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The method of upper and lower solutions for a class of fractional differential coupled systems
Advances in Difference Equations volume 2021, Article number: 263 (2021)
Abstract
In this paper, we investigate a class of nonlocal boundary value problems of nonlinear fractional functional differential coupled systems with state dependent delays. The method of upper and lower solutions is established and some new results for the multiplicity of solutions of the boundary value problem are obtained. An example is also presented to illustrate our main results.
1 Introduction
It is well know that fractional calculus is the generalization of classical calculus from integer to real numbers, even in the complex field. In the past decades, fractional differential equations have been widely used in various research fields, such as chemical engineering, automatic control, and thermoelasticity. In consequence, the theoretical researches of fractional differential equations have been highly valued by more and more scholars; see [1–6].
When there are many state variables depending on each other in the system, the systems are often described as coupled systems; for details and examples, see [7–20] and the references therein. In recent years, the method of upper and lower solutions has played a more and more important role in the theoretical studies of differential equations. There are a large number of publications using the method of upper and lower solutions to study the existence and uniqueness of solutions of fractional differential equations; see [21–25] and the references therein.
In [10], the authors considered the three-point boundary value problems of nonlinear fractional coupled systems,
where \(D_{0^{+} }^{\alpha }\) is the Riemann–Liouville differentiation, \(1 < \alpha \), \(\beta \le 2\), \(0 < \eta _{1} \), \(\eta _{2} \le 1\), \(\sigma _{1} ,\sigma _{2} > 0\), \(\sigma _{1} \eta _{1} ^{\alpha - 1} = \sigma _{2} \eta _{2} ^{\beta - 1}\), \(f,g \in C([0,1] \times \mathbb{R}^{2} ,\mathbb{R})\). The existence theorems of two solutions of boundary value problems at resonance are given by using the coincidence degree theory.
In scientific research, if the system is assumed to be controlled by an equation containing the current state and the rate of change of state, then we consider ordinary or partial differential equations. However, in some applications, the system may also include the past state of the system. In this case, it would be more accurate to describe them by functional differential equations; see [26–28]. In recent years, the boundary value problems of the fractional functional differential equations have attracted the attention of researchers and many research results have been obtained; see [29–31].
At the same time, some problems may have already occurred for some time before we begin to study them, such as infectious diseases. In this way, we have to consider the values of state variables for the previous period of time. The purpose of this paper is to study the existence of multiple solutions for nonlocal boundary value problems of fractional functional differential coupled systems with time delays,
where \(0 < \gamma _{1} \), \(\gamma _{2} \le 1\), \(1 + \gamma _{1} \le \alpha \le 2\), \(1 + \gamma _{2} \le \beta \le 2\), \(\xi ,\eta \in (0,1)\), \(a,b \in \mathbb{R} \). \(D_{0^{+} }^{\alpha }\), \(D_{0^{+} }^{\beta }\), \(D_{0^{+} }^{\gamma _{1} }\), \(D_{0^{+} }^{\gamma _{2} }\) are the Riemann–Liouville fractional derivative operators. The functions \(f,g \in C([0,1] \times \mathbb{R}\times C[-\tau ,0])\). \(u_{t}=u(t+\theta )\), \(v_{t}=v(t+\theta )\), \(\theta \in [ - \tau ,0]\), \(\phi , \psi \in C([ - \tau ,0])\) and \(\phi (0) = \psi (0) = 0\). Some new results for the existence of at least three solutions for the coupled system are established by using upper and lower solutions methods.
2 Preliminaries
In this section, we present some necessary definitions and lemmas which will be used in the proof of our main results.
Definition 2.1
(see [1])
The Riemann–Liouville fractional integral of a function \(h:(0,\infty ) \to \mathbb{R} \) of order \(\alpha > 0 \) is given by
provided the right side is pointwise defined on \((0,\infty )\).
Definition 2.2
(see [1])
The Riemann–Liouville fractional derivative of order \(\alpha > 0 \) of a continuous function \(h:(0,\infty ) \to \mathbb{R} \) is given by
where \(n=[\alpha ]+1\) and \([\alpha ]\) denote the integer parts of the real number α, provided the right side is pointwise defined on \((0,\infty )\).
Lemma 2.1
(see [1])
(1) If \(h \in L(0,1)\), \(\rho > \sigma > 0\), then
(2) If \(\rho > 0\), \(\lambda > 0\), then
Lemma 2.2
(see [1])
Assume \(\alpha > 0\), then the solution of the equation \(D_{0^{+} }^{\alpha }h(t) = 0\) is given by
where \(c_{i} \in \mathbb{R} \), \(i = 1,2, \ldots ,n\), and \(n\in \mathbb{N}\) with \(n - 1 < \alpha \le n\).
For convenience, we denote
and we always assume that \(\rho _{\alpha }>0\), \(\rho _{\beta }>0\).
Lemma 2.3
Suppose \(a_{0}\), \(b_{0}\) are constants. Then for any given functions \(x, y \in C[0,1]\), the boundary value problem of the linear fractional differential system
has a unique solution \((u,v) = (u(t),v(t))\) as
where
Proof
Assume \((u,v) = (u(t),v(t))\) is a solution of the linear fractional system (2.2). By applying Lemma 2.2,
where \(c_{1} ,c_{2} ,d_{1} ,d_{2} \in \mathbb{R}\). In view of \(u(0) = v(0) = 0\), we have \(c_{2} = 0\), \(d_{2} = 0\). Then
By Lemma 2.1, we have
And
By the boundary conditions \(D_{0^{+} }^{\gamma _{1} } u(1) = aD_{0^{+} }^{\gamma _{1} } u(\xi ) - a_{0}\), \(D_{0^{+} }^{\gamma _{2} } v(1) = bD_{0^{+} }^{\gamma _{2} } v(\eta ) - b_{0}\), we have
So,
Similarly,
On the other hand, we can easily see that \((u,v) = (u(t),v(t))\) is a solution of the linear fractional system (2.2) if \(u = u(t)\), \(v=v(t)\) for \(t \in [0,1]\) satisfy (2.3) and (2.4), respectively. □
Lemma 2.4
Assume \(\rho _{\alpha }> 0\), \(\rho _{\beta }> 0\), then \(G_{\alpha }(t,s)\), \(G_{\beta }(t,s)\) defined by (2.5) and (2.6), respectively, have the following properties:
(1) \(G_{\alpha }(t,s)\), \(G_{\beta }(t,s)\) are continuous on \((t,s) \in [0,1] \times [0,1]\);
(2) \(0 < G_{\alpha }(t,s) \le \max_{0 \le t \le 1} G_{\alpha }(t,s) \le w_{1} (s)\), \(t,s \in (0,1)\);
(3) \(0 < G_{\beta }(t,s) \le \max_{0 \le t \le 1} G_{\beta }(t,s) \le w_{2} (s)\), \(t,s \in (0,1)\), where \(w_{1} (s)= \frac{{(1-s)^{\alpha - \gamma _{1} - 1} }}{{\Gamma (\alpha )}}(s^{ \alpha - 1} + \frac{{a\xi ^{\alpha - \gamma _{1} - 1} }}{{1 - a\xi ^{\alpha - \gamma _{1} - 1} }})\), \(w_{2} (s)= \frac{{(1 - s)^{\beta - \gamma _{2} - 1} }}{{\Gamma (\beta )}}(s^{ \beta - 1} + \frac{{b\eta ^{\beta - \gamma _{2} - 1} }}{{1 - b\eta ^{\beta - \gamma _{2} - 1} }})\).
Proof
(1) The continuity of \(G_{\alpha }(t,s)\), \(G_{\beta }(t,s)\) is obvious by (2.5) and (2.6).
(2) Let
It is clear that
and for \(0< s \le t<1\),
and for \(0< t \le s<1\),
which imply that \(g_{1} (t,s) > 0\), and \(g_{1} (t,s)\) is decreasing with respect to t as \(s \le t\) and increasing with respect to t as \(t \le s\).
So,
For \(0< s \le \min \{ t,\xi \}<1\),
In a similar way we show \(G_{\alpha }(t,s)>0\) for \(0< t < s \le \xi <1\) or \(0<\xi \le s<t<1\) or \(0<\max \{ t,\xi \} \le s<1\). Hence, \(G_{\alpha }(t,s)>0\) for \(t,s \in (0,1)\), and it is obvious \(G_{\alpha }(1,s)>0\) for \(s \in (0,1)\).
Next, we will prove that \(\max_{0 \le t \le 1} G_{\alpha }(t,s) \le w_{1} (s)\).
If \(0\le s \le \min \{ t,\xi \}\le 1\), we have
If \(0<\xi \le s < t\le 1\), by (2.7), we get
If \(0\le t < s \le \xi <1\), we can see that
If \(0\le \max \{ t,\xi \} \le s\le 1\), then
Hence,
(3) Similarly, we can prove the inequality. □
By Lemma 2.3, let \(a_{0}=b_{0}=0\), and we can get the following lemma.
Lemma 2.5
The fractional differential coupled system (1.1) is equivalent to the systems of integral systems
Let \(E = \{ (u,v):u,v \in C[ - \tau ,1]\}\) and be endowed with norm
and \(C[ - \tau ,0]\) endowed with the norm \(\|x\|_{\tau }=\max_{t \in [-\tau ,0]} |x(t)|\). Then \((E,\|\cdot \|_{E} )\) and \((C[-\tau ,0],\|\cdot \|_{\tau })\) are Banach spaces.
Let
be endowed with norm
and \(P=\{(r,z)\in E_{0}: r(t)\ge 0,z(t) \ge 0,t \in (0,1]\} \). Obviously, \(E_{0}\subset E\) and \((E_{0} ,\| \cdot \|_{E_{0} } )\) is a Banach space, and \(P\subset E_{0}\) is a normal solid cone.
For \((u_{1},v_{1} ),(u_{2},v_{2} ) \in E_{0}\), \((u_{1},v_{1})\preceq (u_{2},v_{2})\) if and only if \((u_{2}-u_{1},v_{2}-v_{1}) \in P\). Hence, \((E_{0},\preceq )\) is a partial order Banach space. We denote \((u_{1} ,v_{1} ) \prec (u_{2} ,v_{2} )\) if \((u_{1} ,v_{1} )\preceq (u_{2} ,v_{2} ) \in E_{0}\) and \((u_{1} ,v_{1} ) \ne (u_{2} ,v_{2} )\); we denote \((u_{1} ,v_{1} ) \prec \prec (u_{2} ,v_{2} )\) if \((u_{2} - u_{1} ,v_{2} - v_{1} ) \in \mathop{P}^{\circ }\).
Redefine functions \(\phi (t)\) and \(\psi (t)\) on \(t \in [0,1]\). Let \(\phi (t) = \psi (t) = 0\), \(t \in [0,1]\). Obviously, \(\phi ,\psi \in E\).
For any \((r,z) \in E_{0}\), let
and
Then for \(t \in [0,1]\), \(u_{t}=\phi _{t}+r_{t}=\phi (t+\theta )+r(t+\theta )\), \(v_{t}=\psi _{t}+z_{t}= \psi (t+\theta )+z(t+\theta )\), \(\theta \in [-\tau ,0]\).
It is easy to see that the following lemma holds.
Lemma 2.6
\((u,v) \in E\) is a solution of systems (2.9) and (2.10) if and only if \((r,z) \in E_{0}\) is a solution of the following integral systems:
and
which implies that \((u,v) \in E\) is a solution of system (1.1) if and only if \((r,z) \in E_{0}\) is a solution of the following coupled system:
Define an operator \(T:E_{0} \to E_{0} \) by
where
It is clear that the Lemma 2.7 holds.
Lemma 2.7
A solution of the system (1.1) on E is equivalent to a fixed point of operator T on \(E_{0}\).
Lemma 2.8
The operator \(T:E_{0} \to E_{0}\) is completely continuous.
Proof
Firstly, we prove that operator T is continuous on \(E_{0}\).
Let \(\{ r_{n} ,z_{n} \} \subset E_{0}\), \((r,z) \in E_{0}\) such that \(\|(r_{n} ,z_{n} ) - (r,z)\|_{E_{0} } \to 0\) as \(n \to \infty \). Then, there exists a constant \(M_{0} \ge 0\) such that \(\|(r_{n} ,z_{n} )\|_{E_{0} } \le M_{0}\) for \(n = 1,2, \ldots \) and \(\|(r,z)\|_{E_{0}}\le M_{0}\), and then \(\|r_{nt}\|_{\tau }\le M_{0}\), \(\|z_{nt}\|_{\tau }\le M_{0}\), \(\|r_{t}\|_{\tau }\le M_{0}\) and \(\|z_{t}\|_{\tau }\le M_{0}\) for \(t \in [0,1]\). Due to \(\theta \in [ - \tau ,0]\), we have \(\theta + t \in [ - \tau ,1]\) for \(t \in [0,1]\). Therefore, we have \(\|(r_{nt} ,z_{nt} ) - (r_{t},z_{t})\|_{E_{0} } \to 0\) as \(n \to \infty \).
By the continuity of f, g,
And there exist constants \(M_{1} ,M_{2} > 0\) such that
and
Then
It follows from Lemma 2.4 that for \(t\in [-\tau ,1]\)
and
By Lebesgue’s dominated convergence theorem, as \(n \to \infty \),
Consequently, the operator T is continuous.
Assume that \(S \subset E_{0}\) is a bounded set, and there exists a constant \(l_{1} > 0\) such that we have \(\|(r,z)\|_{E_{0} } \le l_{1}\) for any \((r,z) \in S\). There exist constants \(M_{3} > 0\) and \(M_{4} > 0\) such that
Therefore, by Lemma 2.4, we have
Similarly, we can prove that
Therefore, there exists a constant \(l > 0\), such that \(\|T(r,z)\|_{E_{0} } \le l\) for \((r,z) \in S\). So \(T(S)\) is uniformly bounded.
By the continuity of \(G_{\alpha }(t,s)\) and \(G_{\beta }(t,s)\) on \([0,1] \times [0,1]\), we have \(G_{\alpha }(t,s)\) and \(G_{\beta }(t,s)\) are uniformly continuous on \([0,1] \times [0,1]\). Therefore, for any \(\varepsilon > 0\), there exists \(\delta > 0\) such that
whenever \(t_{1},t_{2},s\in [0,1]\) and \(|t_{1}-t_{2}|<\delta \).
If \({t_{1}}, {t_{2}} \in [0,1]\) and \(|t_{1} - t_{2} | < \delta \), we get
Similarly,
And if \({t_{1}},{t_{2}} \in [ - \tau ,0]\) and \(|t_{1} - t_{2} | < \delta \), we have
Hence, \(T(S)\) is equicontinuous. According to the Arzela–Ascoli theorem, \(T(S)\) is a relative compact set.
So the operator T is completely continuous on \(E_{0}\). □
Lemma 2.9
(see [2])
Let E be a Banach space, and \(P \subset E\) be a normal solid cone. Suppose that there exist \(y_{1},z_{1},y_{2},z_{2} \in E\), with \(y_{1} \prec z_{1} \prec y_{2} \prec z_{2}\), and \(A:[y_{1} ,z_{2} ] \to E\) is a completely continuous strongly increasing operator such that
Then the operator A has at least three fixed points \(x_{1}\), \(x_{2}\), \(x_{3}\) such that
3 Multiple solutions of the coupled systems
Definition 3.1
\((u,v) \in E_{0}\cap (AC^{2}(0,1)\times AC^{2}(0,1))\) is called an upper solution of coupled system (2.13), if it satisfies
Definition 3.2
\((x,y) \in E_{0}\cap (AC^{2}(0,1)\times AC^{2}(0,1))\) is called a lower solution of coupled system (2.13), if it satisfies
Lemma 3.1
Let \((u,v) \in E_{0}\cap (AC^{2}(0,1)\times AC^{2}(0,1))\), if
then \(u(t) \ge 0\) and \(v(t) \ge 0\), \(t \in [ 0 ,1]\).
Proof
Let \(D_{0^{+} }^{\alpha }u(t) = - x(t) \le 0\), \(D_{0^{+} }^{\alpha }v(t) = - y(t) \le 0\), \(D_{0^{+} }^{\gamma _{1} } u(1) - aD_{0^{+} }^{\gamma _{1} } u( \xi ) = a_{0} \le 0\), \(D_{0^{+} }^{\gamma _{2} } z(1) - bD_{0^{+} }^{ \gamma _{2} } z(\eta ) = b_{0} \le 0\). Then \(x(t) \ge 0\), \(y(t) \ge 0\), \(a_{0} \le 0\), \(b_{0} \le 0\). By Lemma 2.3, the coupled system
has a unique solution
By Lemma 2.4, \(u(t) \ge 0\) and \(v(t) \ge 0\), \(t \in [ 0 ,1]\). □
For convenience, we assume that the functions f and g satisfy the following properties.
(H) For any \(x_{1} ,x_{2} \in [0, + \infty )\) with \(0 \le x_{1} \le x_{2}\) and any \(p_{1} ,p_{2} \in C([-\tau ,0])\) with \(p_{1}\leq p_{2}\),
when at least \(x_{1}< x_{2}\) and/or \(p_{1}< p_{2}\) holds,
Lemma 3.2
Suppose (H) holds, then T is a strongly increasing operator.
Proof
For any \((h_{1} ,k_{1} ),(h_{2} ,k_{2} ) \in E_{0}\) with \((h_{1} ,k_{1} ) \prec (h_{2} ,k_{2} )\), i.e., \(h_{1} (t) \le h_{2} (t)\), \(k_{1} (t) \le k_{2} (t)\) and \(h_{1} (t)\not \equiv h_{2} (t)\) or \(k_{1} (t)\not \equiv k_{2} (t)\) for \(t\in [-\tau ,1]\).
By (H), we have
Since \(h_{1} (t)\not \equiv h_{2} (t)\) and \(k_{1} (t)\not \equiv k_{2} (t)\), there exist two intervals \([a_{1} ,b_{1} ],[a_{2} ,b_{2} ] \subset [0,1]\) such that \(k_{1} (t) < k_{2} (t)\) for \(t \in [a_{1} ,b_{1} ]\) or \(h_{1} (t) < h_{2} (t)\) for \(t \in [a_{2} ,b_{2} ]\). Then
and
From (2.14), (2.15), (3.3), (3.4) and Lemma 2.4, for any \(t \in (0,1]\),
For any \(t \in [ - \tau ,0]\), we have
In conclusion, we have \(T(h_{1},k_{1} )\prec \prec T(h_{2},k_{2} )\), T is a strongly increasing operator. □
Theorem 3.3
Suppose (H) holds, and there exist two lower solutions \((x_{1} ,y_{1} )\), \((x_{2} ,y_{2} )\) and two upper solutions \((u_{1} ,v_{1} )\), \((u_{2} ,v_{2} )\) of coupled system (2.13) such that \((x_{1},y_{1} )\), \((u_{2},v_{2} )\) are not solutions of the coupled system (2.13) with
Then the coupled system (1.1) has at least three distinct solutions \((r_{1}+\phi ,z_{1}+\psi ),(r_{2}+\phi ,z_{2}+\psi ),(r_{3}+\phi ,z_{3}+ \psi )\in E\) and for \(t \in [0,1]\),
Proof
By Lemma 2.8 and Lemma 3.2, we see that \(T:E_{0}\to E_{0}\) is a completely continuous strongly increasing operator.
Let \(A(x_{1} ,y_{1} ): = x_{1}^{(1)}\), \(B(x_{1} ,y_{1} ): = y_{1}^{(1)}\), then from the definition of T,
It is clear that
By Lemma 3.1,
Therefore,
Similarly, we can prove \(T(x_{2} ,y_{2} )\preceq (x_{2} ,y_{2} )\). Because \((x_{2} ,y_{2} )\) is a lower solution of coupled system (2.13) and not a solution of (2.13), we have \(T(x_{2} ,y_{2} ) \ne (x_{2} ,y_{2} )\). Thus
In the same way, we get
It follows that T has at least three fixed points \((r_{1} ,z_{1} ),(r_{2} ,z_{2} ),(r_{3} ,z_{3} ) \in [(x_{1} ,y_{1} ),(u_{2} ,v_{2} )]\) from Lemma 2.9.
Hence, by Lemma 2.6, the coupled system (1.1) has at least three distinct solutions \((r_{1}+\phi ,z_{1}+\psi ),(r_{2}+\phi ,z_{2}+\psi ),(r_{3}+\phi ,z_{3}+ \psi )\in E\), and for \(t \in [0,1]\),
□
4 Illustration
To illustrate the applicability of the conclusion, we consider the following nonlinear differential fractional coupled system:
The coupled system (4.1) can be regarded as the form of (1.1), where \(\alpha = \frac{3}{2}\), \(\beta = \frac{5}{4}\), \(\tau = \frac{1}{2}\), \({\gamma _{1}} = \frac{3}{8}\), \({\gamma _{2}} = \frac{5}{8}\), \(\xi = \frac{1}{2}\), \(\eta = \frac{1}{4}\), \(a = 1\), \(b = \frac{3}{5}\), \(f(t,v(t),{v_{t}}) = \frac{1}{\pi }\arctan (\sqrt{t} v(t)) + 0.01\| {v_{t}}\|_{\tau }\), \(g(t,u(t),{u_{t}}) = \frac{1}{\pi }\arctan ({t^{\frac{1}{4}}}u(t)) + 0.01\|{u_{t}}\|_{\tau }\), \(\varphi (t) = {t^{2}}\), \(\psi (t) = {t^{4}}\).
It is obvious that
Take
By simple computations, we have
which show \(({u_{1}},{v_{1}})\) and \(({u_{2}},{u_{2}})\) are upper solutions, \(({x_{1}},{y_{1}})\) and \(({x_{2}},{y_{2}})\) are lower solutions of the coupled systems (4.1), and it is not hard to get \(({x_{1}},{y_{1}}) \prec ({u_{1}},{v_{1}}) \prec ({x_{2}},{y_{2}}) \prec ({u_{2}},{v_{2}})\).
It is easy to obtain
and
Similarly,
Then condition (H) is satisfied. And all conditions of Theorem 3.3 are satisfied. In view of Theorem 3.3, the coupled system (4.1) has at least three distinct solutions \((r_{1}+\phi ,z_{1}+\psi ),(r_{2}+\phi ,z_{2}+\psi ),(r_{3}+\phi ,z_{3}+ \psi )\in [({x_{1}},{y_{1}}),({u_{2}},{v_{2}})]\) and, moreover,
5 Conclusion
In this paper, we present the method of upper and lower solutions for a class of fractional coupled systems including state dependent delays with nonlocal boundary conditions. By using the method of upper and lower solutions and fixed point theorems on the normal cone, the multiplicity results for the boundary value problem are established. The method and main results obtained in this paper can also be extended to the boundary value problems of function fractional differential high dimensional systems of the form
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This work is supported by the National Natural Science Foundation of China (No.11171220).
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Quan, H., Liu, X. & Jia, M. The method of upper and lower solutions for a class of fractional differential coupled systems. Adv Differ Equ 2021, 263 (2021). https://doi.org/10.1186/s13662-021-03419-4
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DOI: https://doi.org/10.1186/s13662-021-03419-4