© Q.Zou and P.Weng. 2009
Received: 7 July 2009
Accepted: 15 October 2009
Published: 20 October 2009
The variational method and critical point theory are employed to investigate the existence of solutions for 2 th-order difference equation for with boundary value condition by constructing a functional, which transforms the existence of solutions of the boundary value problem (BVP) to the existence of critical points for the functional. Some criteria for the existence of at least one solution and two solutions are established which is the generalization for BVP of the even-order difference equations.
Difference equations have been applied as models in vast areas such as finance insurance, biological populations, disease control, genetic study, physical field, and computer application technology. Because of their importance, many literature deals with its existence and uniqueness problems. For example, see [1–10].
We notice that the existing results are usually obtained by various analytical techniques, for example, the conical shell fixed point theorem [1, 6], Banach contraction map method , Leray-Schauder fixed point theorem [2, 10], and the upper and lower solution method . It seems that the variational technique combining with the critical point theory  developed in the recent decades is one of the effective ways to study the boundary value problems of difference equations. However because the variational method requires a "symmetrical'' functional, it is hard for the odd-order difference equations to create a functional satisfying the "symmetrical'' property. Therefore, the even-order difference equations have been investigated in most references.
Let , be integers, and , be a discrete interval in . Inspired by [5, 8], in this paper, we try to investigate the following th-order boundary value problem (BVP) of difference equation via variational method combining with some traditional analytical skills:
where is the forward difference operator; for and A variational functional for BVP (1.1)-(1.2) is constructed which transforms the existence of solutions of the boundary value problem (BVP) to the existence of critical points of this functional. In order to prove the existence criteria of critical points of the functional, some lemmas are given in Section 2. Two criteria for the existence of at least one solution and two solutions for BVP (1.1)-(1.2) are established in Section 3 which is the generalization for BVP of the even-order difference equations. The existence results obtained in this paper are not found in the references, to the best of our knowledge.
For convenience, we will use the following notations in the following sections:
2. Variational Structure and Preliminaries
Lemma 2.2 (mountain-pass lemma).
The following lemma will be used in the proof of Lemma 2.4.
with the norm
Hence is an -dimensional Hilbert space. For any , let then one can show that there exist constants s.t. ; that is, is an equivalent norm of (see [9, page 68]).
However, how to find the in Lemma 2.4 is a skillful and challenging task. The following lemma from  offers some help for the estimation of .
Lemma 2.5 (Brualdi ).
In Lemma 2.5, is the complex number set, and the denotations , , , can be found in . Since is positive defined, all eigenvalues are positive real numbers. Therefore, by Lemma 2.5, let
Furthermore, for any we have , By using the following formula (e.g., see [14, page 28]):
Repeating the above process, we obtain
3. Main Results
Now we present our main results of this paper.
then BVP (1.1)-(1.2) has at least one solution.
Then BVP (1.1)-(1.2) has at least two solutions.
We first show that satisfies the P-S condition. Let satisfy that is bounded and If is unbounded, it possesses a divergent subseries, say as . However from (ii), we get (3.4) or (3.8), hence as , which is contradictory to the the fact that is bounded.
On the other hand, by Theorem 3.1 or Corollary 3.2, we know that there exists s.t. If the theorem is proved. If on the contrary, , that is, that implies for any , Taking in with , by the continuity of there exist s.t. , Hence , are two different critical points of that is, BVP (1.1)-(1.2) has at least two different solutions.
4. An Example
This research is partially supported by the NSF of China and NSF of Guangdong Province.
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