#### Abstract

We study the following second-order periodic system: where has a singularity and . Under some assumptions on the and , by Moser's twist theorem we obtain the existence of quasiperiodic solutions and boundedness of all the solutions.

#### 1. Introduction and Main Result

In the early 1960s, Littlewood [1] asked whether or not the solutions of the Duffing-type equations are bounded for all time, that is, whether there are resonances that might cause the amplitude of the oscillations to increase without bound. Littlewood suggested studying the following two cases: (i)superlinear case: as ,(ii)sublinear case: and as .

The first positive result of boundedness of solutions in the superlinear case (i) was due to Morris [2]. By means of KAM theorem, Morris proved that every solution of the second-order system (1.1) is bounded if and is piecewise continuous and periodic. This result relies on the fact that the nonlinearity can guarantee the twist condition of KAM theorem. Later, several authors (see [3â€“5]) improved the Morrisâ€™s result and obtained similar results for a large class of superlinear function .

In 1999, the first result in the sublinear case was proved by KÃ¼pper and You [6] in the study of where and . The authors transform (1.2) into a perturbation of an integrable Hamiltonian system and then prove that the PoincarÃ© map of the transformed system is close to a so-called twist map. So, the Moserâ€™s twist theorem guarantees the boundedness of all solutions of (1.2). The general sublinear case was considered by Liu [7] under certain reasonable conditions.

The Littlewood problem for singular potentials is known to be challenging, and there are only very a few results. Recently, Capietto et al. [8] studied with is a -periodic function and , where and is a positive integer. Under the Lazer-Leach assumption that they prove the boundedness of solutions and the existence of quasiperiodic solution by Moserâ€™ twist theorem. It is the first time that the equation of the boundedness of all solution is treated in case of a singular potential.

In this paper, We consider the following sublinearly growing potential: where .

Our main result is the following theorem.

Theorem 1.1. *If is 1-periodic continuous, then all the solutions of (1.5) are bounded.*

The idea for proving the boundedness of solutions of (1.5) is as follows. By means of transformation theory, (1.5) is, outside of a large disc in the -plane, transformed into a perturbation of an integrable Hamiltonian system. Then, PoincarÃ© map of the transformed system is close to a so-called twist map in . The Moserâ€™s twist theorem [9] guarantees the existence of arbitrarily large invariant curves diffeomorphic to circles and surrounding the origin in the -plane. Every such curves is the base of a time-periodic and flow-invariant cylinder in the extended phase space , which confines the solutions in the interior and which leads to a bound of these solutions.

The paper is organized as follows. In Sections 2.1 and 2.2, we give action-angle variables and some estimates which is useful for our proof. In Section 2.3, we will give an asymptotic expression of the PoincarÃ© map and prove the main result by Moserâ€™s twist theorem [9].

#### 2. Proof of Theorem

##### 2.1. Action-Angle Variables and Some Estimates

Without loss of generality and for brevity of arguments, we assume that the average value of vanishes; that is, . Hence the function is also 1-periodic in and is in .

System (1.5) is equivalent to the planar Hamiltonian system where Hamiltonian is .

In order to introduce action and angle variables, we first consider the auxiliary autonomous system which is integrable with the Hamiltonian The closed curves are just the integral curves of (2.2). Denote by the time period of the integral curve : and by the area enclosed by the closed curve . Let Then, .

It is easy to see that Denote Then,

The following estimates on the functions , , and and , , and are crucial for this paper. We first estimate and . Since is the area enclosed by the closed curve and axis when , we can easily prove that

Let , then we get Since , we have

We now give the estimates on the function and .

Lemma 2.1. *One has
**
where , . Note that here and below, one always uses , , or to indicate some constants. *

*Proof. *Now, we estimate the first inequality. We choose as the new variable of integration, then we have
Since and , we have . By direct computation, we have
then we get
When and sufficient large, there exits such that , so we have
Since , we have
then
Observing that there is such that when and , we have
By (2.15)â€“(2.18), we have , .

The proof of the second inequality is similar to the first one, so we only give the brief proof.

We choose as the new variable of integration, so we have
By direct computation, we have
By (2.20), we can easily get
where .

By the similar way in estimating , we get
which means that

Thus, we complete the proof of Lemma 2.1.

*Remark 2.2. *It follows from (2.10) and Lemma 2.1 that
Thus, the time period is dominated by when is sufficiently large. By , we know is dominated by when is sufficiently large.

*Remark 2.3. *It also follows from the definition of , , and Remark 2.2 that
In particular,

*Remark 2.4. *Note that is the inverse function of . By Remark 2.3, we have

We now carry out the standard reduction to the action-angle variables. For this purpose, we define the generating function , where is the part of the the closed curve connecting the point on the -axis and point .

We define the well-known map by It is well-known that the map is symplectic, since From the above discussion, we can easily get In the new variables , the system (2.1) becomes where where is the inverse function of and .

In order to estimate , we need the estimate on the functions and . For this purpose, we first give some definitions which are very similar to those in [4].

Define a function in terms of and by and a linear differential operator acting on functions of , according to where is a smooth function, and we denote .

The following equality (its proof can be found in [4]) is crucial for the proof of the following lemmas:

Before giving the estimates on and , we now prove some lemmas which will be used frequently in the following proof.

Lemma 2.5. * Suppose that there is a constant such that , then one can find a constant and such that, for ,
*

*Proof. *We now prove (2.37). Let
then
By direct calculation,
Since
we have
By , there is such that
that is
which means that
That is,
Thus, we complete the proof (2.37).

Now, we prove (2.38). Let
Then, we have
By direct computation, we have
Since and , it follows that for
so for , we have
which means that
By the definition of , we have
Thus, we complete the proof of (2.38) and Lemma 2.5.

By Lemma 2.5, we have the following Lemma which is important to our estimation.

Lemma 2.6. *One can find a constant such that, for ,
**
where .*

*Proof. *When , we have
By the definitions of and , we have and . By (2.37), we obtain
Suppose that , we have
We now proven that when ,
By direct computation, we have
where denotes linear combination of functions with integer coefficients and .

Since , we have . By (2.37), we obtain
By direct computation, we get
By assumption (2.59), we have
So, we have
By (2.62) and (2.65), we have
Thus, we have proved (2.55).

The inequality (2.56) can be proved by (2.38), and the process of proof is similar to that of (2.55), so we omit it.

Thus, we have proved Lemma 2.6.

Now, we give the estimates of and .

Lemma 2.7. *For sufficient large and , the following estimates hold:
*

*Proof. *We now prove the first inequality. It is sufficient to prove that
Case . Differentiating (2.30) by and noting , we have
Now, we choose as the new variable of integration, so
Observing that
and simplifying, we have
By (2.69)â€“(2.72), we have
Since , by Lemma 2.6, we have
We observe that
where , so , which means that
By (2.74) and (2.76), we have .

We suppose that
where . We will prove .

For this purpose, we firstly estimate We differentiate in (2.73) and using (2.73), then we obtain
Since , we have
By (2.37), we have
By (2.78)â€“(2.80), we have
We suppose that when,
We will prove that when,
By direct computation, we have
where and .

By the assume (2.77), we have
By (2.55), we get
By (2.85), (2.86) and noting the fact that
we obtain
By assumption (2.82), we have
By (2.89) and the fact that , we have
So, by (2.88) and (2.90), we have proved (2.83).

By (2.56), we have
By (2.83) and (2.91), we have
By the assumption (2.77), the fact that , and noting that
where , we have
By (2.92) and (2.94), we have
which means

We now prove
Since
we have
we have proved (2.83), so we have
which means that
The proof of Lemma 2.7 is complete.

*Remark 2.8. *Lemma 2.7 also holds when . Since the idea and the process of the proof is more easily than that of Lemma 2.7, we omit the details.

Now, we give the estimate of .

Lemma 2.9.

*Proof. *This lemma can be proved easily form the definition of and .

##### 2.2. New Action and Angle Variables

Now, we are concerned with the Hamiltonian system (2.32) with Hamiltonian function given by (2.33). Note that This means that if one can solve form (2.32) as a function of ( and as parameters), then is also a Hamiltonian system with Hamiltonian function , and now, the action, angle, and time variables are , , and .

Form Remarks 2.3 and 2.4, we have Hence, by the implicit function theorem, there is a function such that By Lemma 2.9, we have So, there is a function with such that Let Then, From Remark 2.3, we have known the estimate of , so we need to give the estimate of . For this propose, we need firstly the following Lemma on the estimate of .

Lemma 2.10. *The function possesses the following estimates:
**
for .*

*Proof. *From (2.108) and (2.110), it follows that
When , by Lemma 2.9, we have
When , we first denote
By Remark 2.3, we observe that is increasing and
By Lemma 2.9 and (2.115), we have
So,
By direct computation, we have