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Homoclinic orbits for second-order discrete Hamiltonian systems with subquadratic potential
Advances in Difference Equations volume 2013, Article number: 228 (2013)
Abstract
Under the assumptions that is indefinite sign and subquadratic as and satisfies
for some constant , we establish a theorem on the existence of infinitely many homoclinic solutions for the second-order self-adjoint discrete Hamiltonian system
where and are real symmetric matrices for all , and is always positive definite.
MSC:39A11, 58E05, 70H05.
1 Introduction
Consider the second-order self-adjoint discrete Hamiltonian system
where , , is the forward difference, and , is continuously differentiable in x for every .
As usual, we say that a solution of system (1.1) is homoclinic (to 0) if as . In addition, if then is called a nontrivial homoclinic solution.
The existence and multiplicity of nontrivial homoclinic solutions for problem (1.1) have been extensively investigated in the literature with the aid of critical point theory and variational methods; see, for example, [1–13]. Most of them treat the case where is superquadratic as .
Compared to the superquadratic case, as far as the author is aware, there are a few papers [10, 12, 13] concerning the case where has subquadratic growth at infinity. Specifically, [12] and [10] dealt with the existence and multiplicity of homoclinic solutions for (1.1) under the following assumptions on L:
() is an real symmetric positive definite matrix for all and there exists a constant such that
() is an real symmetric positive definite matrix for all and there exists a constant such that
respectively. In the above two cases, since is positive definite, the variational functional associated with system (1.1) is bounded from below, techniques based on the genus properties have been well applied. In particular, Clark’s theorem is an efficacious tool to prove the existence and multiplicity of homoclinic solutions for system (1.1). However, if is not global positive definite on ℤ, the problem is far more difficult as 0 is a saddle point rather than a local minimum of the variational functional, which is strongly indefinite and it is not easy to obtain the boundedness of the Palais-Smale sequence. In a recent paper [13], based on a new direct sum decomposition of the ‘work space’, Tang and Lin proved the following theorem by using a linking theorem which was developed in [14].
Theorem 1.1 [13]
Assume that is an real symmetric positive definite matrix for all , L and W satisfy the following assumptions:
() is an real symmetric matrix for all and there exists a constant such that
(W1) there exist constants and such that
(W2) there exists a function such that
where as , ;
(W3) there exist constants , and such that
(W4) there exist constants , and such that
(W5) , .
Then system (1.1) possesses infinitely many nontrivial homoclinic solutions.
We remark that the condition ‘positive definite’ is removed in (), i.e., is not required to be global positive definite on ℤ. The main goal of this paper is to weaken conditions (W1), (W2), (W3) and (W4) of Theorem 1.1 under assumption ().
To state our result, we first introduce the following assumptions:
(W1′) there exist constants , and with such that
(W2′) there exist two constants , and two functions such that
where as , ;
(W3′) there exist constants , and such that
(W4′) there exist constants , and such that
We are now in a position to state the main result of this paper.
Theorem 1.2 Assume that is an real symmetric positive definite matrix for all , L and W satisfy (), (W1′), (W2′), (W3′), (W4′) and (W5). Then system (1.1) possesses infinitely many nontrivial homoclinic solutions.
2 Preliminaries
In what follows, we always assume that is a real symmetric positive definite matrix for all . As done in [13], we define
and
Then by (), is bounded from below and so is a finite set and
Define
Then, it follows from (2.1), (2.2), (2.3) and (2.4) that
Let
and for , let
Then E is a Hilbert space with the above inner product, and the corresponding norm is
As usual, for , set
and
and their norms are defined by
respectively.
Lemma 2.1 [[9], Lemma 2.2]
For , one has
where .
Set
Lemma 2.2 [[13], Lemma 2.3]
Suppose that L satisfies (). Then
-
(i)
is a bilinear function on E, and there exists a constant such that
(2.8) -
(ii)
(2.9)
By (), there exist an integer and such that
which implies
Lemma 2.3 Suppose that L satisfies (). Then, for and , E is compactly embedded in ; moreover,
and
where
Proof Let . Then . For and , it follows from (2.10), (2.13) and the Hölder inequality that
This shows that (2.11) holds. Hence, from (2.5), (2.11) and the Hölder inequality, one has
This shows that (2.12) holds.
Finally, we prove that E is compactly embedded in . Let be a bounded sequence. Then by (2.6), there exists a constant such that
Since E is reflexive, possesses a weakly convergent subsequence in E. Passing to a subsequence if necessary, it can be assumed that in E. It is easy to verify that
For any given number , we can choose such that
It follows from (2.15) that there exists such that
On the other hand, it follows from (2.11), (2.14) and (2.16) that
Since ε is arbitrary, combining (2.17) with (2.18), we get
This shows that possesses a convergent subsequence in . Therefore, E is compactly embedded in for . □
Lemma 2.4 Suppose that L and W satisfy () and (W1′). Then, for ,
where
Proof For , it follows from (W1′), (2.12), (2.20), (2.21) and (2.22) that
This shows that (2.19) holds. □
Lemma 2.5 Assume that L and W satisfy (), (W1′) and (W2′). Then the functional defined by
is well defined and of class and
Furthermore, the critical points of f in E are the solutions of system (1.1) with .
Proof Lemmas 2.2 and 2.4 imply that f defined by (2.23) is well defined on E. Next, we prove that (2.24) holds. By (W2′), there exist such that
For any , there exists an integer such that for . Then, for any sequence with for and any number , by (W2′), (2.11) and (2.25), we have
where , . Then by (2.23), (2.26) and Lebesgue’s dominated convergence theorem, we have
This shows that (2.24) holds. In view of the proof of [[13], Lemma 2.6], the critical points of f in E are the solutions of system (1.1) with . □
Let us prove now that is continuous. Let in E. Then there exists a constant such that
It follows from (2.6) that
By (W2′), there exist such that
From (2.11), (2.24), (2.27), (2.28), (2.29), (W2′) and the Hölder inequality, we have
which implies the continuity of . The proof is complete. □
Lemma 2.6 [14]
Let X be an infinite dimensional Banach space and let be even, satisfy the (PS)-condition, and . If (direct sum), where is finite dimensional, and f satisfies
-
(i)
f is bounded from below on ;
-
(ii)
for each finite dimensional subspace , there are positive constants and such that and , where .
Then f possesses infinitely many nontrivial critical points.
3 Proof of the theorem
Proof of Theorem 1.2 For , we define two functions as follows:
Set
Then (direct sum) and . Obviously, (W1′) and (W5) imply and f is even. In view of Lemma 2.5, . In what follows, we first prove that f satisfies the (PS)-condition. Assume that is a (PS)-sequence: is bounded and as . From (2.23), (2.24) and (W3′), we have
It follows that there exists a constant such that
Since , it follows that there exists a constant such that
where . Combining (3.3) with (3.4), one has
From (2.19), (2.23) and (3.5), we obtain
Since , , it follows from (3.6) that is bounded. Let such that
So, passing to a subsequence if necessary, it can be assumed that in E. It is easy to verify that
By (W2′), there exist such that
For any given number , we can choose an integer such that
It follows from (3.8) and the continuity of on x that there exists such that
On the other hand, it follows from (2.11), (3.7), (3.9), (3.10) and (W2′) that
Since ε is arbitrary, combining (3.11) with (3.12), we get
It follows from (2.24) that
Since , it follows from (3.8), (3.13) and (3.14) that in E. Hence, f satisfies the (PS)-condition.
Next, for , it follows from (2.9), (2.19) and (2.23) that
as and , since .
Finally, we prove that assumption (ii) in Lemma 2.6 holds. Let be any finite dimensional subspace. Then there exist constants and such that
From (2.9), (2.23), (3.16) and (W4′), one has
Since , the above estimation implies that there exist and such that
This shows that assumption (ii) in Lemma 2.6 holds. By Lemma 2.6, f has infinitely many critical points which are homoclinic solutions for system (1.1). □
4 Example
In this section, we give an example to illustrate our result.
Example 4.1 In system (1.1), let be an real symmetric positive definite matrix for all , , and let
Then L satisfies () with , and
and
Thus all the conditions of Theorem 1.2 are satisfied with
Hence, by Theorem 1.2, system (1.1) has infinitely many nontrivial homoclinic solutions. However, one can see that defined by (4.1) does not satisfy (W1) and (W2).
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Acknowledgements
The author would like to express their thanks to the referees for their helpful suggestions. This work is partially supported by the NNSF (No. 11171351) of China and supported by the Scientific Research Fund of Hunan Provincial Education Department (08A053) and supported by the Hunan Provincial Natural Science Foundation of China (No. 11JJ2005).
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Lin, X. Homoclinic orbits for second-order discrete Hamiltonian systems with subquadratic potential. Adv Differ Equ 2013, 228 (2013). https://doi.org/10.1186/1687-1847-2013-228
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DOI: https://doi.org/10.1186/1687-1847-2013-228