Abstract and Applied Analysis

1. Introduction

The existence of periodic solutions for differential system has received a great deal of attention in the last few decades. Different from ordinary differential equations and partial differential equations that do not contain delay variate, it is very difficult to study the existence of periodic solutions for functional differential equations. For this reason, many mathematicians developed different approaches such as the averaging method [1], the Massera-Yoshizawa theory [2, 3], the Kaplan-York [4] method of coupled systems, the Grafton cone mapping method [5], the Nussbaum method of fixed point theory [6] and Mawhin [7] coincidence degree theory.

However, the critical point theory was rarely used in the literature, and most of the existing results were established for autonomous functional differential equations while little was done for nonautonomous equations via critical point theory.

In this paper, by using critical point and operator equation theories, we study the existence of the following second-order nonlinear and nonautonomous mixed-type functional differential equation:

Our basic assumptions are the following and to be given in Section 4:, and ; there exists a continuously differentiable function such that where and denote and , respectively; for all .

2. Variational Structure

Fix , where is a positive integer and

It is obvious that is a Sobolev space by defining the inner product and the norm as follows:

Moreover, can be expressed as

Let us consider the functional defined on by

For all and , we know that

It is then easy to see that where denotes the Frechet differential of the function . By the periodicity of , , and , we have Similarly, we have Hence, Therefore, the Euler equation corresponding to the functional is

It is not difficult to see that (2.10) is equivalent to (1.1). Thus, system (1.1) is the Euler equation of the functional . It follows that it is possible to obtain -periodic solutions of system (1.1) by seeking critical points of the functional .

Since has neither a supremum nor an infimum, we do not seek critical points of the functional by the extremum method. But we may use operator equation theory. First via the dual variational principle, we obtain new operator equations (see (4.16)) related to (1.1). Then solutions to system (1.1) are obtained by seeking critical points of operator equation (4.16).

In this paper, our main tool is the following.

Lemma 2.1 (Maintain Pass Theorem). Let be a real Banach space. If satisfies the Palais-Smale condition as well as the following additional conditions:(1)there exist constants and such that , where ,(2) and there exists such that , then is a positive critical value of , where

The rest of this paper is organized as follows. Subdifferentiability of lower semicontinuous convex function and its conjugate function are introduced in Section 3. In Section 4, we first give the definition of the weak solution to (1.1), then we establish the new operator equation (4.16) related to (1.1) by the conjugate function of and show that we can obtain solution to (1.1) from the solution to operator equation (4.16). In Section 5, by seeking critical points of operator equation (4.16), we obtain the result that there exist multiple subharmonic periodic solutions to system (1.1). Finally in Section 6, an example and a remark are given to illustrate our result.

3. The Subdifferentiability and the Conjugate Function of the Lower Semicontinuous Convex Function

Let be a space of all given -periodic functions in and a Banach space, where is a positive integer. Denote . Let be a lower semicontinuous convex function. Generally, is not always differentiable in conventional sense, but we may generalize the definition of “derivative” as follows.

Definition 3.1. Let . We say that is a subgradient of at point if For all , the set of all subgradients of at point will be called the sub-differential of at point and will be denoted by .
By the definition of Subdifferentiability of function , we may define its conjugate function by where denotes the duality relation of and . So it is not difficult to obtain the following propositions.

Proposition 3.2. is a lower semicontinuous convex function ( may have functional value , but not functional value ).

Proposition 3.3. If , then .

Proposition 3.4 (Yang inequality). One has

Proposition 3.5. One has

Proposition 3.6. does not always equal .

Proof. Let and such that and . We consider the two convex sets in defined by By Hahn-Banach Theorem, we know that there exists and such that So, we have Thus , and

Since is a lower semicontinuous convex function that does not always equal , we may define its conjugate function by where denotes the duality relation of and .

Theorem 3.7. Let be a lower semicontinuous convex function that does not always equal , then .

Proof. We divide our proof into two parts. First we show that holds when and then holds for all lower semicontinuous convex functions that do not always equal .
(i) The case when .
From the definition of and Yang inequality, it is obvious that holds.
Next, to prove holds, suppose to the contrary that there exist a point , such that holds.
Consider the two convex sets By the Hahn-Banach Theorem, we know that there exist and such that So, it follows that . Let . Using and (3.11), one obtains that Thus, we have Then, by the definition of , we obtain that That is to say, which is a contradiction to (3.12).
(ii) For all , by Proposition 3.6, we know that . Choose , and define function by Then is a lower semicontinuous convex function that is not always equal to and satisfies . By the result of (i), we have that . On the other hand, we have That is, .

Corollary 3.8. Let be a lower semicontinuous convex function that is not always equal to . Then if and only if

Proof. One has

4. Weak Solution to (1.1)

Define an operator . By (2.6) and as well as we may define a weak solution to (1.1) as follows.