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Existence and uniqueness of solutions for fourth-order periodic boundary value problems under two-parameter nonresonance conditions

He Yang1*, Yue Liang2 and Pengyu Chen1

Author affiliations

1 Department of Mathematics, Northwest Normal University, Lanzhou, 730070, People’s Republic of China

2 Science College, Gansu Agricultural University, Lanzhou, 730070, People’s Republic of China

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Citation and License

Boundary Value Problems 2013, 2013:14  doi:10.1186/1687-2770-2013-14


The electronic version of this article is the complete one and can be found online at: http://www.boundaryvalueproblems.com/content/2013/1/14


Received:16 December 2012
Accepted:18 January 2013
Published:4 February 2013

© 2013 Yang et al.; licensee Springer

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

This paper deals with the existence and uniqueness of solutions of the fourth-order periodic boundary value problem

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M1','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M1">View MathML</a>

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M2','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M2">View MathML</a> is continuous. Under two-parameter 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; two-parameter nonresonance condition; equivalent norm

1 Introduction and main results

In mathematics, the equilibrium state of an elastic beam is described by fourth-order boundary value problems. According to the difference of supported condition on both ends, it brings out various fourth-order boundary value problems; see [1]. In this paper, we deal with the periodic boundary value problem (PBVP) of the fourth-order ordinary differential equation

(1)

(2)

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M2','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M2">View MathML</a> 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 [2-6].

Throughout this paper, we denote that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M6','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M6">View MathML</a>, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M7','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M7">View MathML</a>, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M8','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M8">View MathML</a>, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M9','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M9">View MathML</a>, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M10','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M10">View MathML</a>. In [7-10], authors showed the existence of solutions to Eq. (1) under the boundary condition

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M11','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M11">View MathML</a>

(3)

At first, the existence of a solution to two-point 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:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M12','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M12">View MathML</a>

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 pair<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M13','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M13">View MathML</a>satisfies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M14','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M14">View MathML</a>

(4)

and that there are positive constantsa, b, andcsuch that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M15','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M15">View MathML</a>

(5)

andfsatisfies the growth condition

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M16','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M16">View MathML</a>

Then BVP (1)-(3) possesses at least one solution.

Condition (4)-(5) trivially implies that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M17','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M17">View MathML</a>

(6)

It is easy to prove that condition (6) is equivalent to the fact that the rectangle

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M18','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M18">View MathML</a>

does not intersect any of the eigenlines of the two-parameter 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 pair<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M13','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M13">View MathML</a>satisfies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M20','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M20">View MathML</a>

(7)

and that there are positive constantsa, b, andcsuch that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M21','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M21">View MathML</a>

(8)

andfsatisfies the growth condition

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M22','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M22">View MathML</a>

(9)

Then PBVP (1)-(2) has at least one solution.

Condition (7)-(9) concerns a nonresonance condition involving the two-parameter linear eigenvalue problem (LEVP)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M23','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M23">View MathML</a>

(10)

In [2], it has been proved that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M13','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M13">View MathML</a> is an eigenvalue pair of LEVP (10) if and only if <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M25','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M25">View MathML</a>, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M26','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M26">View MathML</a>. Hence, for each <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M26','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M26">View MathML</a>, the straight line

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M28','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M28">View MathML</a>

is called an eigenline of LEVP (10). Condition (7)-(8) trivially implies that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M29','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M29">View MathML</a>

(11)

It is easy to prove that condition (11) is equivalent to the fact that the rectangle <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M30','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M30">View MathML</a> does not intersect any of the eigenline <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M31','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M31">View MathML</a> of LEVP (10). Hence, we call (11) and (9) the two-parameter nonresonance condition described by rectangle, which is a direct extension from a single-parameter nonresonance condition to a two-parameter one.

The purpose of this paper is to improve and extend the above-mentioned results. Different from the two-parameter nonresonance condition described by rectangle, we will present new two-parameter 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 pair<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M13','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M13">View MathML</a>satisfies (7). If there exist positive constantsa, b, andcsuch that (11) and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M33','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M33">View MathML</a>

(12)

hold, then PBVP (1)-(2) has at least one solution.

When the partial derivatives <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M34','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M34">View MathML</a> and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M35','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M35">View MathML</a> exist, if <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M36','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M36">View MathML</a> is large enough such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M37','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M37">View MathML</a>

(13)

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M38','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M38">View MathML</a> is a certain ellipse, and the corresponding close rectangle <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M30','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M30">View MathML</a> satisfies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M40','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M40">View MathML</a>

(14)

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 derivatives<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M34','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M34">View MathML</a>and<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M35','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M35">View MathML</a>exist in<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M43','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M43">View MathML</a>. If there exists an ellipse<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M44','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M44">View MathML</a>such that (13) holds for a positive real number<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M45','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M45">View MathML</a>large enough, and the corresponding close rectangle<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M30','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M30">View MathML</a>satisfies (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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M47','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M47">View MathML</a>

(15)

In this case, we have the following results.

Theorem 2Assume that the pair<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M13','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M13">View MathML</a>satisfies (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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M49','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M49">View MathML</a>

(16)

Condition (16) indicates that the ellipse <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M44','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M44">View MathML</a> does not intersect any of the eigenline <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M31','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M31">View MathML</a> of LEVP (10). Hence, we call (15) and (12) the two-parameter nonresonance condition described by ellipse, which is another extension of a single-parameter nonresonance condition. Similar to Corollary 1, we have the following corollary.

Corollary 2Assume that the partial derivatives<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M34','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M34">View MathML</a>and<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M35','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M35">View MathML</a>exist in<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M43','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M43">View MathML</a>. If there exists an ellipse<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M44','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M44">View MathML</a>such that (13) and (16) hold for a positive real number<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M45','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M45">View MathML</a>large enough, then PBVP (1)-(2) has at least one solution.

Theorem 3Assume that the partial derivatives<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M34','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M34">View MathML</a>and<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M35','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M35">View MathML</a>exist in<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M43','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M43">View MathML</a>. If there exists an ellipse<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M44','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M44">View MathML</a>such that (16) and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M61','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M61">View MathML</a>

(17)

hold, then PBVP (1)-(2) has a unique solution.

In Theorem 2, Theorem 3, and Corollary 2, we present a new two-parameter nonresonance condition described by ellipse, which is another extension of a single-parameter nonresonance condition. As a special case, we replace the ellipse <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M44','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M44">View MathML</a> by a circle

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M63','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M63">View MathML</a>

and obtain the following results.

Corollary 3Assume that there exist a circle<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M64','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M64">View MathML</a>and a positive constantcsuch that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M65','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M65">View MathML</a>

(18)

andfsatisfies the growth condition

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M66','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M66">View MathML</a>

(19)

Then PBVP (1)-(2) has at least one solution.

Condition (18) indicates that the circle <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M64','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M64">View MathML</a> does not intersect any of the eigenline <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M31','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M31">View MathML</a> of LEVP (10). Hence, we call condition (18)-(19) the two-parameter nonresonance condition described by circle, which is also an extension of a single-parameter nonresonance condition. Similarly to Corollary 2 and Theorem 3, we have the following corollaries.

Corollary 4Assume that the partial derivatives<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M34','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M34">View MathML</a>and<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M35','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M35">View MathML</a>exist in<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M43','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M43">View MathML</a>. If there exists a circle<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M64','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M64">View MathML</a>such that (18) and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M73','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M73">View MathML</a>

(20)

hold for a positive real number<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M45','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M45">View MathML</a>large enough, then PBVP (1)-(2) has at least one solution.

Corollary 5Assume that the partial derivatives<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M34','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M34">View MathML</a>and<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M35','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M35">View MathML</a>exist in<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M43','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M43">View MathML</a>. If there exists a circle<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M64','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M64">View MathML</a>such that (18) and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M79','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M79">View MathML</a>

(21)

hold, then PBVP (1)-(2) has a unique solution.

2 Preliminaries

Let <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M13','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M13">View MathML</a> be not eigenvalue pair of LEVP (10), i.e., <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M81','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M81">View MathML</a>. For any <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M82','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M82">View MathML</a>, we consider the linear periodic boundary value problem (LPBVP)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M83','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M83">View MathML</a>

(22)

By the Fredholm alternative, LPBVP (22) has a unique solution <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M84','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M84">View MathML</a>. If <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M85','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M85">View MathML</a>, then the solution <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M86','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M86">View MathML</a>. We define an operator T by

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M87','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M87">View MathML</a>

Then <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M88','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M88">View MathML</a> is a bounded linear operator, and we call it the solution operator of LPBVP (22). By compactness of the embedding <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M89','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M89">View MathML</a>, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M90','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M90">View MathML</a> is a compact linear operator.

Let <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M91','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M91">View MathML</a>. We choose an equivalent norm in the Sobolev space <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M92','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M92">View MathML</a> by

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M93','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M93">View MathML</a>

and denote the Banach space <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M92','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M92">View MathML</a> reendowed norm <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M95','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M95">View MathML</a> by <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M96','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M96">View MathML</a>.

Lemma 1Let<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M97','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M97">View MathML</a>. Then the solution operator of LPBVP (22) <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M98','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M98">View MathML</a>is a compact linear operator and its norm satisfies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M99','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M99">View MathML</a>

(23)

Proof We only need to prove that (23) holds.

Since <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M100','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M100">View MathML</a> is a complete orthogonal system of <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M101','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M101">View MathML</a>, every <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M82','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M82">View MathML</a> can be expressed by the Fourier series expansion

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M103','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M103">View MathML</a>

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M104','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M104">View MathML</a>, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M105','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M105">View MathML</a>. By the Parseval equality, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M106','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M106">View MathML</a>

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M107','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M107">View MathML</a> is the norm in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M101','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M101">View MathML</a>. Now, by uniqueness of the Fourier series expansion, the solution <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M109','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M109">View MathML</a> of LPBVP (22) has the Fourier series expansion

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M110','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M110">View MathML</a>

and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M111','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M111">View MathML</a> can be expressed by the Fourier series expansion

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M112','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M112">View MathML</a>

Hence, by the Parseval equality, we have

(24)

(25)

From (24) and (25), we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M115','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M115">View MathML</a>

This implies that (23) holds. The proof of Lemma 1 is completed. □

Lemma 2Let<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M116','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M116">View MathML</a>and<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M91','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M91">View MathML</a>. Then the rectangle<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M30','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M30">View MathML</a>satisfies condition (14) if and only if condition (11) holds.

Proof Condition (14) holds

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M119','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M119">View MathML</a> and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M120','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M120">View MathML</a> on the same side of every eigenline <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M31','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M31">View MathML</a>,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M122','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M122">View MathML</a> and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M123','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M123">View MathML</a> have the same sign,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M124','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M124">View MathML</a>,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M125','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M125">View MathML</a>.

The proof of Lemma 2 is completed. □

Lemma 3Let<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M116','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M116">View MathML</a>and<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M91','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M91">View MathML</a>. Then the ellipse<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M44','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M44">View MathML</a>satisfies condition (16) if and only if condition (15) holds.

Proof Condition (16) holds

⇔ for <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M129','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M129">View MathML</a>, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M130','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M130">View MathML</a> and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M131','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M131">View MathML</a> on the same side of every eigenline <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M31','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M31">View MathML</a>,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M133','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M133">View MathML</a> and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M134','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M134">View MathML</a> have the same sign,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M135','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M135">View MathML</a>,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M136','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M136">View MathML</a>,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M137','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M137">View MathML</a>,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M138','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M138">View MathML</a>.

The proof of Lemma 3 is completed. □

3 Proof of the main results

Proof of Theorem 1 We define a mapping <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M139','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M139">View MathML</a> by

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M140','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M140">View MathML</a>

(26)

It follows from (12) that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M141','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M141">View MathML</a> is continuous and satisfies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M142','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M142">View MathML</a>

(27)

Therefore, the mapping defined by

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M143','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M143">View MathML</a>

(28)

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 <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M144','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M144">View MathML</a>. We choose <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M145','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M145">View MathML</a>. Let <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M146','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M146">View MathML</a>. Then for any <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M147','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M147">View MathML</a>, from (27) and (28), we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M148','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M148">View MathML</a>

Therefore, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M149','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M149">View MathML</a>. By the Schauder’s fixed point theorem, Q has at least one fixed point in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M150','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M150">View MathML</a>, which is a solution of PBVP (1)-(2). □

By Lemma 2, we can obtain the following existence result:

Corollary 6Assume that the pair<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M13','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M13">View MathML</a>satisfies (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 <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M141','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M141">View MathML</a> be a mapping defined by (26). Then it follows from (12) that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M153','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M153">View MathML</a> is continuous and satisfies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M154','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M154">View MathML</a>

Thus, the mapping <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M155','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M155">View MathML</a> 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 <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M150','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M150">View MathML</a>, which is the solution of PBVP (1)-(2). □

Proof of Theorem 3 Let <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M141','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M141">View MathML</a> be defined by (26). Then <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M141','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M141">View MathML</a> is continuous. For any <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M159','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M159">View MathML</a>, from (17), we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M160','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M160">View MathML</a>

It follows from the above that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M161','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M161">View MathML</a>. Thus, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M155','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M155">View MathML</a> is a continuous mapping and it satisfies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M163','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M163">View MathML</a>

It follows from (16) and Lemma 3 that (15) holds. By (15) and Lemma 1, it is easy to see that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M144','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M144">View MathML</a>. Hence, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M165','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2013/1/14/mathml/M165">View MathML</a> 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 two-parameter 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 NWNU-LKQN-11-3.

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