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Existence and uniqueness of positive solutions for a new class of coupled system via fractional derivatives
Advances in Difference Equations volume 2020, Article number: 111 (2020)
Abstract
In this paper we study the existence of unique positive solutions for the following coupled system:
where the integer number \(n>3\) and \(1\leq \gamma \leq \xi \leq n-2\), \(1\leq \eta \leq \zeta \leq n-2\), \(f_{1},f_{2}:[0,1]\times \mathbb{R^{+}}\times \mathbb{R^{+}} \rightarrow \mathbb{R^{+}}\), \(g_{1},g_{2}:[0,1]\times \mathbb{R^{+}}\rightarrow \mathbb{R^{+}}\) and \(k_{1},k_{2}:\mathbb{R^{+}}\rightarrow \mathbb{R^{+}}\) are continuous functions, \(D_{0^{+}}^{\alpha }\) and \(D_{0^{+}}^{\beta }\) stand for the Riemann–Liouville derivatives. An illustrative example is given to show the effectiveness of theoretical results.
1 Introduction
A lot of fractional differential equations and coupled systems have been studied widely; see [1–19, 24] and the references therein. As is well known, coupled systems with boundary conditions appear in the investigations of many problems such as mathematical biology (see [9, 30]), natural sciences and engineering; for example, we can see beam deformation and steady-state heat flow (see [25, 26]) and heat equations (see [18, 24]). So the subject of coupled systems is gaining much attention and importance. There are a large number of articles dealing with the existence or multiplicity of solutions or positive solutions for some nonlinear coupled systems with boundary conditions; for details, see [7, 8, 10, 11, 20, 21, 27, 29, 32, 33, 35–41].
In [42] Zhang and Tian considered a unique positive solution for the following problem:
where \(n>3\), \(1\leq \gamma \leq \beta \leq n-2\), \(f:[0,1]\times \mathbb{R^{+}} \times \mathbb{R^{+}}\rightarrow \mathbb{R^{+}}\), \(g:[0,1]\times \mathbb{R^{+}}\rightarrow \mathbb{R^{+}}\) and \(k:\mathbb{R^{+}}\rightarrow \mathbb{R^{+}}\) are continuous functions, \(D_{0^{+}}^{\alpha }\) is the Riemann–Liouville fractional derivative and \(w^{(i)}\) represents the ith (ordinary) derivative of w.
Continuing their work, we establish the existence of solutions for the following coupled system:
where the integer number \(n>3\) and \(1\leq \gamma \leq \xi \leq n-2\), \(1\leq \eta \leq \zeta \leq n-2\), \(f_{1},f_{2}:[0,1]\times \mathbb{R^{+}}\times \mathbb{R^{+}} \rightarrow \mathbb{R^{+}}\), \(g_{1},g_{2}:[0,1]\times \mathbb{R^{+}}\rightarrow \mathbb{R^{+}}\) and \(k_{1},k_{2}:\mathbb{R^{+}}\rightarrow \mathbb{R^{+}}\) are continuous functions, \(D_{0^{+}}^{\alpha }\) and \(D_{0^{+}}^{\beta }\) stand for the Riemann–Liouville derivatives.
2 Preliminaries
Suppose \((E,\| \cdot \| )\) is a Banach space which is partially ordered by a cone \(P\subseteq E\). We denote the zero element of E by θ. A cone P is called normal if there exists a constant \(N>0\) such that \(\theta \leq x\leq y\) implies \(\| x \| \leq N \| y \| \).
Definition 2.1
\(A:P\times P\rightarrow P\) is said to be a mixed monotone operator if \(A(x,y)\) is increasing in x and decreasing in y, i.e., for \(x_{i},y_{i} \in P\) (\(i=1,2\)), \(x_{1}\leq x_{2}\), \(y_{1}\geq y_{2}\) imply \(A(x_{1},y_{1})\leq A(x_{2},y_{2})\). The element \(x\in P\) is called a fixed point of A if \(A(x,x)=x\).
An element \(u^{*}\in D\) is called a fixed point of A if it satisfies \(A(u^{*},u^{*})=u^{*}\). Let \(h>\theta \), write \(P_{h}=\{u\in E|\exists \lambda ,\mu >0 : \lambda h\leq u\leq \mu h\}\).
Let Φ be a class of functions \(\varphi :(0,1)\rightarrow (0,1)\) with \(\varphi (\tau )>\tau \) for \(\tau \in (0,1)\).
Theorem 2.2
([34])
LetPbe a normal cone inE, \(\alpha \in (0,1)\)\(A:P\rightarrow P\)is an increasing sub-homogeneous, \(B:P\rightarrow P\)is a decreasing operator, \(C:P\times P\rightarrow P\)is a mixed monotone operator and that satisfy the following conditions:
Assume that
- \((i)\):
∃ \(h_{0}\in P_{h}\)such that\(Ah_{0}\in P_{h}\), \(Bh_{0}\in P_{h}\), \(C(h_{0},h_{0})\in P_{h}\);
- \((\mathit{ii})\):
∃ \(\delta _{0}>0\)with\(C(u,v)\geq \delta _{0}(Au+Bv)\)for\(u,v\in P\).
Then
- \((1)\) :
-
\(A:P_{h}\rightarrow P_{h}\), \(B:P_{h}\rightarrow P_{h}\)and\(C:P_{h}\times P_{h}\rightarrow P_{h}\);
- \((2)\) :
-
∃ \(x_{0},y_{0}\in P_{h}\)and\(r\in (0,1)\)with
$$ rx_{0}\leq x_{0}< y_{0}, x_{0}\leq Ax_{0}+By_{0}+C(x_{0},y_{0})\leq Ay_{0}+Bx_{0}+C(y_{0},x_{0}) \leq y_{0}; $$ - \((3)\) :
-
the equation\(Au+Bu+C(u,u)=u\)has a unique solution\(u^{*}\)in\(P_{h}\);
- \((4)\) :
-
for\(x_{0},y_{0}\in P_{h}\), we can construct
$$\begin{aligned} &u_{n}=Ax_{n-1}+By_{n-1}+C(x_{n-1},y_{n-1}), \\ &v_{n}=Ay_{n-1}+Bx_{n-1}+C(y_{n-1},x_{n-1}),\quad n=1,2, \ldots \end{aligned}$$and\(u_{n}\rightarrow u^{*}\)and\(v_{n}\rightarrow v^{*}\).
Definition 2.3
The Riemann–Liouville fractional derivative for a continuous function f is defined by
where the right-hand side is point-wise defined on \((0,\infty )\).
Definition 2.4
Let \([a,b]\) be an interval in \(\mathbb{R}\) and \(\alpha >0\). The Riemann–Liouville fractional order integral of a function \(f\in L^{1}([a,b],\mathbb{R})\) is defined by
whenever the integral exists.
Lemma 2.5
([42])
Let\(h\in C[0,1]\), then the unique solution of the linear problem
is given by
where
is the Green function.
Lemma 2.6
([42])
The Green function (7) has the following properties:
Lemma 2.7
([36])
\(K_{h}=P_{h_{1}}\times P_{h_{2}}\), where that\(K=P\times P\)and\(h(\tau )=(h_{1},h_{2})\).
3 Main results
Let \(E\times E\subset X\times X\) with \(X=C[0,1]\) such that \(E=\{x|x,D^{\eta }_{0^{+}}x, D^{\gamma }_{0^{+}}x\in X\}\) endowed with the norm \(\| x \| =\max \{\max_{\tau \in [0,1]}{|x(\tau )|}, \max_{\tau \in [0,1]}{D^{\eta }_{0^{+}}|x(\tau )|},\max_{\tau \in [0,1]}{D^{ \gamma }_{0^{+}}|x(\tau )|}\}\). For \((x,y)\in E\times E\), let \(\| (x,y) \| =\max \{\| x \| , \| y \| \}\). It is easy to see that \((E\times E,\| (x,y) \| )\) is a Banach space. Define \(P=\{x\in E:x,D^{\eta }_{0^{+}}x,D^{\gamma }_{0^{+}}x\geq 0\}\), \(K=P\times P\), then K is a normal cone equipped with the following partial order:
and
By Lemma 2.5 in [42], the unique positive solution for the problem (1) is given by
where
is a Green function.
Assume that \(f_{1}(\tau ,x,y)\), \(f_{2}(\tau ,x,y)\) are continuous, then \((x,y)\in X\times X\) is a solution of the system (2) if and only if \((x,y)\) is a solution of the integral equations
where
and
are Green functions.
Let us define the operators \(A_{1}\), \(B_{1}\), \(C_{1}\), \(A_{2}\), \(B_{2}\), \(C_{2}\) by
for \(0\leq \tau \leq 1\).
Theorem 3.1
Assume that
- \((H_{1})\) :
-
\(f_{1},f_{2} :[0,1]\times \mathbb{R^{+}}\times \mathbb{R^{+}} \rightarrow \mathbb{R^{+}}\), \(g_{1},g_{2} :[0,1]\times \mathbb{R^{+}}\rightarrow \mathbb{R^{+}}\)and\(k_{1},k_{2} :\mathbb{R^{+}}\rightarrow \mathbb{R^{+}}\)are continuous, also: \(f_{1}(\tau ,1,0)\not \equiv 0\), \(f_{2}(\tau ,1,0)\not \equiv 0\);
- \((H_{2})\) :
-
\(f_{1}(\tau ,x,y)\)and\(f_{2}(\tau ,x,y)\)are increasing respect to\(x\in \mathbb{R^{+}}\), decreasing respect to\(y\in \mathbb{R^{+}}\), \(g_{1}\), \(g_{2}\)are increasing respect toyfor fixed\(0\leq \tau \leq 1\)and\(k_{1}\), \(k_{2}\)are decreasing with\(k_{1}(y(1)),k_{2}(x(1))\neq 0\);
- \((H_{3})\) :
-
∃ \(\alpha _{1},\alpha _{2}\in (0,1)\)such that
$$\begin{aligned} &f_{1}\bigl(\tau ,\lambda x,\lambda ^{-1} y \bigr)\geq \lambda ^{\alpha _{1}}f_{1}( \tau ,x,y), \qquad f_{2}\bigl(\tau ,\lambda x,\lambda ^{-1} y\bigr)\geq \lambda ^{ \alpha _{2}}f_{2}(\tau ,x,y), \end{aligned}$$(14)and\(g_{1}\), \(g_{2}\), \(k_{1}\), \(k_{2}\)satisfy
$$\begin{aligned} &g_{i}(\tau ,\lambda x)\geq \lambda g_{i}( \tau ,x),\qquad k_{i}\bigl(\lambda ^{-1}x\bigr) \geq \lambda k_{i}(x),\quad i=1,2, \end{aligned}$$(15)for\(\lambda \in (0,1)\), \(0\leq \tau \leq 1\), \(x\in \mathbb{R^{+}}\);
- \((H_{4})\) :
-
\(g_{i}(\tau ,0)\not \equiv 0\)and there exist positive constants\(\delta _{11}\), \(\delta _{12}\), \(\delta _{21}\)and\(\delta _{22}\)such that
$$\begin{aligned} &f_{i}(\tau ,x,y)\geq \delta _{i1}g_{i}(\tau ,x),\\ &f_{i}(\tau ,x,y) \geq \delta _{i2}\geq k_{i}(y),\quad (i=1,2), 0\leq \tau \leq 1,x,y\in \mathbb{R^{+}}. \end{aligned}$$
Then
\((1)\) ∃ \((u_{01},u_{02}),(v_{01},v_{02})\in K\subset E\times E\)and\(r\in (0,1)\)such that
that is,
where\(G_{1}(\tau ,\rho )\), \(G_{2}(\tau ,\rho )\)are defined by (11) and (12), respectively.
\((2)\)The problem (2) has a unique positive solution\((u^{*},v^{*})\)in\(K_{h}\), with\(h(\tau )=(h_{1}(\tau ), h_{2}(\tau ))=(\tau ^{\alpha -1},\tau ^{ \beta -1})\), \(0\leq \tau \leq 1\).
\((3)\)For\((x_{01},x_{02}),(y_{01},y_{02})\in P_{h}\times P_{h}\), there are two iterative sequences\(\{(x_{n1},x_{n2})\}\), \(\{(y_{n1},y_{n2})\}\)for approximating\((x^{*},y^{*})\), that is, \((x_{n1},x_{n2})\rightarrow (x^{*},y^{*})\), \((y_{n1},y_{n2})\rightarrow (x^{*},y^{*})\), where
Proof
By Lemma 2.6 we have
Regarding (16) and \((H_{1})\) in (13) we get \(A_{1},A_{2},B_{1},B_{2}:P\rightarrow P\) and \(C_{1},C_{2}:P\times P\rightarrow P\times P\).
Obviously \(A_{1}\), \(A_{2}\) are increasing and sub-homogeneous, Because \(g_{1}\), \(g_{2}\) are increasing and sub-homogeneous. \(B_{1}\), \(B_{2}\) are decreasing (due to this fact, \(k_{1}\) and \(k_{2}\) are decreasing) and satisfy in conditions \(B_{i}(\lambda ^{-1}x)\geq \lambda B_{i}(x)\), \(i=1,2 \), by (15). For any \((u_{1},v_{1}),(u_{2},v_{2})\in K\) with \((u_{1},v_{1})\preceq (u_{2},v_{2})\), considering that \(f_{1}(\tau ,x,y)\) and \(f_{2}(\tau ,x,y)\) are increasing in x and decreasing in y, we have
also
Set \(A=(A_{1},A_{2}):K\rightarrow K\), \(B=(B_{1},B_{2}):K\rightarrow K\), \(C=(C_{1},C_{2}): K\times K\rightarrow K\). Then A, B, C satisfy Eq. (3) of Theorem 2.2, with replacing the cone K for the cone P.
From Lemma 2.7, we get \(K_{h}=P_{h_{1}}\times P_{h_{2}}\), where \(h(\tau )=(h_{1}(\tau ),h_{2}(\tau ))=(\tau ^{\alpha -1},\tau ^{ \beta -1})\), also by condition \((i)\) of Theorem 2.2, we need prove \(A_{1_{h_{1}}},B_{1_{h_{1}}}\in P_{h_{1}}\), \(A_{2_{h_{2}}},B_{2_{h_{2}}}\in P_{h_{2}}\) and \(C_{1}(h_{1},h_{1})\in P_{h_{1}} \), \(C_{2}(h_{2},h_{2})\in P_{h_{2}}\).
Indeed
Let
Then \(a_{12}\geq a_{11}>0\) and thus
Also,
Let
Then \(a_{22}\geq a_{21}>0\) and thus
therefore
From \(k_{2}(u(1))\not \equiv 0\) and \(k_{1}(v(1))\not \equiv 0\) we get \(B_{1_{h_{1}}}\in P_{h_{1}}\), \(B_{2_{h_{2}}}\in P_{h_{2}}\). We have
We can calculate that
also
Set \(a^{\prime }_{11}=\int _{0}^{1} \frac{(1-\rho )^{\alpha -\zeta -1}[1-(1-\rho )^{\zeta -\eta }]}{\varGamma (\alpha )}g_{1}(\rho ,0)\,d\rho \) and \(a^{\prime }_{12}=\int _{0}^{1} \frac{(1-\rho )^{\alpha -\zeta -1}}{\varGamma (\alpha )}g_{1}(\rho ,1)\,d\rho \), we have
and by (17) and (18) we have \(a^{\prime }_{11}h\leq A_{1}(h)\leq a^{\prime }_{12}h\). So \(\min \{a_{11},a^{\prime }_{11}\}h\preceq A_{1}(h)\preceq \max \{a_{12}, a^{ \prime }_{12}\}h\). Hence \(A_{1}(h)\in P_{h}\).
Again we have
Similarly we set \(a^{\prime }_{21}=\int _{0}^{1} \frac{(1-\rho )^{\beta -\xi -1}[1-(1-\rho )^{\xi -\eta }]}{\varGamma (\beta )}g_{1}(\rho ,0)\,d\rho \) and \(a^{\prime }_{22}=\int _{0}^{1} \frac{(1-\rho )^{\beta -\xi -1}}{\varGamma (\beta )}g_{1}(\rho ,1)\,d\rho \), we have
and by (17) we have \(a^{\prime }_{21}h\leq A_{2}(h)\leq a^{\prime }_{22}h\). So \(\min \{a_{21},a^{\prime }_{21}\}h\preceq A_{2}(h)\preceq \max \{a_{21},a^{ \prime }_{22}\}h\), hence \(A_{2}(h)\in P_{h}\).
Furthermore,
therefore
from \(k_{2}(u(1))\not \equiv 0\) and \(k_{1}(v(1))\not \equiv 0\) we get \(B_{1_{h_{1}}}\in P_{h_{1}}\), \(B_{2_{h_{2}}}\in P_{h_{2}}\).
Set
and
From \((H_{2})\) and \((H_{4})\), it is clear that
Consequently,
Next, we show the proof the condition \((A_{2})\) of Lemma 2.5. By \((H_{4})\),
Then \(C_{1}(y,x)\succeq \delta _{11}A_{1}(x)\).
Then \(C_{2}(y,x)\succeq \delta _{21}A_{2}(y)\). From \((H_{4})\) and Lemma 2.6, we have
That means \(C_{1}(x,y)\succeq \frac{1}{\varGamma (\alpha -\zeta )}( \frac{1}{\alpha -\zeta }-\frac{1}{\alpha -\eta })B_{2}y\). Let
then
That means \(C_{2}(y,x)\succeq \frac{1}{\varGamma (\beta -\xi )}( \frac{1}{\beta -\xi }-\frac{1}{\beta -\gamma })B_{1}x\). Let
Then we have
We see that the conclusion (2) in Lemma 2.5 means that there exist \(u_{01},u_{02},v_{01},v_{02}\in P_{h}\) and \(r\in (0,1)\) such that
\((1)\) ∃ \((u_{01},v_{01}),(u_{02},v_{02})\in K\subset E\times E\) and \(r\in (0,1)\) with
that is,
where \(h(\tau )=(h_{1}(\tau ),h_{2}(\tau ))=(\tau ^{\alpha -1},\tau ^{ \beta -1})\), \(0\leq \tau \leq 1\), and \(G_{1}(\tau ,\rho )\), \(G_{2}(\tau ,\rho )\) are defined by (11) and (12), respectively.
\((2)\) The problem (2) has a unique positive solution \((u^{*},v^{*})\) in \(K_{h}\);
\((3)\) For \((x_{01},x_{02}),(y_{01},y_{02})\in P_{h}\times P_{h}\), there are two iterative sequences \(\{(x_{n1},x_{n2})\}\) and \(\{(y_{n1},y_{n2})\}\) for approximating \((x^{*},y^{*})\), that is, \((x_{n1},x_{n2})\rightarrow (x^{*},y^{*})\) and \((y_{n1},y_{n2})\rightarrow (x^{*},y^{*})\), where
□
3.1 Example
Let us consider
Let \(g_{1}(\tau ,y)=(x(\tau ))^{\frac{1}{4}}+\tau ^{2}\), \(f_{1}(\tau ,x,y)=(x(\tau ))^{\frac{1}{4}}+(y(\tau )+1)^{-\frac{1}{2}}+1\) and \(k_{1}(y)=\frac{1}{1+y^{\frac{1}{2}}}\), also \(g_{2}(\tau ,x)=\tau ^{3}+\frac{x}{1+x}\), \(f_{2}(\tau ,x,y)=\tau +\frac{x}{1+x}+\frac{1}{y+1}\) and \(k_{2}(x)=x^{-\frac{1}{3}}+5\).
Obviously, \(g_{1},g_{2}:[0,1]\times \mathbb{R^{+}}\rightarrow \mathbb{R^{+}}\), \(f_{1},f_{2}:[0,1]\times \mathbb{R^{+}}\times \mathbb{R^{+}} \rightarrow \mathbb{R^{+}}\) and \(k_{1},k_{2}:\mathbb{R^{+}}\rightarrow \mathbb{R^{+}}\) are continuous. It is easy to check that \(g_{1}(\tau ,y)\), \(g_{2}(\tau ,x)\) are increasing in y, x, respectively, and \(k_{1}(y)\), \(k_{2}(x)\) are decreasing in \(y,x\in \mathbb{R^{+}}\) (respectively) and \(f_{1}(\tau ,x,y)\), \(f_{2}(\tau ,x,y)\) are increasing in x and decreasing in y for fixed \(\tau \in (0,1)\). In addition, for any \(\lambda \in (0,1)\) we get
Besides, \(g_{1}(\tau ,0)=2\not \equiv 0\), \(g_{2}(\tau ,0)=\tau \not \equiv 0\) Moreover, set \(\delta _{1}=\delta _{2}=1\),
Then by Theorem 3.1 we deduce that (21) has a unique positive solution \((x^{*},y^{*})\) in \((P_{h_{1}},P_{h_{2}})\), where \((h_{1},h_{2})=(\tau ^{\frac{5}{2}},\tau ^{\frac{7}{3}})\).
4 Conclusion
In this manuscript, we extend the existence and uniqueness of positive solutions from a class of fractional differential equations with nonlinear boundary conditions for a new class of coupled system of fractional derivatives.
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Afshari, H., Sajjadmanesh, M. & Baleanu, D. Existence and uniqueness of positive solutions for a new class of coupled system via fractional derivatives. Adv Differ Equ 2020, 111 (2020). https://doi.org/10.1186/s13662-020-02568-2
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DOI: https://doi.org/10.1186/s13662-020-02568-2