Abstract
This paper deals with the existence and uniqueness of solutions of the fourthorder periodic boundary value problem
where is continuous. Under twoparameter nonresonance conditions described by rectangle and ellipse, some existence and uniqueness results are obtained by using fixed point theorems. These results improve and extend some existing results.
MSC: 34B15.
Keywords:
existence; uniqueness; twoparameter nonresonance condition; equivalent norm1 Introduction and main results
In mathematics, the equilibrium state of an elastic beam is described by fourthorder boundary value problems. According to the difference of supported condition on both ends, it brings out various fourthorder boundary value problems; see [1]. In this paper, we deal with the periodic boundary value problem (PBVP) of the fourthorder ordinary differential equation
where is continuous. PBVP (1)(2) models the deformations of an elastic beam in equilibrium state with a periodic boundary condition. Owing to its importance in physics, the existence of solutions to this problem has been studied by many authors; see [26].
Throughout this paper, we denote that , , , , . In [710], authors showed the existence of solutions to Eq. (1) under the boundary condition
At first, the existence of a solution to twopoint boundary value problem (BVP) (1)(3) was studied by Aftabizadeh in [7] under the restriction that f is a bounded function. Then, under the following growth condition:
Yang in [[8], Theorem 1] extended Aftabizadeh’s result and showed the existence to BVP (1)(3). Later, Del Pino and Manasevich in [9] further extended the result of Aftabizadeh and Yang in [7,8] and obtained the following existence theorem.
Theorem AAssume that the pairsatisfies
and that there are positive constantsa, b, andcsuch that
andfsatisfies the growth condition
Then BVP (1)(3) possesses at least one solution.
Condition (4)(5) trivially implies that
It is easy to prove that condition (6) is equivalent to the fact that the rectangle
does not intersect any of the eigenlines of the twoparameter linear eigenvalue problem corresponding to BVP (1)(3).
In [2], Ma applied Theorem A to PBVP (1)(2) successfully and obtained the following existence theorem.
Theorem BAssume that the pairsatisfies
and that there are positive constantsa, b, andcsuch that
andfsatisfies the growth condition
Then PBVP (1)(2) has at least one solution.
Condition (7)(9) concerns a nonresonance condition involving the twoparameter linear eigenvalue problem (LEVP)
In [2], it has been proved that is an eigenvalue pair of LEVP (10) if and only if , . Hence, for each , the straight line
is called an eigenline of LEVP (10). Condition (7)(8) trivially implies that
It is easy to prove that condition (11) is equivalent to the fact that the rectangle does not intersect any of the eigenline of LEVP (10). Hence, we call (11) and (9) the twoparameter nonresonance condition described by rectangle, which is a direct extension from a singleparameter nonresonance condition to a twoparameter one.
The purpose of this paper is to improve and extend the abovementioned results. Different from the twoparameter nonresonance condition described by rectangle, we will present new twoparameter nonresonance conditions described by ellipse and circle. Under these nonresonance conditions, we obtain several existence and uniqueness theorems.
The main results are as follows.
Theorem 1Assume that the pairsatisfies (7). If there exist positive constantsa, b, andcsuch that (11) and
hold, then PBVP (1)(2) has at least one solution.
When the partial derivatives and exist, if is large enough such that
where is a certain ellipse, and the corresponding close rectangle satisfies
by the theorem of differential mean value, we easily see that (7), (11), and (12) hold. Hence, by Theorem 1, we have the following corollary.
Corollary 1Assume that the partial derivativesandexist in. If there exists an ellipsesuch that (13) holds for a positive real numberlarge enough, and the corresponding close rectanglesatisfies (14), then PBVP (1)(2) has at least one solution.
Condition (11) is weaker than condition (8), but condition (12) is stronger than condition (9). Hence, Theorem 1 and Corollary 1 partly improve Theorem B.
In the nonresonance condition of Theorem 1, condition (11) can be weakened as
In this case, we have the following results.
Theorem 2Assume that the pairsatisfies (7). If there exist positive constantsa, b, andcsuch that (12) and (15) hold, then PBVP (1)(2) has at least one solution.
Condition (15) is equivalent to the fact that
Condition (16) indicates that the ellipse does not intersect any of the eigenline of LEVP (10). Hence, we call (15) and (12) the twoparameter nonresonance condition described by ellipse, which is another extension of a singleparameter nonresonance condition. Similar to Corollary 1, we have the following corollary.
Corollary 2Assume that the partial derivativesandexist in. If there exists an ellipsesuch that (13) and (16) hold for a positive real numberlarge enough, then PBVP (1)(2) has at least one solution.
Theorem 3Assume that the partial derivativesandexist in. If there exists an ellipsesuch that (16) and
hold, then PBVP (1)(2) has a unique solution.
In Theorem 2, Theorem 3, and Corollary 2, we present a new twoparameter nonresonance condition described by ellipse, which is another extension of a singleparameter nonresonance condition. As a special case, we replace the ellipse by a circle
and obtain the following results.
Corollary 3Assume that there exist a circleand a positive constantcsuch that
andfsatisfies the growth condition
Then PBVP (1)(2) has at least one solution.
Condition (18) indicates that the circle does not intersect any of the eigenline of LEVP (10). Hence, we call condition (18)(19) the twoparameter nonresonance condition described by circle, which is also an extension of a singleparameter nonresonance condition. Similarly to Corollary 2 and Theorem 3, we have the following corollaries.
Corollary 4Assume that the partial derivativesandexist in. If there exists a circlesuch that (18) and
hold for a positive real numberlarge enough, then PBVP (1)(2) has at least one solution.
Corollary 5Assume that the partial derivativesandexist in. If there exists a circlesuch that (18) and
hold, then PBVP (1)(2) has a unique solution.
2 Preliminaries
Let be not eigenvalue pair of LEVP (10), i.e., . For any , we consider the linear periodic boundary value problem (LPBVP)
By the Fredholm alternative, LPBVP (22) has a unique solution . If , then the solution . We define an operator T by
Then is a bounded linear operator, and we call it the solution operator of LPBVP (22). By compactness of the embedding , is a compact linear operator.
Let . We choose an equivalent norm in the Sobolev space by
and denote the Banach space reendowed norm by .
Lemma 1Let. Then the solution operator of LPBVP (22) is a compact linear operator and its norm satisfies
Proof We only need to prove that (23) holds.
Since is a complete orthogonal system of , every can be expressed by the Fourier series expansion
where , . By the Parseval equality, we have
where is the norm in . Now, by uniqueness of the Fourier series expansion, the solution of LPBVP (22) has the Fourier series expansion
and can be expressed by the Fourier series expansion
Hence, by the Parseval equality, we have
From (24) and (25), we have
This implies that (23) holds. The proof of Lemma 1 is completed. □
Lemma 2Letand. Then the rectanglesatisfies condition (14) if and only if condition (11) holds.
Proof Condition (14) holds
⇔ and on the same side of every eigenline ,
The proof of Lemma 2 is completed. □
Lemma 3Letand. Then the ellipsesatisfies condition (16) if and only if condition (15) holds.
Proof Condition (16) holds
⇔ for , and on the same side of every eigenline ,
The proof of Lemma 3 is completed. □
3 Proof of the main results
Proof of Theorem 1 We define a mapping by
It follows from (12) that is continuous and satisfies
Therefore, the mapping defined by
is a completely continuous mapping. By the definition of the operator T, the solution of PBVP (1)(2) is equivalent to the fixed point of the operator Q.
From (7), (11), and Lemma 1, it follows that . We choose . Let . Then for any , from (27) and (28), we have
Therefore, . By the Schauder’s fixed point theorem, Q has at least one fixed point in , which is a solution of PBVP (1)(2). □
By Lemma 2, we can obtain the following existence result:
Corollary 6Assume that the pairsatisfies (7). If there exist positive constantsa, b, andcsuch that (12) and (14) hold, then PBVP (1)(2) has at least one solution.
Proof of Theorem 2 Let be a mapping defined by (26). Then it follows from (12) that is continuous and satisfies
Thus, the mapping is completely continuous. By using (7), (15), and Lemma 1, a similar argument as in the proof of Theorem 1 shows that Q has at least one fixed point in , which is the solution of PBVP (1)(2). □
Proof of Theorem 3 Let be defined by (26). Then is continuous. For any , from (17), we have
It follows from the above that . Thus, is a continuous mapping and it satisfies
It follows from (16) and Lemma 3 that (15) holds. By (15) and Lemma 1, it is easy to see that . Hence, is a contraction mapping. By the Banach contraction mapping principle, Q has a unique fixed point, which is the unique solution of PBVP (1)(2). □
As in Corollary 6, in Theorem 2 we can use condition (16) to replace condition (15), and in Theorem 3, we use condition (15) to replace condition (16).
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
HY carried out the study of the twoparameter nonresonance conditions for periodic boundary value problems, participated in the proof of the main results and drafted the manuscript. YL participated in the design of the study and performed the coordination. PC participated in the proof of the main results. All authors read and approved the final manuscript.
Acknowledgements
Research supported by the NNSF of China (Grant No. 11261053), the Fundamental Research Funds for the Gansu Universities and the Project of NWNULKQN113.
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