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Existence and uniqueness of positive solutions to boundary value problem with increasing homeomorphism and positive homomorphism operator
Advances in Difference Equations volume 2014, Article number: 20 (2014)
Abstract
In this paper, we consider the following nonlinear boundary value problem: {(\phi ({u}^{\prime}(t)))}^{\prime}+a(t)f(u(t))=0, 0<t<1, u(0)={\sum}_{i=1}^{m2}{\alpha}_{i}u({\xi}_{i}), {u}^{\prime}(1)=0, where \phi :\mathbb{R}\u27f6\mathbb{R} is an increasing homeomorphism and positive homomorphism with \phi (0)=0. By using a fixedpoint theorem on partially ordered sets, we obtain sufficient conditions for the existence and uniqueness of positive and nondecreasing solutions to the above boundary value problem.
MSC:34B18, 34B27.
1 Introduction
In this paper, we consider the existence and uniqueness of a positive and nondecreasing solution to the following boundary value problem:
where \phi :\mathbb{R}\to \mathbb{R} is an increasing homeomorphism and positive homomorphism with \phi (0)=0. Here {\xi}_{i}\in (0,1) with 0<{\xi}_{1}<{\xi}_{2}<\cdots <{\xi}_{m2}<1 and {\alpha}_{i} satisfy {\alpha}_{i}\in [0,+\mathrm{\infty}), 0<{\sum}_{i=1}^{m2}{\alpha}_{i}<1.
A projection \phi :\mathbb{R}\to \mathbb{R} is called an increasing homeomorphism and positive homomorphism, if the following conditions are satisfied:

(1)
\phi (x)\le \phi (y), for all x,y\in \mathbb{R} with x\le y;

(2)
φ is a continuous bijection and its inverse mapping is also continuous;

(3)
\phi (xy)=\phi (x)\phi (y), for all x,y\in {\mathbb{R}}_{+}.
In the above definition, we can replace the condition (3) by the following stronger condition:

(4)
\phi (xy)=\phi (x)\phi (y), for all x,y\in \mathbb{R}, where \mathbb{R}=(\mathrm{\infty},+\mathrm{\infty}).
Remark 1.1 If conditions (1), (2), and (4) hold, then it implies that φ is homogeneous generating a pLaplace operator, i.e. \phi (x)={x}^{p2}x, for some p>1.
Recently, the existence and multiplicity of positive solutions for the pLaplacian operator, i.e., \phi (x)={x}^{p2}x, for some p>1, have received wide attention, see [1–3] and references therein. We know that the oddness of a pLaplacian operator is key to the proof. However, in this paper we define a new operator, which improves and generates a pLaplacian operator for some p>1, and φ is not necessarily odd. Moreover research of increasing homeomorphisms and positive homomorphism operators has proceeded very slowly, see [4, 5].
In [4], Liu and Zhang studied the existence of positive solutions of quasilinear differential equation
where \phi :\mathbb{R}\to \mathbb{R} is an increasing homeomorphism and positive homomorphism and \phi (0)=0. They obtain the existence of one or two positive solutions by using a fixedpoint index theorem in cones. But the uniqueness of the solution is not treated.
In [5], the authors showed that there exist countably many positive solutions by using the fixedpoint index theory and a new fixedpoint theorem in cones. They also assumed that the operator \phi :\mathbb{R}\to \mathbb{R} is an increasing homeomorphism and a positive homomorphism, and \phi (0)=0.
In [6], the authors established the existence and uniqueness of a positive and nondecreasing solution to a singular boundary value problem of a class of nonlinear fractional differential equation. Their analysis relies on a fixedpoint theorem in partially ordered sets. The existence of a fixed point in partially ordered sets has been considered recently in [6–10].
But whether or not we can obtain the existence and uniqueness of a positive and nondecreasing solution to the boundary value problem (1.1)(1.2) still remains unknown. So, motivated by all the works above, we will prove the existence and uniqueness of a positive and nondecreasing solution for the boundary value problems (1.1)(1.2) by using a fixedpoint theorem on partially ordered sets.
2 Some definitions and fixedpoint theorems
Definition 2.1 Let (E,\parallel \cdot \parallel ) be a real Banach space. A nonempty, closed, convex set P\subset E is said to be a cone provided the following are satisfied:

(a)
if y\in P and \lambda \ge 0, then \lambda y\in P;

(b)
if y\in P and y\in P, then y=0.
If P\subset E is a cone, we denote the order induced by P on E by ≤, that is, x\le y if and only if yx\in P.
The following fixedpoint theorems in partially ordered sets are fundamental and important to the proofs of our main results.
Theorem 2.1 ([7])
Let (E,\le ) be a partially ordered set and suppose that there exists a metric d in E such that (E,d) is a complete metric space. Assume that E satisfies the following condition:
Let T:E\to E be a nondecreasing mapping such that
where \psi :[0,+\mathrm{\infty})\to [0,+\mathrm{\infty}) is a continuous and nondecreasing function such that ψ is positive in (0,+\mathrm{\infty}), \psi (0)=0 and {lim}_{t\to \mathrm{\infty}}\psi (t)=\mathrm{\infty}. If there exists {x}_{0}\in E with {x}_{0}\le T({x}_{0}), then T has a fixed point.
If we consider that (E,\le ) satisfies the following condition:
then we have the following result.
Theorem 2.2 ([8])
Adding condition (2.2) to the hypotheses of Theorem 2.1, we obtain uniqueness of the fixed point.
3 Main results
The basic space used in this paper is E=C[0,1]. Then E is a real Banach space with the norm \parallel u\parallel ={max}_{0\le t\le 1}u(t). Note that this space can be equipped with a partial order given by
In [8] it is proved that (C[0,1],\le ) with the classic metric given by
satisfies condition (2.1) of Theorem 2.1. Moreover, for x,y\in C[0,1] as the function max\{x,y\}\in C[0,1], (C[0,1],\le ) satisfies condition (2.2).
The main result of this paper is the following.
Theorem 3.1 The boundary value problem (1.1)(1.2) has a unique positive solution u(t) which is strictly increasing if the following conditions are satisfied:

(A)
a(t) is a nonnegative measurable function defined in [0,1] and a(t) does not identically vanish on any subinterval of [0,1] and
0<{\int}_{0}^{1}a(t)\phantom{\rule{0.2em}{0ex}}dt<+\mathrm{\infty};
(f_{1}) f:[0,+\mathrm{\infty})\to [0,+\mathrm{\infty}) is continuous and nondecreasing respect to u and f(u(t))\not\equiv 0 for t\in Z\subset [0,1] with \mu (Z)>0 (μ denotes the Lebesgue measure);
(f_{2}) there exists 1<\lambda +1<\frac{1{\sum}_{i=1}^{m2}{\alpha}_{i}}{{\phi}^{1}({\int}_{0}^{1}a(\tau )\phantom{\rule{0.2em}{0ex}}d\tau )} such that for u,v\in [0,+\mathrm{\infty}) with u\ge v and t\in [0,1]
Proof Consider the cone
As K is a closed set of C[0,1], K is a complete metric space with the distance given by d(u,v)={sup}_{t\in [0,1]}u(t)v(t).
Now, we consider the operator T defined by
By conditions (A), (f_{1}), we have T(K)\subset K.
We now show that all the conditions of Theorem 2.1 and Theorem 2.2 are satisfied.
Firstly, by condition (f_{1}), for u,v\in K and u\ge v, we have
This proves that T is a nondecreasing operator. On the other hand, for u\ge v and by (f_{2}) we have
Since the function h(x)=ln(x+1) is nondecreasing, and condition (f_{2}), then we have
Let \psi (x)=xln(x+1). Obviously \psi :[0,+\mathrm{\infty})\to [0,+\mathrm{\infty}) is continuous, nondecreasing, positive in (0,+\mathrm{\infty}), \psi (0)=0, and {lim}_{x\to +\mathrm{\infty}}\psi (x)=+\mathrm{\infty}. Thus, for u\ge v, we have
By conditions (A) and (f_{1}), we know that
Therefore, by Theorem 2.1 we know that problem (1.1)(1.2) has at least one nonnegative solution. As (K,\le ) satisfies condition (2.2), thus, Theorem 2.2 implies the uniqueness of the solution. By definition of T and conditions (A), (f_{1}), it is easy to prove that this solution u(t) is strictly increasing. □
4 Example
Example 4.1 Consider the boundary value problem
where
a(t)=\frac{1}{5}{t}^{4} and f(x)={[ln(x+2)]}^{2} for x\in [0,+\mathrm{\infty}).
Proof Note that f is a continuous function and f(x)>0. Moreover, f is nondecreasing with respect to x since \frac{\partial f}{\partial x}=\frac{2}{x+2}ln(x+2)>0. On the other hand, for u\ge v, we have
In this case, \lambda =1 because 1<\lambda +1<\frac{1{\sum}_{i=1}^{m2}{\alpha}_{i}}{{\phi}^{1}({\int}_{0}^{1}a(\tau )\phantom{\rule{0.2em}{0ex}}d\tau )}=\frac{5}{2}. Thus Theorem 3.1 implies that the boundary value problem (4.1) has a unique positive solution which is strictly increasing. □
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Acknowledgements
The authors would like to express sincere thanks to the anonymous referee for his/her carefully reading the manuscript and valuable comments and suggestions. The authors are supported by the National Natural Science Foundation of China (Grant No. 11301038), Research Foundation during the 12st FiveYear Plan Period of Department of Education of Jilin Province, China (Grant [2013] No. 252), Youth Foundation for Science and Technology Department of Jilin Province (20130522100JH).
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Miao, F., Zhou, C. & Song, Y. Existence and uniqueness of positive solutions to boundary value problem with increasing homeomorphism and positive homomorphism operator. Adv Differ Equ 2014, 20 (2014). https://doi.org/10.1186/16871847201420
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DOI: https://doi.org/10.1186/16871847201420
Keywords
 partially ordered sets
 fixedpoint theorem
 positive solution