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Parameter interval of positive solutions for a system of fractional difference equation
Advances in Difference Equations volume 2020, Article number: 247 (2020)
Abstract
This paper deals with a typical system of Caputo fractional difference equations. Using the Guo–Krasnosel’skii fixed point theorem, we find a parameter interval for which at least one positive solution of the system exists. We give two examples to illustrate the results.
1 Introduction
Fractional calculus, as a generalization of classical calculus, is one of those mathematical topics that received much attention. It has been shown for many years that the using of this emerging tool in modeling and design helps to improve the efficiency of various sciences. On the other hand, in recent years the fractional difference equations have been of great interest, there is much work focused on studying the existence and uniqueness of solutions [6, 19, 20]. The theory of discrete version of fractional calculus is very similar and parallel to the theory of continuous case.
Kutter was the first one studied the time differences of fractional order [16]. Diaz and Osler introduced a discrete fractional difference operator defined as an infinite series [7]. Grey and Zhang developed a fractional calculus for the discrete nabla difference operator [13]. At the same time, Miller and Ross defined a fractional sum via the solution of a linear difference equation [17]. Atici and Eloe introduced the Riemann–Liouville like fractional difference, and developed some of its properties that allow one to obtain solutions of certain fractional difference equations [5]. Ferreira introduced the concept of left and right fractional sum/difference and started a fractional discrete-time theory of the calculus of variations [8, 9]. Holm developed and applied the tools of discrete fractional calculus to the area of fractional difference equations [14, 15]. Abdeljawad obtained dual identities in fractional difference calculus which they relate the delta and nabla and the left and right fractional sums and differences [1–3]. Goodrich and Peterson develop basic theoretical results in the field of discrete fractional calculus [11]. Moreover, Goodrich studied existence of positive solutions for discrete fractional systems and geometrical properties [10, 12].
Our objective is to explore the existence and uniqueness results for the following system of fractional difference equations:
where \(\alpha \in (k-1,k]\) and \(a_{i}^{0} \geq 0, T\geq k\geq 2\) are real numbers, \(\Delta _{c}^{\alpha }\) is the standard Caputo difference, \(f_{i} :[0, \infty )\times [0, \infty ) \times \cdots\times [0, \infty ) \rightarrow [0, \infty ) \) is continuous.
As we stated in the abstract, our objective is to use fixed point theory in special normed spaces to achieve an interval for parameter λ for which the problem (1) may or may not have a positive solution.
2 Preliminaries and basic notations
Here, we give some basic definitions and properties of the discrete fractional calculus theory which can be found in [11].
Definition 2.1
Let \(\mathbb{N}_{a}:=\{a, a+1,\ldots\}, a\in \mathbb{R}\) and f: \(\mathbb{N}_{a}\rightarrow \mathbb{R}\) be a real function. The difference operator Δ acts on f by
Definition 2.2
The falling fractional power \(x^{\underline{\alpha }}\) is given by
Theorem 2.3
According to the definition of Δ and the falling fractional power we have
Definition 2.4
The fractional sum of order α for a given function h, for \(\alpha > 0\), is defined by
for \(x \in \{ \alpha +a, \alpha +a+1, \ldots\} := \mathbb{N}_{a+\alpha }\) and \(\sigma (i)=i+1\). The αth fractional difference for \(\alpha >0\) is defined by \(\Delta ^{\alpha }h(x)=\Delta ^{n}\Delta ^{\alpha -n}h(x)\), where \(x\in \mathbb{N}_{a+n-\alpha }\) and \(n\in \mathbb{N}\) such that \(0\leq n-1<\alpha \leq n\).
Furthermore the Caputo fractional difference for \(\alpha > 0\) is defined by
where \(0\leq n-1<\alpha \leq n\).
Lemma 2.5
Let\(\alpha >0\)andhbe defined on\(\mathbb{N}_{a}\), then
where\(c_{i}\in \mathbb{R}, i=0,1,2,\ldots,n-1\), and\(n-1<\alpha \leq n\).
3 Representation of the solution by Green’s function
Now we are ready to represent the solution of the problem (1) by Green’s function.
Lemma 3.1
The discrete fractional boundary value problem
has a unique solution given by
where \(G(t,s)\) is the Green’s function given by
Proof
Using Lemma 2.5
for \(c_{i}\in \mathbb{R}, i=0,1,2,\ldots ,k-1\).
Taking difference operator we find
From \(\Delta ^{j}y(\alpha -k)=0,j=2,3,\ldots ,k-1\), we get \(c_{2}=c_{3}=\cdots =c_{k-1}=0\), and by \(\Delta y(\alpha +T)=0,y(\alpha -k)=a_{0}\), we have
and \(c_{0}=-(\alpha -k)c_{1}+a_{0}\), then we have
Therefore, the solution of (1) is
□
Lemma 3.2
The Green’s functionGgiven in Lemma 3.1satisfies in the following conditions:
- (i)
\(G(t,s)>0, (t,s)\in [\alpha -(k-1), \alpha +T]_{\mathbb{N}_{\alpha -(k-1)}} \times [0,T+1]_{\mathbb{N}_{0}} \).
- (ii)
\(\max_{t\in [\alpha -(k-1), \alpha +T]_{\mathbb{N}_{\alpha -(k-1)}}} G(t,s)=G(T+ \alpha ,s)\).
- (iii)
\(\min_{t\in [\frac{(\alpha + T)}{4}, \frac{3(\alpha + T)}{4}]_{\mathbb{N}_{\alpha -(k-1)}}} G(t,s)\geq \frac{1}{4} \max_{t \in [\alpha -(k-1), \alpha +T]_{\mathbb{N}_{\alpha -(k-1)}}} G(t,s) =\frac{1}{4} G(T+\alpha ,s)\).
Proof
The proof is similar to the proof of Lemma 2.4 in [6]. □
4 Existence of positive solutions
In this section, we define the Banach space
with norm
It is clear that \(\mathcal{E}_{i}\) is a Banach space. Put \(\mathcal{E}:=\mathcal{E}_{1}\times \mathcal{E}_{2}\times \cdots\times \mathcal{E}_{n}\). By equipping \(\mathcal{E}\) with the norm
it follows that \((\mathcal{E},\| \cdot \|)\) is a Banach space. It is clear that \((y_{1},y_{2},\ldots,y_{n})\) is solution of (1) if and only if \(y_{i}\) satisfies
Let \(\mathcal{T}_{i}: \mathcal{E}\rightarrow \mathcal{E}_{i}\) be the operator defined by
Define the operator \(\mathcal{T} : \mathcal{E}\rightarrow \mathcal{E}\) by
Let \(\mathcal{P}\) be a cone defined by
Lemma 4.1
Assume that\(\mathcal{T}\)is the operator defined in (7). Then\(\mathcal{T} : \mathcal{P}\rightarrow \mathcal{P}\).
Proof
By definition of \(\mathcal{T}_{i}\), for \((y_{1},y_{2}, \ldots, y_{n}) \in \mathcal{E} \), we have
We show that
for \((y_{1},y_{2}, \ldots, y_{n})\in \mathcal{E}\). By Lemma 3.2(iii), we have
This proves that \(\mathcal{T} : \mathcal{P}\rightarrow \mathcal{P}\). □
Theorem 4.2
Let\(f_{i} :[0,\infty )\times [0,\infty ) \times \cdots\times [0,\infty ) \rightarrow [0, \infty ) \)be given for\(i=1,2,\ldots,n\). If\((y_{1},y_{2},\ldots,y_{n})\in \mathcal{E}\)is a fixed point of\(\mathcal{T}\). Then\((y_{1},y_{2},\ldots,y_{n})\in \mathcal{E}\)is a solution of (1).
Proof
Let \((y_{1},y_{2},\ldots,y_{n})\in \mathcal{E}\) be a fixed point of \(\mathcal{T}\), then we have
where \(\mathcal{T}_{i}\) is defined as in (6). It is easy to see that
and
Finally, when \(0< t-\alpha +1\leq s \leq T+1\),
then
Therefore, we conclude
which completes the proof. □
Theorem 4.3
([20])
Let\(\mathcal{E}\)be a Banach space, and let\(\mathcal{P}\subset \mathcal{E}\)be a cone in\(\mathcal{E}\). Assume\(\varOmega _{1},\varOmega _{2}\)are open subsets of\(\mathcal{E}\)with\(0 \in \varOmega _{1}\subset \overline{\varOmega _{1}} \subset \varOmega _{2}\)and let\(S : \mathcal{P}\rightarrow \mathcal{P} \)be a completely continuous operator such that either
- (A1)
\(\Vert \mathcal{T} w\Vert \leq \Vert w\Vert , w\in \mathcal{P} \cap \partial \varOmega _{1}, \Vert \mathcal{T}w\Vert \geq \Vert w \Vert , w \in \mathcal{P} \cap \partial \varOmega _{2}\), or
- (A2)
\(\Vert \mathcal{T}w\Vert \geq \Vert w\Vert , w\in \mathcal{P} \cap \partial \varOmega _{1}, \Vert \mathcal{T}w\Vert \leq \Vert w \Vert , w \in \mathcal{P} \cap \partial \varOmega _{2}\).
Then\(\mathcal{T}\)has a fixed point in\(\mathcal{P}\cap ( \overline{\varOmega _{2}} \setminus \varOmega _{1})\).
Now we find the parameter interval for which (1) has a positive solution. We use the following notations:
In this section without loss of generality, we consider the operator \(\mathcal{T}_{i}\), without \(a_{i}^{0}\).
Theorem 4.4
If\(\frac{1}{16}f_{i}^{\infty }Kl>F_{i}^{0 }K(T+1), f_{i}^{\infty }Kl \neq 0\)hold, then for each
the problem (1) has at least one positive solution. Note that we assume\(( f_{i}^{\infty }Kl)^{-1} =0\)if\(f_{i}^{\infty } =+\infty \)and\((F_{i}^{0 }K(T+1))^{-1}=+ \infty \)if\(F_{i}^{0 }=0\).
Proof
If \(\lambda _{i} \) satisfies in (8) and \(\varepsilon >0 \) is given such that
then, using the notation with \(F_{i}^{0 }\), there exists \(r_{1} > 0\) such that for \((y_{1},y_{2}, \ldots, y_{n})\in \varOmega _{r_{1}}\)
So if \((y_{1},y_{2}, \ldots, y_{n})\in \partial \mathcal{P}\) with \(\Vert (y_{1},y_{2}, \ldots, y_{n})\Vert =r_{1} \) then, by (9) and (10), we have
Hence for \((y_{1},y_{2},\ldots,y_{n})\in \mathcal{P} \cap \partial \varOmega _{r_{1}}\)
Let \(r_{3} > 0\) be such that for \(y_{i}\geq r_{3}\)
If \((y_{1},y_{2}, \ldots, y_{n})\in \partial \mathcal{P} \) and \(\Vert (y_{1},y_{2}, \ldots, y_{n}) \Vert =r_{2}=\max \lbrace 2r_{1},r_{3} \rbrace \) then using (9) and (12) implies
Then for \((y_{1},y_{2},\ldots,y_{n}) \in \mathcal{P} \cap \partial \varOmega _{r_{2}}\)
Now, from (11), (13), and Theorem 4.3, we see that \(\mathcal{T}\) has a fixed point \((y_{1},y_{2},\ldots,y_{n}) \in \mathcal{P}\cap (\overline{\varOmega _{r_{2}}} \setminus \varOmega _{r_{1}}) \), where \(r_{1}\leq \Vert (y_{1},y_{2},\ldots,y_{n}) \Vert \leq r_{2} \), and clearly \((y_{1},y_{2},\ldots,y_{n})\) is a positive solution of the problem (1). □
Theorem 4.5
If\(\frac{1}{16}f_{i}^{0}Kl> F_{i}^{\infty }K(T+1) , f_{i}^{0}Kl\neq 0 \)hold, then for each
the problem (1) has at least one positive solution. Note that we assume\(( f_{i}^{0 }Kl)^{-1} =0\)if\(f_{i}^{0} =+\infty \)and\((F_{i}^{\infty }K(T+1))^{-1}=+ \infty \)if\(F_{i}^{\infty }=0\).
Proof
Suppose \(\lambda _{i} \) satisfies in (14) and \(\varepsilon >0 \) is such that
By using the notation of \(f_{i}^{0} \), there exists \(r_{1} > 0\) such that for \((y_{1},y_{2},\ldots,y_{n})\in \varOmega _{ r_{1}} \)
So if \((y_{1},y_{2},\ldots,y_{n})\in \partial \mathcal{P} \) with \(\Vert (y_{1},y_{2},\ldots,y_{n})\Vert =r_{1} \) then analogous to Theorem 4.4, we deduce
Hence, for \((y_{1},y_{2},\ldots,y_{n}) \in \mathcal{P}\cap \partial \varOmega _{r_{1}}\)
Next, we may choose \(R_{1} > 0\) such that for \(y_{i}\geq R_{1}\)
Case 1. If \(f_{i}\) is bounded, then, for some \(N_{i} > 0\), we have
Now let \(r_{3}=\max \lbrace 2r_{1},\lambda N_{i} K(T+1)\rbrace \) and \((y_{1},y_{2},\ldots,y_{n})\in \mathcal{P}\) with \(\Vert (y_{1},y_{2},\ldots,y_{n}) \Vert =r_{3} \), thus
Hence, for \((y_{1},y_{2},\ldots,y_{n}) \in \partial \varOmega _{r_{3}}\),
Case 2. If \(f_{i}\) is not bounded. Then for some \(r_{4}>\max \lbrace 2r_{1}, R_{1}\rbrace \) we have
If \((y_{1},y_{2},\ldots,y_{n}) \in \mathcal{P} \) with \(\Vert (y_{1},y_{2},\ldots,y_{n})\Vert =r_{4}\), then, by (15) and (18), we have
Thus (19) holds.
For \((y_{1},y_{2},\ldots,y_{n}) \in \mathcal{P} \cap \partial \varOmega _{r_{2}}\), we have
Theorem 4.3 implies that \(\mathcal{T} \) has a fixed point \((y_{1},y_{2},\ldots,y_{n}) \in \mathcal{P} \cap (\overline{\varOmega _{r_{2}}} \setminus \varOmega _{r_{1}}) \), where \(r_{1}\leq \Vert (y_{1},y_{2},\ldots,y_{n}) \Vert \leq r_{2}\), and easily \((y_{1},y_{2},\ldots,y_{n})\) is a positive solution of (1). □
Theorem 4.6
Suppose there exist\(r_{2} > r_{1} > 0\)such that, for\(\lambda _{i}>0\),
Then (1) has a positive solution\((y_{1},y_{2},\ldots,y_{n})\in \mathcal{P} \), where\(r_{1} \leq \Vert (y_{1},y_{2},\ldots,y_{n})\Vert \leq r_{2} \).
Proof
If \((y_{1},y_{2},\ldots,y_{n})\in \mathcal{P} \cap \partial \varOmega _{r_{1}} \), we have
That is, for \((y_{1},y_{2},\ldots,y_{n})\in \mathcal{P} \cap \partial \varOmega _{r_{1}} \),
Also, for \((y_{1},y_{2},\ldots,y_{n}) \in \mathcal{P} \cap \partial \varOmega _{r_{2}} \), we have
That is, for \((y_{1},y_{2},\ldots,y_{n})\in \mathcal{P} \cap \partial \varOmega _{r_{2}} \),
Thus, by Theorem 4.3, the problem (1) has a positive solution \((y_{1},y_{2},\ldots,y_{n}) \in \mathcal{P}\), where \(r_{1}\leq \Vert (y_{1},y_{2},\ldots,y_{n})\Vert \leq r_{2} \). □
5 Nonexistence results
In order to find some nonexistence results for problem (1), we consider the following condition:
- (H)
\(\sup_{r>0}\min_{y_{i} \in (0, r)}f_{i}(y_{1},y_{2},\ldots,y_{n})>0\).
Theorem 5.1
Assume the condition\(\mathrm{(H)}\)is true. If\(F_{i}^{0}<+\infty \)and\(F_{i}^{\infty } <+\infty \), then there is a positive real number\(\lambda _{i}^{0}>0\)such that for\(0< \lambda _{i} <\lambda _{i}^{0}\)the problem (1) does not have a positive solution.
Proof
Since \(F_{i}^{0}, F_{i}^{\infty } \) are finite, we can find positive real numbers \(l_{i}^{1}, l_{i}^{2}, r_{1},r_{2}\), where \(r_{1} < r_{2}\) and
Let \(L_{i}= \max \lbrace l_{i}^{1}, l_{i}^{2}, \max_{r_{1}\leq y_{i} \leq r_{2}} \lbrace \frac{f_{i}(y_{1},y_{2},\ldots,y_{n})}{y_{i}} \rbrace \rbrace \). Then we have
Assume \((w_{1},w_{2},\ldots,w_{n})(t)\) is a positive solution of (1). We find a contradiction for \(0 <\lambda _{i} < \lambda _{i}^{0}:= (L_{i}K(T+1))^{-1}\). Since \(\mathcal{T}(w_{1},w_{2},\ldots,w_{n})(t) = (w_{1},w_{2},\ldots,w_{n})(t) \) for \(t \in [\alpha -k,\alpha +T]_{\mathbb{Z}_{\alpha -k}}\),
which is a contradiction. □
Theorem 5.2
Suppose\(\mathrm{(H)}\)holds. If\(f_{i}^{0}>0\)and\(f_{i}^{\infty } >0 \), then there is a real number\(\lambda _{i}^{0}>0\)such that for\(\lambda _{i} >\lambda _{i}^{0}\)the problem (1) does not have a positive solution.
Proof
Since \(f_{i}^{0}, f_{i}^{\infty }\) are positive, there exist \(\gamma _{i}^{1},\gamma _{i}^{2}, r_{1},r_{2}>0\), where \(r_{1} < r_{2}\), and
Let \(\gamma _{i} = \min \lbrace \gamma _{i}^{1}, \gamma _{i}^{2}, \min_{r_{1}\leq y_{i} \leq r_{2}} \lbrace \frac{f_{i}(y_{1},y_{2},\ldots,y_{n})}{ y_{i}}\rbrace \rbrace >0\). Then we get
Assume \((w_{1},w_{2},\ldots,w_{n})\) is a positive solution of (1). We find a contradiction for \(\lambda _{i} >\lambda _{i}^{0}:= ( \frac{1}{16}\gamma _{i} Kl)^{-1}\). Since \(\mathcal{T} (w_{1},w_{2},\ldots,w_{n})(t) = (w_{1},w_{2},\ldots,w_{n})(t) \) for \(t \in [\alpha -k,\alpha +T]_{\mathbb{Z}_{\alpha -k}}\),
which is a contradiction. □
6 Uniqueness
Theorem 6.1
Assume that the Banach space\(\mathcal{E}:=\mathcal{E}_{1}\times \mathcal{E}_{2}\times \cdots\times \mathcal{E}_{n}\)is endowed with the norm
and\(f_{i}\)satisfying the Lipschitz condition
where\(y_{i} ,w_{i}\in \mathcal{E}_{i} , L_{i}>0\). Then the problem (1) has exactly one positive solution provided
where\(L=\max \{L_{i},i=0,1,2,\ldots,n\}\), \(K=\max G(t,s)\)and\(\Vert (\lambda _{1},\ldots,\lambda _{n}) \Vert =\max \{|\lambda _{i} |,i=0,1,2,\ldots,n\}\).
Proof
For any \((y_{1},y_{2}, \ldots, y_{n}), (w_{1},w_{2},\ldots,w_{n}) \in \mathcal{E}\), using the assumption (21), we have
\(i=1,2,\ldots ,n\). That is,
Since \(L=\max \{L_{i},i=0,1,2,\ldots,n\}\) and \(\|(\lambda _{1},\ldots,\lambda _{n})\|=\max \{|\lambda _{i}|,i=0,1,2,\ldots,n \}\), for \(L(T+1) K \Vert (\lambda _{1},\ldots,\lambda _{n}) \Vert <1\), the operator \(\mathcal{T}\) is the contraction mapping. Therefore, the problem (1) has exactly one positive solution. □
7 Example
Example 7.1
Suppose that \(\alpha =1.8, T=10,\lambda _{i}>0\), and \(a_{i}^{0}>0\) is an integer and
where
Clearly \(f_{i}(y_{1},y_{2}): \mathbb{R}^{+}\times \mathbb{R}^{+}\rightarrow \mathbb{R}^{+}\) is continuous. Moreover, it is easy to prove that
Therefore, from Theorem 4.5, for each
the boundary value problem (22) has at least one positive solution.
Example 7.2
In this example we focus on the linearized system as follows:
where \(\alpha =2.98, T=10,\lambda _{i}>0\), and \(a_{i}^{0}>0\) is an integer and
Clearly \(f_{i}(y_{1},y_{2},y_{3},y_{4}): \mathbb{R}^{+}\times \mathbb{R}^{+}\times \mathbb{R}^{+}\times \mathbb{R}^{+}\rightarrow \mathbb{R}^{+}\) is continuous. Take \(l_{i}^{1}=l_{i}^{2}=l_{i}^{3}=l_{i}^{4}=\frac{1}{1000}\). It is easy to prove that
Thus, the conditions of Theorem 5.1 are satisfied. Therefore the problem (23) does not have a positive solution for \(0< \lambda _{i}<\lambda _{i}^{0}\). Moreover, we have
Thus, the conditions of Theorem 5.2 are satisfied. Therefore the problem (23) does not have a positive solution for \(\lambda _{i}>\lambda _{i}^{0}\).
8 Conclusion
In this research we consider a typical system of Caputo fractional difference equations of the form (1). Using the Guo–Krasnosel’skii fixed point theorem, we find a parameter interval for existence and nonexistence of positive solutions dependent on the parameter λ and two examples are given to illustrate the main results.
In this paper we used Caputo discrete fractional operators on the time scale \(\mathbb{Z}\). It could be interesting to extend this work to the time scale \(h\mathbb{Z}\). Working on \(h\mathbb{Z}, 0< h<1\) rather than on \(\mathbb{Z}\) makes it possible to guarantee the convergence of solutions for a larger class of fractional difference initial value problems [4, 18].
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Ghanbari, K., Haghi, T. Parameter interval of positive solutions for a system of fractional difference equation. Adv Differ Equ 2020, 247 (2020). https://doi.org/10.1186/s13662-020-02706-w
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DOI: https://doi.org/10.1186/s13662-020-02706-w