# Periodic Problem with a Potential Landesman Lazer Condition

- Petr Tomiczek
^{1}Email author

**Received: **6 January 2010

**Accepted: **22 September 2010

**Published: **26 September 2010

## Abstract

We prove the existence of a solution to the periodic nonlinear second-order ordinary differential equation with damping , , . We suppose that , the nonlinearity satisfies the potential Landesman Lazer condition and prove that a critical point of a corresponding energy functional is a solution to this problem.

## 1. Introduction

where , the nonlinearity is a Caratheodory function and .

To state an existence result to (1.1) Amster [1] assumes that is a nondecreasing function (see also [2]). He supposes that the nonlinearity satisfies the growth condition , for , , where is the first eigenvalue of the problem , and there exist such that An interval is centered in with the radius where , and is a solution to the problem with .

In [3, 4] authors studied (1.1) with a constant friction term and results with repulsive singularities were obtained in [5, 6].

To obtain our result we use variational approach even if the linearization of the periodic problem (1.1) is a non-self-adjoint operator.

## 2. Preliminaries

Notation 2.

We will use the classical space of functions whose th derivative is continuous and the space of measurable real-valued functions whose th power of the absolute value is Lebesgue integrable. We denote the Sobolev space of absolutely continuous functions such that and with the norm . By a solution to (1.1) we mean a function such that is absolutely continuous, satisfies the boundary conditions and (1.1) is satisfied a.e. in .

Now we prove that any critical point of the functional is a solution to (1.1) mentioned above.

Lemma 2.1.

Let the condition (1.2) be satisfied. Then any critical point of the functional is a solution to (1.1).

Proof.

We say that
satisfies the *Palais-Smale condition* (PS) if every sequence
for which
is bounded in
and
(as
possesses a convergent subsequence.

To prove the existence of a critical point of the functional we use the Saddle Point Theorem which is proved in Rabinowitz [9] (see also [10]).

Theorem 2.2 (Saddle Point Theorem).

Let , and . Let be a functional such that and

(a) there exists a bounded neighborhood of in and a constant such that ,

(b) there is a constant such that ,

## 3. Main Result

Theorem 3.1.

Under the assumptions (1.2), (3.2), (3.3), problem (1.1) has at least one solution.

Proof.

We verify that the functional satisfies assumptions of the Saddle Point Theorem 2.2 on , then has a critical point and due to Lemma 2.1 is the solution to (1.1).

It is easy to see that . Let then and .

a contradiction to (3.2). We proceed for the case Then assumption (a) of Theorem 2.2 is verified.

Since the function is strictly positive equality (3.10) implies that the functional is bounded from below.

Using (3.4), (3.10) we see that there exists a bounded neighborhood of in , a constant such that , and there is a constant such that .

Hence we obtain the strong convergence in . This shows that satisfies the Palais-Smale condition and the proof of Theorem 3.1 is complete.

## Declarations

### Acknowledgment

This work was supported by Research Plan MSM 4977751301.

## Authors’ Affiliations

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## Copyright

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