (5.13) holds by (2.3) and the Fatou lemma. Then . According to , we have . Then is a ground state for (NLS). Below we argue analogously with the proof of Theorem 1.1 to get a positive ground state for (NLS). The proof is complete. □
Proof of Theorem 1.3 By Lemma 5.3, repeating the argument of Theorem 1.2, we show the existence of a ground state for (NLS) and then look for a positive ground state as the argument of Theorem 1.1. □
The authors declare that they have no competing interests.
The paper is a joint work of all the authors who contributed equally to the final version of the paper. All authors read and approved the final manuscript.
The authors would like to express their sincere gratitude to the referee for helpful and insightful comments. Hui Zhang was supported by the Research and Innovation Project for College Graduates of Jiangsu Province with contract number CXLX12_0069, Junxiang Xu and Fubao Zhang were supported by the National Natural Science Foundation of China with contract number 11071038.
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