Research | Open | Published:

# Nontrivial solutions for a boundary value problem with integral boundary conditions

*Boundary Value Problems***volume 2014**, Article number: 15 (2014)

## Abstract

This paper concerns the existence of nontrivial solutions for a boundary value problem with integral boundary conditions by topological degree theory. Here the nonlinear term is a sign-changing continuous function and may be unbounded from below.

## 1 Introduction

Consider the following Sturm-Liouville problem with integral boundary conditions

where $(Lu)(t)={(\tilde{p}(t){u}^{\prime}(t))}^{\prime}+q(t)u(t)$, $\tilde{p}(t)\in {C}^{1}[0,1]$, $\tilde{p}(t)>0$, $q(t)\in C[0,1]$, $q(t)<0$, *α* and *β* are right continuous on $[0,1)$, left continuous at $t=1$ and nondecreasing on $[0,1]$ with $\alpha (0)=\beta (0)=0$; ${\gamma}_{0},{\gamma}_{1}\in [0,\pi /2]$, ${\int}_{0}^{1}u(\tau )\phantom{\rule{0.2em}{0ex}}d\alpha (\tau )$ and ${\int}_{0}^{1}u(\tau )\phantom{\rule{0.2em}{0ex}}d\beta (\tau )$ denote the Riemann-Stieltjes integral of *u* with respect to *α* and *β*, respectively. Here the nonlinear term $f:[0,1]\times (-\mathrm{\infty},+\mathrm{\infty})\to (-\mathrm{\infty},+\mathrm{\infty})$ is a continuous sign-changing function and *f* may be unbounded from below, $h:(0,1)\to [0,+\mathrm{\infty})$ with $0<{\int}_{0}^{1}h(s)\phantom{\rule{0.2em}{0ex}}ds<+\mathrm{\infty}$ is continuous and is allowed to be singular at $t=0,1$.

Problems with integral boundary conditions arise naturally in thermal conduction problems [1], semiconductor problems [2], hydrodynamic problems [3]. Integral BCs (BCs denotes boundary conditions) cover multi-point BCs and nonlocal BCs as special cases and have attracted great attention, see [4–14] and the references therein. For more information about the general theory of integral equations and their relation with boundary value problems, we refer to the book of Corduneanu [4], Agarwal and O’Regan [5]. Yang [6], Boucherif [8], Chamberlain *et al.* [10], Feng [11], Jiang *et al.* [14] focused on the existence of positive solutions for the cases in which the nonlinear term is nonnegative. Although many papers investigated two-point and multi-point boundary value problems with sign-changing nonlinear terms, for example, [15–20], results for boundary value problems with integral boundary conditions when the nonlinear term is sign-changing are rarely seen except for a few special cases [7, 12, 13].

Inspired by the above papers, the aim of this paper is to establish the existence of nontrivial solutions to BVP (1.1) under weaker conditions. Our findings presented in this paper have the following new features. Firstly, the nonlinear term *f* of BVP (1.1) is allowed to be sign-changing and unbounded from below. Secondly, the boundary conditions in BVP (1.1) are the Riemann-Stieltjes integral, which includes multi-point boundary conditions in BVPs as special cases. Finally, the main technique used here is the topological degree theory, the first eigenvalue and its positive eigenfunction corresponding to a linear operator. This paper employs different conditions and different methods to solve the same BVP (1.1) as [7]; meanwhile, this paper generalizes the result in [17] to boundary value problems with integral boundary conditions. What we obtain here is different from [6–20].

## 2 Preliminaries and lemmas

Let $E=C[0,1]$ be a Banach space with the maximum norm $\parallel u\parallel ={max}_{0\le t\le 1}|u(t)|$ for $u\in E$. Define $P=\{u\in E\mid u(t)\ge 0,t\in [0,1]\}$ and ${B}_{r}=\{u\in E\mid \parallel u\parallel <r\}$. Then *P* is a total cone in *E*, that is, $E=\overline{P-P}$. ${P}^{\ast}$ denotes the dual cone of *P*, namely, ${P}^{\ast}=\{g\in {E}^{\ast}\mid g(u)\ge 0,\text{for all}u\in P\}$. Let ${E}^{\ast}$ denote the dual space of *E*, then by Riesz representation theorem, ${E}^{\ast}$ is given by

We assume that the following condition holds throughout this paper.

(H_{1}) $u(t)\equiv 0$ *is the unique* ${C}^{2}$ *solution of the linear boundary value problem*

Let $\phi ,\psi \in {C}^{2}([0,1],{\mathbf{R}}^{+})$ solve the following inhomogeneous boundary value problems, respectively:

Let ${\kappa}_{1}=1-{\int}_{0}^{1}\phi (\tau )\phantom{\rule{0.2em}{0ex}}d\alpha (\tau )$, ${\kappa}_{2}={\int}_{0}^{1}\psi (\tau )\phantom{\rule{0.2em}{0ex}}d\alpha (\tau )$, ${\kappa}_{3}={\int}_{0}^{1}\phi (\tau )\phantom{\rule{0.2em}{0ex}}d\beta (\tau )$, ${\kappa}_{4}=1-{\int}_{0}^{1}\psi (\tau )\phantom{\rule{0.2em}{0ex}}d\beta (\tau )$.

(H_{2}) ${\kappa}_{1}>0$, ${\kappa}_{4}>0$, $k={\kappa}_{1}{\kappa}_{4}-{\kappa}_{2}{\kappa}_{3}>0$.

**Lemma 2.1** ([7])

*If* (H_{1}) *and* (H_{2}) *hold*, *then BVP* (1.1) *is equivalent to*

*where* $G(t,s)\in C([0,1]\times [0,1],{\mathbf{R}}^{+})$ *is the Green function for* (1.1).

Define an operator $A:E\to E$ as follows:

It is easy to show that $A:E\to E$ is a completely continuous nonlinear operator, and if $u\in E$ is a fixed point of *A*, then *u* is a solution of BVP (1.1) by Lemma 2.1.

For any $u\in E$, define a linear operator $K:E\to E$ as follows:

It is easy to show that $K:E\to E$ is a completely continuous nonlinear operator and $K(P)\subset P$ holds. By [7], the spectral radius $r(K)$ of *K* is positive. The Krein-Rutman theorem [21] asserts that there are $\varphi \in P\setminus \{0\}$ and $\omega \in {P}^{\ast}\setminus \{0\}$ corresponding to the first eigenvalue ${\lambda}_{1}=1/r(K)$ of *K* such that

and

Here ${K}^{\ast}:{E}^{\ast}\to {E}^{\ast}$ is the dual operator of *K* given by:

The representation of ${K}^{\ast}$, the continuity of *G* and the integrability of *h* imply that $\omega \in {C}^{1}[0,1]$. Let $e(t):={\omega}^{\prime}(t)$. Then $e\in P\setminus \{0\}$, and (2.4) can be rewritten equivalently as

**Lemma 2.2** ([7])

*If* (H_{1}) *holds*, *then there is* $\delta >0$ *such that* ${P}_{0}=\{u\in P\mid {\int}_{0}^{1}u(t)e(t)\phantom{\rule{0.2em}{0ex}}dt\ge \delta \parallel u\parallel \}$ *is a subcone of* *P* *and* $K(P)\subset {P}_{0}$.

**Lemma 2.3** ([22])

*Let* *E* *be a real Banach space and* $\mathrm{\Omega}\subset E$ *be a bounded open set with* $0\in \mathrm{\Omega}$. *Suppose that* $A:\overline{\mathrm{\Omega}}\to E$ *is a completely continuous operator*. (1) *If there is* ${y}_{0}\in E$ *with* ${y}_{0}\ne 0$ *such that* $u\ne Au+\mu {y}_{0}$ *for all* $u\in \partial \mathrm{\Omega}$ *and* $\mu \ge 0$, *then* $deg(I-A,\mathrm{\Omega},0)=0$. (2) *If* $Au\ne \mu u$ *for all* $u\in \partial \mathrm{\Omega}$ *and* $\mu \ge 1$, *then* $deg(I-A,\mathrm{\Omega},0)=1$. *Here* deg *stands for the Leray*-*Schauder topological degree in* *E*.

**Lemma 2.4** *Assume that* (H_{1}), (H_{2}) *and the following assumptions are satisfied*:

(C_{1}) *There exist* $\varphi \in P\setminus \{0\}$, $\omega \in {P}^{\ast}\setminus \{0\}$ *and* $\delta >0$ *such that* (2.3), (2.4) *hold and* *K* *maps* *P* *into* ${P}_{0}$.

(C_{2}) *There exists a continuous operator* $H:E\to P$ *such that*

(C_{3}) *There exist a bounded continuous operator* $F:E\to E$ *and* ${u}_{0}\in E$ *such that* $Fu+{u}_{0}+Hu\in P$ *for all* $u\in E$.

(C_{4}) *There exist* ${v}_{0}\in E$ *and* $\zeta >0$ *such that* $KFu\ge {\lambda}_{1}(1+\zeta )Ku-KHu-{v}_{0}$ *for all* $u\in E$.

*Let* $A=KF$, *then there exists* $R>0$ *such that*

*where* ${B}_{R}=\{u\in E\mid \parallel u\parallel <R\}$.

*Proof* Choose a constant ${L}_{0}={(\delta {\lambda}_{1})}^{-1}(1+{\zeta}^{-1})+\parallel K\parallel >0$. From (C_{2}), for $0<{\epsilon}_{0}<{L}_{0}^{-1}$, there exists ${R}_{1}>0$ such that $\parallel u\parallel >{R}_{1}$ implies

Now we shall show

provided that *R* is sufficiently large.

In fact, if (2.7) is not true, then there exist ${u}_{1}\in \partial {B}_{R}$ and ${\mu}_{1}\ge 0$ satisfying

Since $\varphi \in P\setminus \{0\}$, $e(t)\in P\setminus \{0\}$, ${\int}_{0}^{1}\varphi (t)e(t)\phantom{\rule{0.2em}{0ex}}dt>0$. Multiply (2.8) by $e(t)$ on both sides and integrate on $[0,1]$. Then, by (C_{4}), (2.5), we get

Thus,

By (2.9), ${\int}_{0}^{1}(KH{u}_{1})(t)e(t)\phantom{\rule{0.2em}{0ex}}dt=r(K){\int}_{0}^{1}(H{u}_{1})(t)e(t)\phantom{\rule{0.2em}{0ex}}dt$ holds. Then (2.3), (2.6) and (2.10) imply

where ${L}_{1}={\zeta}^{-1}{\int}_{0}^{1}{v}_{0}(t)e(t)\phantom{\rule{0.2em}{0ex}}dt+{\int}_{0}^{1}(K{u}_{0})(t)e(t)\phantom{\rule{0.2em}{0ex}}dt$ is a constant.

(C_{3}) shows $F{u}_{1}+{u}_{0}+H{u}_{1}\in P$ and (C_{1}) implies ${\mu}_{1}\varphi ={\mu}_{1}{\lambda}_{1}K{\phi}_{1}\in {P}_{0}$. Then (C_{1}), (2.8) and Lemma 2.2 tell us that

The definition of ${P}_{0}$ yields

It follows from (2.6), (2.11) and (2.12) that

where ${L}_{2}=\parallel K{u}_{0}\parallel +{L}_{1}{\delta}^{-1}$ is a constant.

Since $0<{\epsilon}_{0}{L}_{0}<1$, then (2.13) deduces that (2.7) holds provided that *R* is sufficiently large such that $R>max\{{L}_{2}/(1-{\epsilon}_{0}{L}_{0}),{R}_{1}\}$. By (2.13) and Lemma 2.3, we have

□

## 3 Main results

**Theorem 3.1** *Assume that* (H_{1}), (H_{2}) *hold and the following conditions are satisfied*:

(A_{1}) *There exist two nonnegative functions* $b(t),c(t)\in C[0,1]$ *with* $c(t)\not\equiv 0$ *and one continuous even function* $B:\mathbf{R}\to {\mathbf{R}}^{+}$ *such that* $f(t,x)\ge -b(t)-c(t)B(x)$ *for all* $x\in \mathbf{R}$. *Moreover*, *B* *is nondecreasing on* ${\mathbf{R}}^{+}$ *and satisfies* ${lim}_{x\to +\mathrm{\infty}}\frac{B(x)}{x}=0$.

(A_{2}) $f:[0,1]\times \mathbf{R}\to \mathbf{R}$ *is continuous*.

(A_{3}) ${lim\hspace{0.17em}inf}_{x\to +\mathrm{\infty}}\frac{f(t,x)}{x}>{\lambda}_{1}$ *uniformly on* $t\in [0,1]$.

(A_{4}) ${lim\hspace{0.17em}sup}_{x\to 0}|\frac{f(t,x)}{x}|<{\lambda}_{1}$ *uniformly on* $t\in [0,1]$.

*Here* ${\lambda}_{1}$ *is the first eigenvalue of the operator* *K* *defined by* (2.2).

*Then BVP* (1.1) *has at least one nontrivial solution*.

*Proof* We first show that all the conditions in Lemma 2.4 are satisfied. By Lemma 2.2, condition (C_{1}) of Lemma 2.4 is satisfied. Obviously, $B:E\to P$ is a continuous operator. By (A_{1}), for any $\epsilon >0$, there is $L>0$ such that when $x>L$, $B(x)<\epsilon x$ holds. Thus, for $u\in E$ with $\parallel u\parallel >L$, $B(\parallel u\parallel )<\epsilon \parallel u\parallel $ holds. The fact that *B* is nondecreasing on ${\mathbf{R}}^{+}$ yields $(Bu)(t)\le B(\parallel u\parallel )$ for any $u\in P$, $t\in [0,1]$. Since $B:\mathbf{R}\to {\mathbf{R}}^{+}$ is an even function, for any $u\in E$ and $t\in [0,1]$, $(Bu)(t)\le B(\parallel u\parallel )$ holds, which implies $\parallel Bu\parallel \le B(\parallel u\parallel )$ for $u\in E$. Therefore,

that is, ${lim}_{\parallel u\parallel \to +\mathrm{\infty}}\frac{\parallel Bu\parallel}{\parallel u\parallel}=0$. Take $Hu={c}_{0}Bu$, for any $u\in E$, where ${c}_{0}={max}_{t\in [0,1]}c(t)>0$. Obviously, ${lim}_{\parallel u\parallel \to +\mathrm{\infty}}\frac{\parallel Hu\parallel}{\parallel u\parallel}=0$ holds. Therefore *H* satisfies condition (C_{2}) in Lemma 2.4.

Take ${u}_{0}(t)\equiv b={max}_{t\in [0,1]}b(t)>0$ and $(Fu)(t)=f(t,u(t))$ for $t\in [0,1]$, $u\in E$, then it follows from (A_{1}) that

which shows that condition (C_{3}) in Lemma 2.4 holds.

By (A_{3}), there exist ${\epsilon}_{1}>0$ and a sufficiently large number ${l}_{1}>0$ such that

Combining (3.1) with (A_{1}), there exists ${b}_{1}\ge 0$ such that

and so

Since *K* is a positive linear operator, from (3.2) we have

So condition (C_{4}) in Lemma 2.4 is satisfied.

According to Lemma 2.4, we derive that there exists a sufficiently large number $R>0$ such that

From (A_{4}) it follows that there exist $0<{\epsilon}_{2}<1$ and $0<r<R$ such that

Thus

Next we will prove that

If there exist ${u}_{1}\in \partial {B}_{r}$ and ${\mu}_{1}\in [0,1]$ such that ${u}_{1}={\mu}_{1}A{u}_{1}$. Let $z(t)=|{u}_{1}(t)|$. Then $z\in P$ and by (3.4), $z\le (1-{\epsilon}_{2}){\lambda}_{1}Kz$. The *n* th iteration of this inequality shows that $z\le {(1-{\epsilon}_{2})}^{n}{\lambda}_{1}^{n}{K}^{n}z$ ($n=1,2,\dots $), so $\parallel z\parallel \le {(1-{\epsilon}_{2})}^{n}{\lambda}_{1}^{n}\parallel {K}^{n}\parallel \cdot \parallel z\parallel $, that is, $1\le {(1-{\epsilon}_{2})}^{n}{\lambda}_{1}^{n}\parallel {K}^{n}\parallel $. This yields $1-{\epsilon}_{2}=(1-{\epsilon}_{2}){\lambda}_{1}r(K)=(1-{\epsilon}_{2}){\lambda}_{1}{lim}_{n\to \mathrm{\infty}}\sqrt[n]{\parallel {K}^{n}\parallel}\ge 1$, which is a contradictory inequality. Hence, (3.5) holds.

It follows from (3.5) and Lemma 2.3 that

By (3.3), (3.6) and the additivity of Leray-Schauder degree, we obtain

So *A* has at least one fixed point on ${B}_{R}\setminus {\overline{B}}_{r}$, namely, BVP (1.1) has at least one nontrivial solution. □

**Corollary 3.1** *Using* (${\mathrm{A}}_{1}^{\ast}$) *instead of* (A_{1}), *the conclusion of Theorem * 3.1 *remains true*.

(${\mathrm{A}}_{1}^{\ast}$) *There exist three constants* $b>0$, $c>0$ *and* $\alpha \in (0,1)$ *such that*

## References

- 1.
Cannon JR: The solution of the heat equation subject to the specification of energy.

*Q. Appl. Math.*1963, 21: 155-160. - 2.
Ionkin NI: Solution of a boundary value problem in heat conduction theory with nonlocal boundary conditions.

*Differ. Equ.*1977, 13: 294-304. - 3.
Chegis RY: Numerical solution of a heat conduction problem with an integral boundary condition.

*Litov. Mat. Sb.*1984, 24: 209-215. - 4.
Corduneanu C:

*Integral Equations and Applications*. Cambridge University Press, Cambridge; 1991. - 5.
Agarwal RP, O’Regan D:

*Infinite Interval Problems for Differential, Difference and Integral Equations*. Kluwer Academic, Dordrecht; 2001. - 6.
Yang ZL: Existence and nonexistence results for positive solutions of an integral boundary value problem.

*Nonlinear Anal.*2006, 65: 1489-1511. 10.1016/j.na.2005.10.025 - 7.
Yang ZL: Existence of nontrivial solutions for a nonlinear Sturm-Liouville problem with integral boundary value conditions.

*Nonlinear Anal.*2008, 68: 216-225. 10.1016/j.na.2006.10.044 - 8.
Boucherif A: Second-order boundary value problems with integral boundary conditions.

*Nonlinear Anal.*2009, 70: 364-371. 10.1016/j.na.2007.12.007 - 9.
Kong LJ: Second order singular boundary value problems with integral boundary conditions.

*Nonlinear Anal.*2010, 72: 2628-2638. 10.1016/j.na.2009.11.010 - 10.
Chamberlain J, Kong LJ, Kong QK: Nodal solutions of boundary value problems with boundary conditions involving Riemann-Stieltjes integrals.

*Nonlinear Anal.*2011, 74: 2380-2387. 10.1016/j.na.2010.11.040 - 11.
Feng MQ: Existence of symmetric positive solutions for a boundary value problem with integral boundary conditions.

*Appl. Math. Lett.*2011, 24: 1419-1427. 10.1016/j.aml.2011.03.023 - 12.
Li YH, Li FY: Sign-changing solutions to second-order integral boundary value problems.

*Nonlinear Anal.*2008, 69: 1179-1187. 10.1016/j.na.2007.06.024 - 13.
Li HT, Liu YS: On sign-changing solutions for a second-order integral boundary value problem.

*Comput. Math. Appl.*2011, 62: 651-656. 10.1016/j.camwa.2011.05.046 - 14.
Jiang JQ, Liu LS, Wu YH: Second-order nonlinear singular Sturm-Liouville problems with integral boundary conditions.

*Appl. Math. Comput.*2009, 215: 1573-1582. 10.1016/j.amc.2009.07.024 - 15.
Sun JX, Zhang GW: Nontrivial solutions of singular superlinear Sturm-Liouville problem.

*J. Math. Anal. Appl.*2006, 313: 518-536. 10.1016/j.jmaa.2005.06.087 - 16.
Han GD, Wu Y: Nontrivial solutions of singular two-point boundary value problems with sign-changing nonlinear terms.

*J. Math. Anal. Appl.*2007, 325: 1327-1338. 10.1016/j.jmaa.2006.02.076 - 17.
Liu LS, Liu BM, Wu YH: Nontrivial solutions of

*m*-point boundary value problems for singular second-order differential equations with a sign-changing nonlinear term.*J. Comput. Appl. Math.*2009, 224: 373-382. 10.1016/j.cam.2008.05.007 - 18.
Liu LS, Liu BM, Wu YH: Nontrivial solutions for higher-order

*m*-point boundary value problem with a sign-changing nonlinear term.*Appl. Math. Comput.*2010, 217: 3792-3800. 10.1016/j.amc.2010.09.038 - 19.
Graef JR, Kong LJ: Periodic solutions for functional differential equations with sign-changing nonlinearities.

*Proc. R. Soc. Edinb. A*2010, 140: 597-616. 10.1017/S0308210509000523 - 20.
Wang YQ, Liu LS, Wu YH: Positive solutions for a class of fractional boundary value problem with changing sign nonlinearity.

*Nonlinear Anal.*2011, 74: 6434-6441. 10.1016/j.na.2011.06.026 - 21.
Krein MG, Rutman MA: Linear operators leaving invariant a cone in a Banach space.

*Transl. Am. Math. Soc.*1962, 10: 199-325. - 22.
Guo DJ, Lakshmikantham V:

*Nonlinear Problems in Abstract Cones*. Academic Press, Orlando; 1988.

## Acknowledgements

The first two authors were supported financially by the National Natural Science Foundation of China (11201473, 11271364) and the Fundamental Research Funds for the Central Universities (2013QNA35, 2010LKSX09, 2010QNA42). The third author was supported financially by the National Natural Science Foundation of China (11371221, 11071141), the Specialized Research Foundation for the Doctoral Program of Higher Education of China (20123705110001) and the Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province.

## Author information

## Additional information

### Competing interests

The authors declare that no conflict of interest exists.

### Authors’ contributions

All authors participated in drafting, revising and commenting on the manuscript. All authors read and approved the final manuscript.

## Rights and permissions

## About this article

#### Received

#### Accepted

#### Published

#### DOI

### Keywords

- Nonlinear Term
- Nontrivial Solution
- Continuous Operator
- Real Banach Space
- Positive Linear Operator