# Existence and uniqueness of positive and nondecreasing solutions for a class of fractional boundary value problems involving the p-Laplacian operator

## Abstract

In this article, we investigate the existence of a solution arising from the following fractional q-difference boundary value problem by using the p-Laplacian operator: $$D_{q}^{\gamma}(\phi_{p}(D_{q}^{\delta}y(t)))+f(t,y(t))=0$$ ($$0< t<1$$; $$0<\gamma<1$$; $$3<\delta<4$$), $$y(0)=(D_{q}y)(0)=(D_{q}^{2}y)(0) =0$$, $$a_{1}(D_{q}y)(1)+a_{2}(D_{q}^{2}y)(1)=0$$, $$a_{1} +\vert a_{2}\vert \neq0$$, $$D_{0+}^{\gamma}y(t)|_{t=0}=0$$. We make use of such a fractional q-difference boundary value problem in order to show the existence and uniqueness of positive and nondecreasing solutions by means of a familiar fixed point theorem.

## Introduction, definitions, and preliminaries

Recently, many mathematicians, physicists and engineers have extensively studied various families of fractional differential equations and their applications. The development of the theory of fractional calculus stems from the applications in many widespread disciplines such as engineering, economics and other fields. Jackson  introduced the q-difference calculus (or the so-called quantum calculus), which is an old subject. New developments in this theory were made. These include (for example) the q-analogs of the fractional integral and the fractional derivative operators, the q-analogs of the Laplace, Fourier, and other integral transforms, and so on (see, for details, , and ; see also a very recent work  dealing with q-calculus).

Throughout our present investigation, we make use of the following notations:

$$\mathbb{N}:= \{ 1,2,3,\ldots \} \quad\mbox{and}\quad \mathbb{N}_{0}:= \mathbb{N}\cup \{ 0 \} .$$

Moreover, as usual, denotes the set of real numbers, $$\mathbb{R}_{+}$$ denotes the set of positive real numbers, $$\mathbb{Z}_{-}$$ denotes the set of negative integers, and denotes the set of complex numbers.

Al-Salam  and Agarwal  investigated several properties and results for some fractional q-integrals and fractional q-derivatives which are based on the q-analog of the ordinary integral:

$$\int_{a}^{x}f ( t )\,dt.$$

Atici and Eloe  constructed interesting links between the fractional q-calculus in the existing literature and the fractional q-calculus on a time scale given by

$$T_{t_{0}}= \bigl\{ t:t=t_{0}q^{n}\ (n\in \mathbb{N}_{0}; t_{0}\in \mathbb{R}; 0< q<1) \bigr\} .$$

They also derived some properties of a q-Laplace transform, which are used to solve fractional q-difference equations. Benchohra et al.  investigated the existence of solutions for fractional-order functional equations by means of the Banach fixed point theorem and its nonlinear alternative of Leray-Schauder type. El-Sayed et al.  studied the stability, existence, uniqueness, and numerical solution of the fractional-order logistic equation. The work of El-Shahed  was concerned with the existence and non-existence of positive solutions for some nonlinear fractional boundary value problems. Ferreira (see  and ) investigated the existence of nontrivial solutions to some nonlinear q-fractional boundary value problems by applying a fixed point theorem in cones. For more information on the positive solutions (or nontrivial solutions) for a class of boundary value problems with the fractional differential equations (or q-fractional differential equations), we refer the reader to such earlier works as (for example) [5, 10, 2231], and .

We now review briefly some concepts of the quantum calculus.

For $$q\in(0,1)$$, the q-integer $$[\lambda]_{q}$$ is defined by

$$[\lambda]_{q}=\frac{1-q^{\lambda}}{1-q} \quad( \lambda \in \mathbb{R} ) .$$

Clearly, we have

$$\lim_{q\rightarrow1^{-}} [ \lambda ] _{q}=\lambda,$$

so we say that $$[ \lambda ] _{q}$$ is a q-analog of the number λ. The q-analog of the binomial formula $$(a-b)^{n}$$ is given by

$$(a-b)^{0}=1 \quad\mbox{and}\quad (a-b)^{n}=\prod _{k=0}^{n-1}\bigl(a-bq^{k}\bigr) \quad( a,b\in \mathbb {R}; n\in\mathbb{N}_{0} ) .$$

More generally, we have

$$(a-b)^{(\delta)}=a^{\delta}\prod_{n=0}^{\infty} \biggl( \frac {a-bq^{n}}{a-bq^{\delta+n}} \biggr) \quad(\delta\in\mathbb{R}).$$
(1.1)

Clearly, if we set $$b=0$$ in Eq. (1.1), it reduces immediately to

$$a^{(\delta)}=a^{\delta} \quad(\delta\in\mathbb{R}).$$

The q-gamma function is defined as follows:

$$\Gamma_{q}(x)=\frac{(1-q)^{(x-1)}}{(1-q)^{x-1}} \quad \bigl( x\in \mathbb{R} \setminus \bigl\{ \{ 0 \} \cup\mathbb {Z}_{-} \bigr\} \bigr)$$

and satisfies the formula:

$$\Gamma_{q}(x+1)=[x]\Gamma_{q}(x).$$

The q-derivative of a function $$f(x)$$ is given by

$$(D_{q}f) (x)=\frac{f(x)-f(qx)}{(1-q)x}\quad \mbox{and}\quad \lim_{q\rightarrow1^{-}}(D_{q}f) (x)=f^{\prime}(x)=\frac{d}{dx}\bigl\{ f(x)\bigr\} .$$

For the q-derivatives of higher order, we have

$$\bigl(D_{q}^{0}f\bigr) (x)=f(x) \quad\mbox{and}\quad \bigl(D_{q}^{n}f\bigr) (x)=D_{q} \bigl(D_{q}^{n-1}f\bigr) (x) \quad( n\in\mathbb {N} ).$$

Suppose now that $$0< a<b$$. Then the definite q-integral is defined as follows:

$$(I_{q}f) (x)=\int_{0}^{x}f(t) \,d_{q}t=x(1-q)\sum_{n=0}^{\infty }f \bigl(xq^{n}\bigr)q^{n} \quad\bigl( x\in [0,b] \bigr)$$

and

$$\int_{a}^{b}f(t)\,d_{q}t=\int _{0}^{b}f(t)\,d_{q}t-\int _{0}^{a}f(t)\,d_{q}t.$$

The operator $$I_{q}^{n}$$ can be defined by

$$\bigl(I_{q}^{0}f\bigr) (x)=f(x) \quad\mbox{and}\quad \bigl(I_{q}^{n}f\bigr) (x)=I_{q} \bigl(I_{q}^{n-1}f\bigr) (x) \quad( n\in\mathbb {N} ).$$

The Fundamental Theorem of Calculus does indeed apply mutatis mutandis to the operators $$I_{q}$$ and $$D_{q}$$. We thus have

$$(D_{q}I_{q}f) (x)=f(x),$$

and if f is continuous at $$x=0$$, then

$$(I_{q}D_{q}f) (x)=f(x)-f(0).$$

Denoting by $$_{x}D_{q}$$ the q-derivative with respect to the variable x, we now recall the following three formulas which will be used in the remainder of this paper:

\begin{aligned}& \bigl[ a(t-s)\bigr]^{(\delta)}=a^{\delta}(t-s)^{(\delta)} , \end{aligned}
(1.2)
\begin{aligned}& {_{x}}D_{q}(t-s)^{(\delta)}=[\delta]_{q}(x-s)^{(\delta-1)} , \end{aligned}
(1.3)
\begin{aligned}& \biggl({_{x}}D_{q}\int_{0}^{x}f(x,t) \,d_{q}t \biggr) (x)=\int_{0}^{x}{_{x}}D_{q}f(x,t) \,d_{q}t+f(qx,x). \end{aligned}
(1.4)

### Definition 1

(see )

Let $$\delta\geqq0$$ and f be a function defined on $$[0,1]$$. The fractional q-integral of the Riemann-Liouville type is given by

$$\bigl(I_{q}^{0}f\bigr) (x)=f(x)$$

and

$$\bigl(I_{q}^{\delta}f\bigr) (x)=\frac{1}{\Gamma_{q}(\delta)}\int _{0}^{x}(x-qt)^{( \delta-1)}f(t)\,d_{q}t\quad \bigl( \delta>0; x\in [ 0,1] \bigr).$$

### Definition 2

(see  and )

The fractional q-derivative of the Riemann-Liouville type of order δ ($$\delta\geqq0$$) is defined by

$$\bigl(D_{q}^{0}f\bigr) (x)=f(x)$$

and

$$\bigl(D_{q}^{\delta}f\bigr) (x)=\bigl(D_{q}^{m}I_{q}^{m-\delta}f \bigr) (x) \quad( \delta >0 ),$$

where m is the smallest integer greater than or equal to δ.

### Lemma 1

(see )

Let $$\delta \geqq0$$, $$\beta\geqq0$$, and f be a function defined on $$[0,1]$$. Then the following two formulas hold true:

1. (1)

$$(I_{q}^{\beta}I_{q}^{\delta }f)(x)=(I_{q}^{\delta +\beta}f)(x)$$;

2. (2)

$$(D_{q}^{\delta}I_{q}^{\delta}f)(x)=f(x)$$.

### Lemma 2

(see  and )

Let $$\delta>0$$ and p be a positive integer. Then the following equality holds:

$$\bigl(I_{q}^{\delta}D_{q}^{p}f\bigr) (x)= \bigl(D_{q}^{p}I_{q}^{\delta }f\bigr) (x)- \sum_{k=0}^{p-1}\frac{x^{\delta-p+k}}{\Gamma_{q}(\delta+k-p+1)} \bigl(D_{q}^{k}f\bigr) (0).$$

### Theorem 1

(see [33, 34], and )

(a) Let $$( E,\leqq )$$ 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 condition that, if $$\{ x_{n} \}$$ is a nondecreasing sequence in E such that $$x_{n}\rightarrow x$$, then

$$x_{n}\leqq x \quad( n\in\mathbb{N} ).$$

Let $$T:E\rightarrow E$$ be a nondecreasing mapping such that

$$d ( Tx,Ty ) \leqq d ( x,y ) -\psi \bigl( d ( x,y ) \bigr) \quad(x\geqq y),$$

where

$$\psi:[ 0,+\infty ) \rightarrow [ 0,+\infty )$$

is a continuous and nondecreasing function such that ψ is positive in $$( 0,\infty )$$, $$\psi ( 0 ) =0$$, and

$$\lim_{t\rightarrow\infty}\psi ( t ) =\infty.$$

If there exists $$x_{0}\in E$$ with $$x_{0}\leqq T ( x_{0} )$$, then T has a fixed point.

(b) If we assume that $$( E,\leqq )$$ satisfies the condition that, for $$x\in E$$ and $$y\in E$$, there exists $$z\in E$$ which is comparable to x and y and the hypothesis of (a), then it leads to the uniqueness of the fixed point.

Mena et al.  investigated the existence and uniqueness of positive and nondecreasing solutions for the following singular fractional boundary value problem:

\begin{aligned}& D_{0^{+}}^{\alpha}u(t)+f\bigl(t,u(t)\bigr)=0 \quad(0< t<1; 2<\alpha <3 ),\\& u(0)=u^{\prime} ( 1 ) =u^{\prime\prime} ( 0 ) =0. \end{aligned}

Miao and Liang , on the other hand, studied the existence and uniqueness of a positive and nondecreasing solution for the following fractional q-difference boundary value problem:

\begin{aligned}& D_{q}^{\gamma}\bigl(\phi_{p}\bigl(D_{q}^{\alpha}u(t) \bigr)\bigr)+f\bigl(t,u(t)\bigr)=0\quad (0< t<1; 2<\alpha<3 ),\\& u(0)=(D_{q}u) (0)=0,\qquad (D_{q}u) (1)=0, \quad\mbox{and}\quad D_{0+}^{\gamma}u(t)|_{t=0}=0. \end{aligned}

Motivated essentially by the aforementioned work by Miao and Liang , we introduce and investigate here the following q-difference boundary value problem by using the p-Laplacian operator:

\begin{aligned}& D_{q}^{\gamma}\bigl(\phi_{p}\bigl(D_{q}^{\delta}y(t) \bigr)\bigr)+f\bigl(t,y(t)\bigr)=0 \quad( 0< t<1; 3<\delta<4 ) , \end{aligned}
(1.5)
\begin{aligned}& \left \{ \begin{array}{@{}l} y(0)=(D_{q}y)(0)=(D_{q}^{2}y)(0)=0, \\ a_{1}(D_{q}y)(1)+a_{2}(D_{q}^{2}y)(1)=0, \quad\mbox{and}\quad D_{0+}^{\gamma}y(t)|_{t=0}=0.\end{array} \right . \end{aligned}
(1.6)

We prove the existence and uniqueness of a positive and nondecreasing solution for the boundary value problem given by Eqs. (1.5) and (1.6) by means of a fixed point theorem involving partially ordered sets.

## Fractional boundary value problem

Throughout of this paper, we always make use of the usual space $$E=C[0,1]$$ which is known as the space of continuous functions on $$[ 0,1 ]$$. We note that E is a real Banach space with the norm given by

$$\| u\|=\max_{0\leqq t\leqq1}\bigl|u(t)\bigr|.$$

Suppose that $$x\in C[0,1]$$ and $$y\in C[0,1]$$. Then we have

$$x\leqq y\quad\Leftrightarrow\quad x(t)\leqq y(t) \quad\bigl( \forall t\in [ 0,1] \bigr).$$

We know from the recent work  that $$(C[0,1],\leqq)$$ with the familiar metric:

$$d(x,y)=\sup_{0\leqq t\leqq1}\bigl\{ \bigl|x(t)-y(t)\bigr|\bigr\}$$

satisfies the hypothesis of Theorem 1(a). Moreover, for $$x\in C[0,1]$$ and $$y\in C[0,1]$$ such that $$\max\{x,y\}\in C[0,1]$$, $$(C[0,1],\leqq )$$ satisfies the condition of Theorem 1(b).

We first demonstrate Lemma 3.

### Lemma 3

If $$h\in C[0,1]$$, the following boundary value problem:

\begin{aligned}& \bigl(D_{q}^{\delta}y\bigr) (t)+h(t)=0 \quad(0< t<1; 3<\delta<4 ) , \end{aligned}
(2.1)
\begin{aligned}& \left \{ \begin{array}{@{}l} u(0)=(D_{q}u)(0)=(D_{q}^{2}u)(0)=0, \\ a_{1}(D_{q}u)(1)+a_{2}(D_{q}^{2}u)(1)=0 \quad ( \vert a_{1}\vert +\vert a_{2}\vert \neq0 )\end{array} \right . \end{aligned}
(2.2)

has a unique solution given by

$$u(t)=\int_{0}^{1}G(t,qs)h(s)\,d_{q}s ,$$
(2.3)

where

\begin{aligned} G(t,s)={}&\frac{1}{ ( a_{1}+a_{2}[\delta-2]_{q} ) \Gamma _{q}(\delta )} \\ &{} \times \left \{ \begin{array}{@{}l} ( a_{1}(1-s)^{(\delta-2)}+a_{2}[\delta-2]_{q}(1-s)^{(\delta -3)} ) t^{\delta-1} \\ \quad{}- ( a_{1}+a_{2}[\delta-2]_{q} ) (t-s)^{(\delta-1)} \quad (0\leqq s\leqq t\leqq1), \\ ( a_{1}(1-s)^{(\delta-2)}+a_{2}[\delta-2]_{q}(1-s)^{(\delta -3)} ) t^{\delta-1} \quad ( 0\leqq t\leqq s\leqq1). \end{array} \right . \end{aligned}
(2.4)

### Proof

By applying Lemma 1, Lemma 2 (with $$p=4$$) and Eq. (2.1), we have

$$\bigl(I_{q}^{\delta}D_{q}^{4}I_{q}^{4-\delta}u \bigr) (x)=-I_{q}^{\delta}f\bigl(t,u(t)\bigr)$$

and

$$u(t)=c_{1}t^{\delta-1}+c_{2}t^{\delta-2}+c_{3}t^{\delta -3}+c_{4}t^{\delta -4}- \int_{0}^{t}\frac{(t-qs)^{(\delta-1)}}{\Gamma_{q}(\delta)}h(s) \,d_{q}s.$$
(2.5)

From Eq. (2.2), we get $$c_{4}=0$$. Thus, upon differentiating both sides of Eq. (2.5), if we make use of Eqs. (1.2) and (1.3), we see that

$$(D_{q}u) (t)=[\delta-1]_{q}c_{1}t^{\delta-2}+[ \delta -2]_{q}c_{2}t^{\delta -3}+c_{3}t^{\delta-4}- \frac{[\delta-1]_{q}}{\Gamma_{q}(\delta)} \int_{0}^{t}(t-qs)^{(\delta-2)}h(s) \,d_{q}s.$$
(2.6)

Using the boundary condition (2.2), we have $$c_{3}=0$$. Moreover, by differentiating both sides of Eq. (2.6), and using Eqs. (1.2) and (1.3), we obtain

\begin{aligned} \bigl(D_{q}^{2}u\bigr) (t)={}&[\delta-1]_{q}[ \delta-2]_{q}c_{1}t^{\delta -3}+[\delta -2]_{q}[\delta-3]_{q}c_{2}t^{\delta-4} \\ &{} -\frac{[\delta-1]_{q}[\delta-2]_{q} }{\Gamma _{q}(\delta)}\int_{0}^{t}(t-qs)^{(\delta-3)}h(s) \,d_{q}s. \end{aligned}

Similarly, by using the boundary condition (2.2), we have $$c_{2}=0$$ and

$$c_{1}=\frac{a_{1}\int_{0}^{1}(1-qs)^{(\delta -2)}h(s)\,d_{q}s+a_{2}[\delta -2]_{q}\int_{0}^{1}(1-qs)^{(\delta-3)}h(s)\,d_{q}s}{ ( a_{1}+a_{2}[\delta -2]_{q} ) \Gamma_{q}(\delta)}.$$

Consequently, we have the following unique solution of the boundary value problem given by Eqs. (2.1) and (2.2):

\begin{aligned} u(t) ={}&{-}\int_{0}^{t}\frac{(t-qs)^{(\delta-1)}}{\Gamma_{q}(\delta)} h(s) \,d_{q}s \\ &{} +\frac{ ( a_{1}\int_{0}^{1}(1-qs)^{(\delta -2)}h(s)\,d_{q}s+a_{2}[\delta-2]_{q}\int_{0}^{1}(1-qs)^{(\delta -3)}h(s)\,d_{q}s ) t^{\delta-1}}{ ( a_{1}+a_{2}[\delta -2]_{q} ) \Gamma_{q}(\delta)} \\ ={}&\frac{1}{\Gamma_{q}(\delta)}\int_{0}^{t} \biggl( \frac{ ( a_{1}(1-qs)^{(\delta-2)}+a_{2}[\delta-2]_{q}(1-qs)^{(\delta -3)} ) t^{\delta-1}}{ ( a_{1}+a_{2}[\delta-2]_{q} ) }-(t-qs)^{(\delta -1)} \biggr) h(s)\,d_{q}s \\ &{} +\int_{t}^{1}\frac{ ( a_{1}(1-qs)^{(\delta -2)}+a_{2}[\delta -2]_{q}(1-qs)^{(\delta-3)} ) t^{\delta-1}}{ ( a_{1}+a_{2}[\delta -2]_{q} ) \Gamma_{q}(\delta)}h(s) \,d_{q}s \\ ={}&\int_{0}^{1}G(t,qs)h(s)\,d_{q}s. \end{aligned}

We thus arrive at the desired result asserted by Lemma 3. □

By using the method in  mutatis mutandis, it can easily be proven that, if $$f\in C([0,1]\times[0,+\infty ),[0,+\infty))$$, then the boundary value problem given by Eqs. (1.5) and (1.6) is equivalent to the following integral equation:

$$u(t)=\int_{0}^{1}G(t,qs)\phi_{p}^{-1} \biggl( \int_{0}^{s}\frac {(s-\tau )^{(\gamma-1)}f(\tau,u(\tau))}{ ( a_{1}+a_{2}[\delta -2]_{q} ) \Gamma_{q}(\gamma)} \,d_{q}\tau \biggr) \,d_{q}s,$$
(2.7)

where $$G(t,s)$$ is defined, as before, by Eq. (2.4).

### Lemma 4

The function $$G(t,s)$$ given by Eq. (2.4) has the following properties:

1. (1)

$$G(t,s)$$ is a continuous function and $$G(t,qs)\geqq0$$;

2. (2)

$$G(t,s)$$ is strictly increasing in the first variable t.

### Proof

The continuity of $$G(t,s)$$ can easily be checked. We, therefore, omit the details involved. Next, for $$0\leqq s\leqq t\leqq1$$, we let

\begin{aligned} g_{1}(t,s) ={}& \bigl( a_{1}(1-s)^{\delta-2}+a_{2}[ \delta -2]_{q}(1-s)^{\delta -3} \bigr) t^{\delta-1} \\ &{} - \bigl( a_{1}+a_{2}[\delta-2]_{q} \bigr) (t-s)^{\delta-1} \end{aligned}

and, for $$0\leqq t\leqq s\leqq1$$, we suppose that

$$g_{2}(t,s)= \bigl( a_{1}(1-s)^{\delta-2}+a_{2}[ \delta -2]_{q}(1-s)^{\delta -3} \bigr) t^{\delta-1}.$$

Then it is not difficult to see that

$$g_{2}(t,qs)\geqq0.$$

Now, for $$g_{1}(0,qs)=0$$, $$\delta>0$$, and $$a\leqq b\leqq t$$, we have

$$(t-a)^{(\delta)}\geqq(t-b)^{(\delta)}\quad (t\neq0).$$

We thus find that

\begin{aligned} g_{1}(t,qs) ={}& \bigl( a_{1}(1-qs)^{\delta-2}+a_{2}[ \delta -2]_{q}(1-qs)^{\delta-3} \bigr) t^{\delta-1} \\ &{} - \bigl( a_{1}+a_{2}[\delta-2]_{q} \bigr) \biggl(1-q\frac {s}{t}\biggr)t^{\delta -1} \\ \geqq{}& \bigl[ \bigl( a_{1}(1-qs)^{\delta-2}+a_{2}[ \delta -2]_{q}(1-qs)^{\delta-3} \bigr) \\ &{} - \bigl( a_{1}+a_{2}[\delta-2]_{q} \bigr) (1-qs)^{\delta-1} \bigr] t^{\delta-1} \\ \geqq{}& \bigl[ \bigl( a_{1}(1-qs)^{\delta-1}+a_{2}[ \delta -2]_{q}(1-qs)^{\delta-1} \bigr) \\ &{} - \bigl( a_{1}+a_{2}[\delta-2]_{q} \bigr) (1-qs)^{\delta-1} \bigr] t^{\delta-1}=0. \end{aligned}

So, clearly, $$G(t,qs)\geqq0$$ for all $$(t,s)\in[0,1]\times [ 0,1]$$. This completes the proof of Lemma 4(1).

Next, for a fixed $$s\in[0,1]$$, we see that

\begin{aligned} _{t}D_{q}g_{1}(t,qs) ={}&[\delta-1]_{q} \bigl( a_{1}(1-qs)^{(\delta -2)}+a_{2}[ \delta-2]_{q}(1-qs)^{(\delta-3)} \bigr) t^{\delta-2} \\ &{} -[\delta-1]_{q} \bigl( a_{1}+a_{2}[ \delta-2]_{q} \bigr) (t-qs)^{\delta-2} \\ ={}&[\delta-1]_{q} \bigl( a_{1}(1-qs)^{(\delta-2)}+a_{2}[ \delta -2]_{q}(1-qs)^{(\delta-3)} \bigr) t^{\delta-2} \\ &{} -[\delta-1]_{q} \bigl( a_{1}+a_{2}[ \delta-2]_{q} \bigr) \biggl(1-q\frac{s}{t}\biggr)^{\delta-2}t^{\delta-2} \\ \geqq{}&[\delta-1]_{q}(1-qs)^{(\delta-2)} \bigl( a_{1}+a_{2}[\delta -2]_{q} \bigr) t^{\delta-2} \\ &{} -[\delta-1]_{q}(1-qs)^{(\delta-2)} \bigl( a_{1}+a_{2}[ \delta -2]_{q} \bigr) t^{\delta-2} \\ ={}&0. \end{aligned}

This implies that $$g_{1}(t,qs)$$ is an increasing function of the first argument t. Furthermore, obviously, $$g_{2}(t,qs)$$ is an increasing function of the first argument t. Therefore, $$G(t,qs)$$ is an increasing function of t for a fixed $$s\in[0,1]$$. This completes the proof of Lemma 4. □

## Uniqueness of positive solutions

For notational convenience, we write

$$M:=\phi_{p}^{-1} \biggl( \frac{1}{\Gamma_{q}(\gamma) ( a_{1}+a_{2}[\delta-2]_{q} ) } \biggr) \sup_{0\leqq t\leqq 1}\int_{0}^{1}G(t,qs) \,d_{q}s>0.$$
(3.1)

The main result of this paper is the assertion in Theorem 2.

### Theorem 2

The boundary value problem given by Eqs. (1.5) and (1.6) has a unique positive and increasing solution $$u(t)$$ if each of the following two conditions is satisfied:

1. (i)

the function $$f:[0,1]\times[0,\infty )\rightarrow[0,\infty)$$ is continuous and nondecreasing with respect to the second variable;

2. (ii)

there exist λ and M given by Eq. (3.1) ($$0<\lambda+1<M$$) such that, for $$u\in[0,\infty)$$ and $$v\in [0,\infty)$$ with $$u\geqq v$$ and $$t\in[0,1]$$,

$$\phi_{p}\bigl(\ln(v+2)\bigr)\leqq f(t,v)\leqq f(t,u)\leqq \phi_{p}\bigl(\ln (u+2) (u-v+1)^{\lambda}\bigr).$$

Furthermore, if $$f(t,0)>0$$ for $$t\in[0,1]$$, then the solution $$u(t)$$ of the boundary value problem given by Eqs. (1.5) and (1.6) is strictly increasing on $$[0,\infty)$$.

### Proof

First of all, we set

$$u:=u ( t ) \quad\mbox{and}\quad v:=v ( t ).$$

We then consider the set K (called a cone) given by

$$K=\bigl\{ u: u\in C[0,1] \mbox{ and } u(t)\geqq0\bigr\} .$$

Since K is a closed set, K is a complete metric space in accordance with the usual metric

$$d(u,v)=\sup_{t\in[0,1]}\bigl|u(t)-v(t)\bigr|.$$

Let us now consider the operator T as follows:

$$Tu(t)=\int_{0}^{1}G(t,qs)\phi_{p}^{-1} \biggl( \frac{1}{ ( a_{1}+a_{2}[\delta-2]_{q} ) \Gamma_{q}(\gamma)}\int_{0}^{s}(s-\tau )^{(\gamma-1)}f\bigl(\tau,u(\tau)\bigr)\,d_{q}\tau \biggr) \,d_{q}s.$$

Then, by applying Lemma 4 and the condition (i) of Theorem 2, we see that $$T(K)\subset K$$.

We now show that all conditions of Theorem 1 are satisfied. Firstly, by the condition (i) of Theorem 2, for $$u,v\in K$$ and $$u\geqq v$$, we have

\begin{aligned} Tu(t)& =\int_{0}^{1}G(t,qs)\phi_{p}^{-1} \biggl( \frac{1}{ ( a_{1}+a_{2}[\delta-2]_{q} ) \Gamma_{q}(\gamma)}\int_{0}^{s}(s-\tau )^{(\gamma-1)}f\bigl(\tau,u(\tau)\bigr)\,d_{q}\tau \biggr) \,d_{q}s \\ & \geqq\int_{0}^{1}G(t,qs)\phi_{p}^{-1} \biggl( \frac{1}{ ( a_{1}+a_{2}[\delta-2]_{q} ) \Gamma_{q}(\gamma)}\int_{0}^{s}(s-\tau )^{(\gamma-1)}f\bigl(\tau,v(\tau)\bigr)\,d_{q}\tau \biggr) \,d_{q}s \\ & =Tv(t). \end{aligned}

This shows that T is a nondecreasing operator. On the other hand, for $$u\geqq v$$ and by the condition (ii) of Theorem 2, we have

\begin{aligned} d(Tu,Tv) ={}&\sup_{0\leqq t\leqq1}\bigl|(Tu) (t)-(Tv) (t)\bigr| \\ ={}&\sup_{0\leqq t\leqq1} \bigl((Tu) (t)-(Tv) (t) \bigr) \\ \leqq{}&\sup_{0\leqq t\leqq1} \biggl[\int_{0}^{1}G(t,qs) \phi_{p}^{-1} \biggl(\frac{1}{ ( a_{1}+a_{2}[\delta-2]_{q} ) \Gamma_{q}(\gamma)}\\ &{}\times \int _{0}^{s}(s-\tau)^{(\gamma-1)}f\bigl(\tau,u(\tau) \bigr)\,d_{q}\tau \biggr)\,d_{q}s \\ &{} -\int_{0}^{1}G(t,qs)\phi_{p}^{-1} \biggl(\frac{1}{ ( a_{1}+a_{2}[\delta-2]_{q} ) \Gamma_{q}(\gamma)}\\ &{}\times\int_{0}^{s}(s-\tau )^{(\gamma-1)}f\bigl(\tau,v(\tau)\bigr)\,d_{q}\tau \biggr) \,d_{q}s \biggr] \\ \leqq{}& \bigl(\ln(u+2) (u-v+1)^{\lambda}-\ln(v+2) \bigr) \\ &{} \times\sup_{0\leqq t\leqq1}\int_{0}^{1}G(t,qs) \phi _{p}^{-1} \biggl( \frac{1}{ ( a_{1}+a_{2}[\delta-2]_{q} ) \Gamma_{q}(\gamma)} \int _{0}^{s}(s-\tau)^{(\gamma-1)}\,d_{q} \tau \biggr)\,d_{q}s \\ \leqq{}&\ln\frac{(u+2)(u-v+1)^{\lambda}}{v+2}\phi_{p}^{-1} \biggl( \frac{1}{ ( a_{1}+a_{2}[\delta-2]_{q} ) \Gamma_{q}(\gamma)} \biggr)\sup_{0\leqq t\leqq1}\int _{0}^{1}G(t,qs)\,d_{q}s \\ \leqq{}& (\lambda+1 )\ln(u-v+1)\phi_{p}^{-1} \biggl( \frac {1}{ ( a_{1}+a_{2}[\delta-2]_{q} ) \Gamma_{q}(\gamma)} \biggr)\sup_{0\leqq t\leqq1}\int_{0}^{1}G(t,qs) \,d_{q}s. \end{aligned}

Since the function $$h(x)=\ln(x+1)$$ is nondecreasing, from the condition (ii) of Theorem 2, we have

\begin{aligned} d(Tu,Tv)& \leqq(\lambda+1)\ln\bigl(\| u-v\|+1\bigr)\phi_{p}^{-1} \biggl(\frac{1}{ ( a_{1}+a_{2}[\delta-2]_{q} ) \Gamma_{q}(\gamma)} \biggr) \sup_{0\leqq t\leqq1}\int _{0}^{1}G(t,qs)\,d_{q}s \\ & =(\lambda+1)\ln\bigl(\| u-v\|+1\bigr)M \\ & \leqq\| u-v\|-\bigl(\| u-v\|-\ln\bigl(\| u-v\|+1\bigr)\bigr). \end{aligned}

We now let $$\psi(x)=x-\ln(x+1)$$. Then, obviously, the function ψ given by

$$\psi:[0,+\infty)\rightarrow[0,+\infty)$$

is continuous, nondecreasing, and positive in $$(0,\infty)$$. It is also clearly seen that $$\psi(x)$$ satisfies the following conditions:

$$\psi(0)=0 \quad\mbox{and}\quad \lim_{x\rightarrow+\infty}\psi (x)=\infty.$$

Thus, for $$u\geqq v$$, we have

$$d(Tu,Tv)\leqq d(u,v)-\psi\bigl(d(u,v)\bigr).$$

As $$G(t,qs)\geqq0$$ and $$f\geqq0$$, we have

$$(T0) (t)=\int_{0}^{1}G(t,qs)f(s,0) \,d_{q}s \geqq0.$$

Consequently, in view of Theorem 1, the boundary value problem given by Eqs. (1.5) and (1.6) has at least one nonnegative solution. Since $$(K,\leqq)$$ satisfies the condition (ii) of Theorem 2, Theorem 1 implies the uniqueness of the solution. Thus, clearly, the proof of the last assertion of Theorem 2 follows immediately from the proof of a well-known result in [10, Theorem 4.2]. Our proof Theorem 2 is thus completed. □

## Concluding remarks and observations

Our present study was motivated by several aforementioned recent works. Here, we have successfully addressed the problem involving the existence and uniqueness of positive and nondecreasing solutions of a family of fractional q-difference boundary value problems given by Eqs. (1.5) and (1.6). The proof of our main result asserted by Theorem 2 of the preceding section has made use of some familiar fixed point theorems. We have also indicated the relevant connections of the results derived in this investigation with those in earlier works on the subject.

## References

1. 1.

Jackson, FH: On q-definite integrals. Pure Appl. Math. Q. 41, 193-203 (1910)

2. 2.

Agarwal, RP: Certain fractional q-integrals and q-derivatives. Proc. Camb. Philos. Soc. 66, 365-370 (1969)

3. 3.

Atici, FM, Eloe, PW: Fractional q-calculus on a time scale. J. Nonlinear Math. Phys. 14, 333-344 (2007)

4. 4.

Ernst, T: The history of q-Calculus and a new method. U.U.D.M. Report 2000:16, Department of Mathematics, Uppsala University (2000)

5. 5.

Han, Z-H, Lu, H-L, Sun, S-R, Yang, D-W: Positive solutions to boundary-value problems of p-Laplacian fractional differential equations with a parameter in the boundary. Electron. J. Differ. Equ. 2012, 213 (2012)

6. 6.

Hilfer, R (ed.): Applications of Fractional Calculus in Physics. World Scientific, Singapore (2000)

7. 7.

Kilbas, AA, Srivastava, HM, Trujillo, JJ: Theory and Applications of Fractional Differential Equations. North-Holland Mathematical Studies, vol. 204. Elsevier, Amsterdam (2006)

8. 8.

Lakshmikantham, V: Theory of fractional functional differential equations. Nonlinear Anal. 69, 3337-3343 (2008)

9. 9.

Lakshmikantham, V, Vatsala, AS: Basic theory of fractional differential equations. Nonlinear Anal. 69, 2677-2682 (2008)

10. 10.

Miao, F-G, Liang, S-H: Uniqueness of positive solutions for fractional q-difference boundary value problems with p-Laplacian operator. Electron. J. Differ. Equ. 2013, 174 (2013)

11. 11.

Petráš, I: Fractional-Order Nonlinear Systems: Modeling, Analysis and Simulation. Springer Series on Nonlinear Physical Science. Springer, Berlin (2011)

12. 12.

Podlubny, I: Fractional Differential Equations: An Introduction to Fractional Derivatives, Fractional Differential Equations, to Methods of Their Solution and Some of Their Applications. Mathematics in Science and Engineering, vol. 198. Academic Press, New York (1999)

13. 13.

Rajković, PM, Marinković, SD, Stanković, MS: Fractional integrals and derivatives in q-calculus. Appl. Anal. Discrete Math. 1, 311-323 (2007)

14. 14.

Samko, SG, Kilbas, AA, Marichev, OI: Fractional Integrals and Derivatives: Theory and Applications. Gordon & Breach, New York (1993)

15. 15.

Srivastava, HM: Some generalizations and basic (or q-) extensions of the Bernoulli, Euler and Genocchi polynomials. Appl. Math. Inf. Sci. 5, 390-444 (2011)

16. 16.

Al-Salam, WA: Some fractional q-integrals and q-derivatives. Proc. Edinb. Math. Soc. 15, 135-140 (1966/1967)

17. 17.

Benchohra, M, Henderson, J, Ntouyas, SK, Ouahab, A: Existence results for fractional order functional differential equations with infinite delay. J. Math. Anal. Appl. 338, 1340-1350 (2008)

18. 18.

El-Sayed, AMA, El-Mesiry, AEM, El-Saka, HAA: On the fractional-order logistic equation. Appl. Math. Lett. 20, 817-823 (2007)

19. 19.

El-Shahed, M: Positive solutions for boundary value problem of nonlinear fractional differential equation. Abstr. Appl. Anal. 2007, Article ID 10368 (2007)

20. 20.

Ferreira, RAC: Nontrivial solutions for fractional q-difference boundary-value problems. Electron. J. Qual. Theory Differ. Equ. 2010, 70 (2010)

21. 21.

Ferreira, RAC: Positive solutions for a class of boundary value problems with fractional q-differences. Comput. Math. Appl. 61, 367-373 (2011)

22. 22.

Agarwal, RP, O’Regan, D, Wong, PJY: Positive Solutions of Differential, Difference and Integral Equations. Kluwer Academic, Dordrecht (1999)

23. 23.

Chai, G: Positive solutions for boundary value problem of fractional differential equation with p-Laplacian operator. Bound. Value Probl. 2012, Article ID 18 (2012)

24. 24.

Lakshmikantham, V, Vatsala, AS: General uniqueness and monotone iterative technique for fractional differential equations. Appl. Math. Lett. 21, 828-834 (2008)

25. 25.

Liang, S-H, Zhang, J-H: Positive solutions for boundary value problems of nonlinear fractional differential equation. Nonlinear Anal. 71, 5545-5550 (2009)

26. 26.

Li, CF, Luo, XN, Zhou, Y: Existence of positive solutions of the boundary value problem for nonlinear fractional differential equations. Comput. Math. Appl. 59, 1363-1375 (2010)

27. 27.

Mena, JC, Harjani, J, Sadarangani, K: Existence and uniqueness of positive and nondecreasing solutions for a class of singular fractional boundary value problems. Bound. Value Probl. 2009, Article ID 421310 (2009)

28. 28.

Şen, E, Açikgöz, M, Seo, JJ, Araci, S, Oruçoğlu, K: Existence and uniqueness of positive solutions of boundary value problems for fractional differential equations with p-Laplacian operator and identities on the some special polynomials. J. Funct. Spaces Appl. 2013, Article ID 753171 (2013)

29. 29.

Yang, W-G: Positive solution for fractional q-difference boundary value problems with ϕ-Laplacian operator. Bull. Malays. Math. Soc. 36, 1195-1203 (2013)

30. 30.

Zhang, S-Q: Positive solutions for boundary-value problems of nonlinear fractional differential equations. Electron. J. Differ. Equ. 2006, 36 (2006)

31. 31.

Zhang, S-Q: Existence of solution for a boundary value problem of fractional order. Acta Math. Sci. 26, 220-228 (2006)

32. 32.

Zhou, Y: Existence and uniqueness of fractional functional differential equations with unbounded delay. Int. J. Dyn. Syst. Differ. Equ. 1, 239-244 (2008)

33. 33.

Harjani, J, Sadarangani, K: Fixed point theorems for weakly contractive mappings in partially ordered sets. Nonlinear Anal. 71, 3403-3410 (2009)

34. 34.

Nieto, JJ, Rodríguez-López, R: Contractive mapping theorems in partially ordered sets and applications to ordinary differential equations. Order 22, 223-239 (2005)

35. 35.

O’Regan, D, Petrusel, A: Fixed point theorems for generalized contractions in ordered metric spaces. J. Math. Anal. Appl. 341, 1241-1252 (2008)

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