Research

# Subharmonic solutions for a class of second-order impulsive Lagrangian systems with damped term

Xingyong Zhang

Author Affiliations

Department of Mathematics, Faculty of Science, Kunming University of Science and Technology, Kunming, Yunnan, 650500, P.R. China

Boundary Value Problems 2013, 2013:218  doi:10.1186/1687-2770-2013-218

 Received: 16 July 2013 Accepted: 27 August 2013 Published: 7 November 2013

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

In this paper, by using the mountain pass theorem, we investigate the existence of subharmonic weak solutions for a class of second-order impulsive Lagrangian systems with damped term under asymptotically quadratic conditions. Some new existence criteria are established. Finally, an example is presented to verify our results.

MSC: 37J45, 34C25, 70H05.

##### Keywords:
impulsive Lagrangian systems; damped term; subharmonic weak solutions; mountain pass theorem

### 1 Introduction and main results

In this paper, we investigate the existence of subharmonic weak solutions for the following second-order impulsive Lagrangian system with damped term:

(1.1)

where , , , , , satisfying and , B is a skew-symmetric constant matrix, and are symmetric and continuous matrix-value functions on ℝ satisfying and , and satisfies , where K, W are T-periodic in their first variable, and the following assumption:

(A) is measurable intfor everyand continuously differentiable inxfor a.e., and there existandwithsuch that

for alland a.e..

Lagrangian systems are applied extensively to study the fluid mechanics, nuclear physics and relativistic mechanics. Especially, as a special case of Lagrangian systems, the following second-order Hamiltonian systems are considered by many authors:

(1.2)

where satisfies , and the existence and multiplicity of periodic solutions, subharmonic solutions and homoclinic solutions are obtained via variational methods. We refer readers to [1-14]. Especially, in 2010, under the asymptotically quadratic conditions, Tang and Jiang [10] obtained the following interesting result.

Theorem A (see [10], Theorem 1.1)

Assume thatFsatisfies

(F) andareT-periodic in their first variable with, and thatKandWsatisfy the following assumptions:

(H1) There exist constantsandsuch that

(H2) for;

(H3) uniformly for;

(H4) There exists a functionsuch that

and

(H5) There exist constantsandsuch that

(H6) There existssuch that

Then system (1.2) has a nontrivialT-periodic solution.

In recent years, variational methods have been applied to study the existence and multiplicity of solutions for impulsive differential equations and lots of interesting results have been obtained, see [15-20].

In [15], Nieto and O’Regan considered a one-dimensional Dirichlet boundary value problem with impulses. They obtained that the solutions of the impulsive problem minimize some (energy) functional and the critical points of the functional are indeed solutions of the impulsive problem.

In [16], Nieto introduced a variational formulation for the following one-dimensional damped nonlinear Dirichlet problem with impulses:

(1.3)

and gave the concept of a weak solution for such a problem. They obtained that the weak solutions of problem (1.3) are indeed the critical points of the functional:

(1.4)

where and . In [17] and [18], the authors also dealt with some one-dimensional impulsive problems with damped term by variational methods.

For higher dimensional dynamical systems, some interesting results have also been obtained (see [21-23]). In [21], Zhou and Li investigated the second-order Hamiltonian system with impulsive effects:

(1.5)

By using the least action principle and the saddle point theorem, they obtained some existence results of solutions under sublinear condition and some reasonable conditions. In [22], system (1.5) with , where , was also investigated. By using variational methods, the authors obtained that system (1.5) has at least three weak solutions. In [23], the authors investigated system (1.5) with . They obtained that system (1.5) has infinitely many solutions under the assumptions that nonlinear term is superquadratic, asymptotically quadratic and subquadratic, respectively.

In recent years, via variational methods, some authors have been interested in studying the existence and multiplicity of periodic solutions and homoclinic solutions for the following Lagrangian systems with damped term:

(1.6)

where is a symmetric and continuous matrix-valued function, B is a skew-symmetric constant matrix and . They obtained some interesting results. We refer readers to [24-27].

In 2010, Li et al.[28] investigated the following system, more general than system (1.6), with :

(1.7)

Motivated by [28], in [29], we investigated the following system, more general than system (1.7):

(1.8)

By variational methods, under superquadratic or subquadratic conditions, we obtained that system (1.8) has infinitely many solutions. One can see more details of our results and more research background of system (1.8) in [29].

In [32], Luo et al. investigated the existence of subharmonic solutions with prescribed minimal period for the following one-dimensional second-order impulsive differential equation:

(1.9)

where , , , , , and if , while if .

In this paper, motivated by [10,15,16,21,28,29] and [32], we focus on the existence of subharmonic weak solutions for system (1.1), which is of impulsive conditions, and we study the problem under asymptotically quadratic conditions. To the best of our knowledge, there are few papers that consider such a problem for system (1.1). We call a solution u subharmonic if u is kT-periodic for some .

Let

and

In this paper, we make the following assumptions:

(P) There exists a constant such that the matrix satisfies

(K1) There exist constants and such that

(K2) for all and a.e. ;

(K3) There exists such that

(W1) uniformly for a.e. ;

(W2) There exist constants and such that

(W3) There exists a function such that

and

where ;

(W4) There exists such that

(W5) There exists a constant such that

(I1) There exist constants () such that

(I2) and for all ;

(I3) There exists a constant C such that

This paper is organized as follows. In Section 2, we present the definition of a subharmonic classical solution, a subharmonic weak solution and the variational structure for system (1.1) and make some preliminaries. In Section 3, we present our main theorems and their proofs. In Section 4, an example is given to verify our main theorems.

### 2 Preliminaries

In this section, we present the variational structure of system (1.1), which is motivated by [15-17,28] and [29].

Let

Define

and

for each . Then is a Hilbert space. It is well known that

is also a norm on . Obviously, if the condition (P) holds, and are equivalent. Moreover, there exists such that

(see Proposition 1.1 in [1]). Hence, there exist positive constants , such that

(2.1)

For any , define

If , then may not hold, which leads to impulsive effects.

Definition 2.1 Assume that and the limits and () exist. If u satisfies system (1.1), then we say that u is a subharmonic classical solution of system (1.1).

Remark 2.1 In [32], impulsive effects may occur periodically in , . In order to obtain a sequence of distinct subharmonic weak solutions (see Theorem 3.2 below), different from [32], in Definition 2.1, we assume that the impulsive effects only occur in , , which belong to . In other words, u is absolutely continuous on ℝ and is absolutely continuous on . Moreover, note that . Then it is easy to see that .

Note that . Then, by T-periodicity of q, we have . Moreover, obviously, is continuous on ℝ. We transform system (1.1) into the following system:

(2.2)

Then system (2.2) is equivalent to system (1.1) and its solutions are the solutions of system (1.1).

By the idea in [15], we take and multiply the two sides of the equality

by v and integrate it from 0 to kT. Then we obtain

(2.3)

Note that , is continuous on ℝ and . By integration by parts and the continuity of v, we obtain

(2.4)

Definition 2.2 is called a subharmonic weak solution of system (1.1) if

holds for any .

Lemma 2.1Ifis a subharmonic weak solution of system (1.1), thenuis a subharmonic classical solution of system (1.1).

Proof Motivated by [15], for , choose with for every . Then, by Definition 2.2, we obtain

(2.5)

Choose with for every . Then we obtain

(2.6)

Equations (2.5) and (2.6) imply that and u satisfies

Multiplying the above equality by v and integrating between 0 and kT, combining the argument of (2.4) and Definition 2.2, we obtain that

Hence, for every . This completes the proof. □

For every , define by

It follows from assumption (A) and Theorem 1.4 in [1] that the functional is continuously differentiable and

(2.7)

for . Obviously, if is a critical point of , i.e., , then is a subharmonic weak solution of system (1.1).

We will use the following mountain pass theorem to prove our results.

Lemma 2.2 (see [30])

LetEbe a real Banach space, and letsatisfy the (PS) condition. Ifϕsatisfies the following conditions:

(i) ;

(ii) There exist constantssuch that;

(iii) There existssuch that, thenϕpossesses a critical valuegiven by

whereis an open ball inEof radiusρcentered at 0, and

Remark 2.2 As shown in [31], a deformation lemma can be proved by replacing the usual (PS)-condition with the condition (C), and it turns out that Lemma 2.2 holds true under the condition (C). We say that ϕ satisfies the condition (C), i.e., for every sequence , has a convergent subsequence if is bounded and as .

### 3 Main results

Theorem 3.1Assume that (P), (K1), (K2), (W1)-(W4) and (I1)-(I3) hold. Then, for every, system (1.1) has at least onekT-periodic weak solution in.

Proof We use Lemma 2.2 to prove the theorem. Let .

Step 1. We prove that satisfies assumption (ii) of Lemma 2.2. It follows from (W1) and (W2) that there exist , and such that

(3.1)

Choose such that . Then it follows from (K1), (I2), (3.1) and (2.1) that for all with ,

Step 2. We prove that satisfies assumption (iii) of Lemma 2.2. Set for . By the argument in [10], we know that (W3) implies that

(3.2)

and (K2) implies that

(3.3)

It follows from (3.2), (3.3), (W3) and (I1) that for sufficiently large s,

By (W4), we can choose sufficiently large such that and . Let . Then satisfies assumption (iii) of Lemma 2.2.

Step 3. We prove that φ satisfies the (C)-condition on . The proof is motivated by [10]. For every , assume that there exists a constant such that

(3.4)

Then it follows from antisymmetry of B, (K2) and (I3) that

(3.5)

Next we prove that is bounded. Assume that as . Let . Then , and so there exists a subsequence, still denoted by , such that on . Then, by Proposition 1.2 in [1], we get . Hence, we have and for a.e. . Thus, by conditions (P), (W2) and (I2), we have

Hence, we have

Let . Then, by (3.4), we get

(3.6)

Then it follows from and (3.6) that and so . Let and . Then and

(3.7)

Let . Then (3.7) and T-periodicity of in t imply that

(3.8)

It follows from (3.8) and Lemma 1 in [6] that there exists a subset of with such that

(3.9)

By (W3), we have

Let . Then by Fatou’s lemma and (3.9), we have

which contradicts (3.5). Hence is bounded. Going if necessary to a subsequence, assume that in . Then, by Proposition 1.2 in [1], we have and so as . Similar to the argument of Theorem 3.1 in [28], it is easy to obtain that . Hence, as . Hence, satisfies the (C)-condition.

Finally, (K1), (W1) and (I2) imply that . Hence, combining Step 1-Step 3 with Lemma 2.2 and Remark 2.2, we obtain that has at least a critical point in and . Then system (1.1) has at least one kT-periodic solution in . This completes the proof. □

Remark 3.1 It is easy to see that Theorem 3.1 generalizes Theorem A. To be precise, when , , , , and , Theorem 3.1 reduces to Theorem A.

Theorem 3.2Assume (P), (K1)-(K3), (W1)-(W5) and (I1)-(I3) hold. Then system (1.1) has a sequence of distinct subharmonic weak solutions with periodsatisfyingandas.

Proof By Theorem 3.1, we know that for every , system (1.1) has at least one kT-periodic solution in and . By Lemma 2.2, we have

where

Let , . Obviously, . Hence, by (K3), (W5) and (I1), we have

(3.10)

Hence, is uniformly bounded for all .

Obviously, we can find such that , then we claim that is distinct to for all . In fact, if for some , it is easy to check that

Then, by (3.10), we have , a contradiction. We can also find such that for all . Otherwise, if for some , we have . Then by (3.10), we have , a contradiction. In the same way, we can obtain that system (1.1) has a sequence of distinct periodic solutions with period satisfying and as . This completes the proof. □

### 4 Example

The following example is inspired partially by Example 3.1 in [10]. Let , . Consider the following impulsive Lagrangian system with damped term:

(4.1)

where

Obviously, the condition (P) holds and , and (K1), (K2), (W1) and (W2) hold with and .

Then (W3) holds with . Moreover,

Hence, it is easy to see that there exists such that (W4) holds by the above inequality. Obviously, (I1) and (I2) hold. Note that

So (I3) holds. Hence, by Theorem 3.1, we obtain that system (4.1) has at least one kT-periodic solution for every .

Moreover, it is easy to see that (K3) holds with . Since

Choose . Then (W5) holds. Hence, by Theorem 3.2, we obtain that system (4.1) has a sequence of distinct subharmonic weak solutions with period satisfying and as .

### Competing interests

The author declares that he has no competing interests.

### Author’s contributions

The author read and approved the final manuscript.

### Acknowledgements

This work is supported by Tianyuan Fund for Mathematics of the National Natural Science Foundation of China (No. 11226135) and the Fund for Fostering Talents in Kunming University of Science and Technology (No. KKSY201207032).

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