Symmetric positive solutions to a second-order boundary value problem with integral boundary conditions

The theory of boundary value problems with integral boundary conditions for ordinary differential equations arises in different areas of applied mathematics and physics. For example, heat conduction, chemical engineering, underground water flow, thermo-elasticity, and plasma physics can be reduced to the nonlinear problems with integral boundary conditions; we refer readers to [1–3] for examples and references.

At the same time, boundary value problems with integral boundary conditions constitute a very interesting and important class of problems. They include two, three, multipoint and nonlocal boundary value problems as special cases.

Hence, increasing attention is paid to boundary value problems with integral boundary conditions [4–8]. Generally, the Guo-Krasnosel’ skii fixed point theorem in a cone, the Leggett-Williams fixed point theorem, the method of upper and lower solutions and the monotone iterative technique play extremely important roles in proving the existence of solutions to boundary value problems. In particular, we would like to mention some excellent results.

where $p,q\in L^1[0,1]$, h and f are continuous. The existence of at least one symmetric positive solution is obtained by the application of the fixed point index in cones.

where ϕ, f, $h_1$ and $h_2$ are continuous, $k_1$ and $k_2$ are nonnegative constants. The existence result was obtained by applying the method of upper and lower solutions and Leray-Schauder degree theory. Theorem 1 (see [7]) supposed that the upper and lower solutions exist, and then, the theory of differential inequalities was used to prove that there is a solution to the boundary value problem between the upper and lower solutions.

And by applying monotone iterative techniques, author proved the existence of n symmetric positive solutions.

where $p\in L^1[0,1]$. We construct a specific form of the symmetric upper and lower solutions, and by applying monotone iterative techniques, we construct successive iterative schemes for approximating solutions.

The difficulty of this paper is that the nonlinear term f depends on $u^{\prime }$, which leads to complexities to prove the properties of the operator T, especially the monotonicity of the operator T. In Lemma 2.2, we skillfully use the cone’s character to overcome the mentioned obstacle. In addition, it is worth stating that the first term of our iterative scheme is a simple function or a constant function. Therefore, the iterative scheme is feasible. Under the appropriate assumptions on nonlinear term, this paper aims to establish a new and general result on the existence of a symmetric positive solution to BVP (1.1) and (1.2).

Definition 2.2 Let $(E,⩽)$ be an ordered Banach space. An operator $\phi :E\to E$ is said to be nondecreasing (nonincreasing) provided that $\phi (u)⩽\phi (v)$ ($\phi (u)⩾\phi (v)$) for all $u,v\in E$ with $u⩽v$. If the inequality is strict, then φ is said to be strictly nondecreasing (nonincreasing).

for any $x_1,x_2\in [0,1]$ and $\lambda \in [0,1]$.

We consider the Banach space $E=C^1[0,1]$ equipped with the norm $\parallel u\parallel =max\{{\parallel u\parallel }_{\mathrm{\infty }},{\parallel u^{\prime }\parallel }_{\mathrm{\infty }}\}$, where ${\parallel u\parallel }_{\mathrm{\infty }}={max}_{x\in [0,1]}|u(x)|$. In this paper, a symmetric positive solution $u^{\ast }$ of (1.1) means a function which is symmetric and positive on $(0,1)$ and satisfies equation (1.1) as well as the boundary conditions (1.2).

In this paper, we always suppose that the following assumptions hold:

(H₁) $f\in C([0,1]\times [0,+\mathrm{\infty })\times R,[0,+\mathrm{\infty }))$, $f(x,u,v)=f(1-x,u,-v)$ for $x\in [\frac{1}{2},1]$, and $f(x,u,v)>0$ for all $(x,u,v)\in [0,1]\times [0,+\mathrm{\infty })\times R$;

(H₂) $f(x,\cdot ,v)$ is nondecreasing for each $(x,v)\in [0,\frac{1}{2}]\times R$, $f(x,u,\cdot )$ is nondecreasing for $(x,u)\in [0,\frac{1}{2}]\times [0,+\mathrm{\infty })$;

(H₃) $p\in L^1[0,1]$ is nonnegative and $0\le \mu <1$, where $\mu ={\int }_0^{1}p(s)ds$.

It is easy to see that P is a cone in E.

for $x\in [0,1]$.

Lemma 2.2 If (H₃) is satisfied, $T:P\to P$ is completely continuous, i.e., T is continuous and compact. Moreover, T is nondecreasing provided that (H₂) holds.

Obviously, Ty is concave. From the expression of Ty, combining with Lemma 2.1, we know that Ty is nonnegative on $[0,1]$. We now prove that Ty is symmetric about $\frac{1}{2}$.

And the similar results can be obtained for $\frac{1}{2}⩽x_1⩽x_2⩽1$ and $0⩽x_1⩽\frac{1}{2}⩽x_2⩽1$.

The Arzelà-Ascoli theorem guarantees that T Ω is relatively compact, which means T is compact.

Finally, we show that Ty is nondecreasing about y.

A similar result can be obtained for $x\in [\frac{1}{2},1]$. And it is easy to see that $(T{y}_2)(x)-(T{y}_1)(x)$ is symmetric about $\frac{1}{2}$. So, $(T{y}_2-T{y}_1)\in P$ and thus T is nondecreasing. □

Proof We denote $P_a=\{w\in P:\parallel w\parallel ⩽a\}$. In what follows, we first prove $T{P}_a\subset P_a$.

Let $w\in P_a$, then $0⩽w(x)⩽{max}_{x\in [0,1]}w(x)={\parallel w\parallel }_{\mathrm{\infty }}⩽a$, ${max}_{x\in [0,1]}|w^{\prime }(x)|=w^{\prime }(0)⩽a$.

So, $w_0(x)-w_1(x)\in P_a$. By Lemma 2.2, we know $T{w}_1⩽T{w}_0$, which means $w_2⩽w_1$, $0<x<1$. By induction, $w_{n+1}⩽w_n$, $0<x<1$ ($n=0,1,2,\dots$). Hence, we assert that $w_n\to w^{\ast }$. Let $n\to \mathrm{\infty }$ in (3.4) to obtain $T{w}^{\ast }=w^{\ast }$ since T is continuous. It is well known that the fixed point of the operator T is the solution of BVP (1.1) and (1.2). Therefore, $w^{\ast }$ is a concave symmetric positive solution of BVP (1.1) and (1.2).

Similarly to ${\{v_n\}}_{n=1}^{\mathrm{\infty }}$, we assert that ${\{v_n\}}_{n=1}^{\mathrm{\infty }}$ has a convergent subsequence ${\{v_{n_k}\}}_{k=1}^{\mathrm{\infty }}$ and there exists $v^{\ast }\in P_a$ such that $v_{n_k}\to v^{\ast }$. Now, since $v_1⩾v_0$, by Lemma 2.2, we know $T{v}_1⩾T{v}_0$, which means $v_2⩾v_1$, $0<x<1$. By induction, $v_{n+1}⩾v_n$, $0<x<1$ ($n=0,1,2,\dots$). Hence, we assert that $v_n\to v^{\ast }$, $T{v}^{\ast }=v^{\ast }$, and $v^{\ast }(x)>0$, $x\in (0,1)$. Therefore, $v^{\ast }$ is a concave symmetric positive solution of BVP (1.1) and (1.2). □

Remark The existence of a solution under the assumptions of Theorem 3.1 is just a consequence of Schauder’s fixed point theorem. The monotone iterative technique adds the information about the approximation sequences.