Abstract
By some new integral inequalities of Henry–Gronwall type, we investigate the existence and uniqueness of positive solutions for fractional differential equations.
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1 Introduction
Fractional differential equations have gained considerable importance due to their applications in various sciences such as physics, mechanics, chemistry, engineering, etc. [1,2,3,4,5,6,7]. In the recent years, there has been a significant development in ordinary and partial differential equations involving fractional derivatives, see the monographs [8,9,10] and the papers in [11,12,13,14,15,16,17]. However, there have been few contributions to the existence and uniqueness of the following fractional differential equations:
In most of the available literature, fractional integral inequalities play an important role in the qualitative analysis of the solutions for fractional differential equations (see [14,15,16,17]). In this paper, by a method introduced by M. Medveď [18], we first study the following Henry–Gronwall integral inequalities:
and
where \(0<\gamma_{1}<\gamma_{2}<1\), which generalize the famous Henry inequalities [19]. Then using a suitable substitution, we construct an equivalent fractional integral equation of equation (1.1). By the above integral inequalities and fixed point theorem, we present the existence and uniqueness of fractional differential equations (1.1). Finally, some examples are given to illustrate the applications of the obtained results.
2 Preliminaries
In this section, we introduce definitions and preliminary facts which are used throughout this paper.
Let \(I=[0,a]\ (0< a<+\infty)\) be a finite interval. \(AC[0,a]\) is the space of functions which are absolutely continuous on I. \(L^{\infty}(0,a)\) is the space of measurable functions \(f: I\rightarrow\Re\) with the norm \(\|f\|_{L^{\infty}}=\inf\{c>0, |f(t)|\leq c,\mbox{ a.e. }t\in I\}\). \(C^{1}[0,a]\) is the space of functions which are continuously differentiable on I.
The Riemann–Liouville fractional integral and derivative of order \(\alpha\in(0,1)\) are defined by
and
The Caputo fractional derivative of order \(\alpha\in(0,1)\) is defined by
In particular, when \(x(t)\in AC[0,a]\),
Lemma 2.1
([8])
Let \(\alpha\in(0,1)\) and \(x\in L^{\infty}(0,a)\) or \(x\in C[0,a]\), then
Lemma 2.2
([8])
Let \(\alpha\in(0,1)\) and \(x\in AC[0,a]\) or \(x\in C^{1}[0,a]\), then
Theorem 2.3
Let \(0<\beta<\alpha<1\) and \(x\in AC[0,a]\) or \(x\in C^{1}[0,a]\), then
Proof
By Lemmas 2.1 and 2.2, we know
□
Theorem 2.4
Let \(0<\beta<\alpha<1\) and \(x=I^{\beta}\mu(t)\), where \(\mu\in C[0,a]\), then
Proof
We know
□
Theorem 2.5
Let \(0<\gamma_{1}<\gamma_{2}<1\), \(a(t)\), \(b_{1}(t)\), \(b_{2}(t)\), \(l_{1}(t)\), and \(l_{2}(t)\) be continuous, nonnegative functions on \([0,+\infty)\), and \(u(t)\) be a continuous, nonnegative function on \([0,+\infty)\) with
Then the following assertions hold:
where \(b(t)=\max\{\frac{b_{1}(t)t^{\gamma_{1}-1+\frac{1}{q}}}{(q(\gamma _{1}-1)+1)^{\frac{1}{q}}}, \frac{b_{2}(t)t^{\gamma_{2}-1+\frac{1}{q}}}{(q(\gamma _{2}-1)+1)^{\frac{1}{q}}} \}\), \(A(t)=\int_{0}^{t}3^{p-1}(l_{1}^{p}(s)+l_{2}^{p}(s))a^{p}(s)\,ds\), \(L(t)= 3^{p-1}b^{p}(t)(l_{1}^{p}(t)+l_{2}^{p}(t))\), and \(p, q\in(1,+\infty)\) such that \(\gamma_{1}+\frac{1}{q}> 1\) and \(\frac{1}{q}+\frac{1}{p}=1\).
Proof
Choose nonnegative constants \(p, q\) such that \(\gamma_{1}+\frac{1}{q}> 1\) and \(\frac{1}{q}+\frac{1}{p}=1\). Using the Hölder inequality, we obtain
Let \(b(t)=\max\{\frac{b_{1}(t)t^{\gamma_{1}-1+\frac{1}{q}}}{(q(\gamma _{1}-1)+1)^{\frac{1}{q}}}, \frac{b_{2}(t)t^{\gamma_{2}-1+\frac{1}{q}}}{(q(\gamma _{2}-1)+1)^{\frac{1}{q}}} \}\). Then
and
Let \(w(t)=\int_{0}^{t}(l_{1}^{p}(s)+l_{2}^{p}(s))u^{p}(s)\,ds\), \(A(t)=\int_{0}^{t}3^{p-1}(l_{1}^{p}(s)+l_{2}^{p}(s))a^{p}(s)\,ds\), and \(L(t)=3^{p-1}b^{p}(t)\times (l_{1}^{p}(t)+l_{2}^{p}(t))\). Then
By Gronwall’s integral inequality, we have
By (2.6) and (2.9) we obtain inequality (2.4) and complete the proof. □
Theorem 2.6
Let \(0<\gamma_{1}<\gamma_{2}<1\), \(a(t)\), \(b_{1}(t)\), \(b_{2}(t)\), \(l_{1}(t)\), and \(l_{2}(t)\) be nondecreasing, nonnegative, and continuous functions on \([0,T)(0< T\leq+\infty)\), \(\varphi_{1}, \varphi_{2}: [0,+\infty)\rightarrow[0,+\infty)\) be continuous, nondecreasing functions, and \(u(t)\) be a continuous, nonnegative function on \([0,T)\) with
Then
where \(A(t)=3^{p-1}a^{p}(t)\), \(B_{1}(t)=3^{p-1}(\frac{b_{1}(t)t^{\gamma_{1}-1+\frac {1}{q}}}{(q(\gamma _{1}-1)+1)^{\frac{1}{q}}})^{p}\), \(B_{2}(t)=3^{p-1}(\frac{b_{2}(t)t^{\gamma_{2}-1+\frac {1}{q}}}{(q(\gamma _{2}-1)+1)^{\frac{1}{q}}})^{p}\), \(\varOmega(x)= \int_{t_{0}}^{x}\frac{1}{\mu_{1}(t)+\mu_{2}(t)}\,dt\), \(\mu_{1}(t)=\varphi_{1}^{p}(t^{\frac{1}{p}})\), \(\mu_{2}(t)=\varphi_{2}^{p}(t^{\frac{1}{p}})\), \(t_{0}>0\), \(\varOmega^{-1}\) is the inverse of Ω, and \(T_{1}\in(0,T)\) is such that \(\varOmega(A(t))+B_{1}(t)\int_{0}^{t}l_{1}^{p}(s)\,ds+B_{2}(t)\int _{0}^{t}l_{2}^{p}(s)\,ds\in \operatorname{Dom}(\varOmega^{-1})\) for all \(t\in[0,T_{1}]\), and \(p, q\in(1,+\infty)\) such that \(\gamma_{1}+\frac{1}{q}> 1\) and \(\frac{1}{q}+\frac{1}{p}=1\).
Proof
Choose nonnegative constants \(p, q\) such that \(\gamma_{1}+\frac{1}{q}> 1\) and \(\frac{1}{q}+\frac{1}{p}=1\). Using the Hölder inequality, we obtain
Then
Let \(w(t)=u^{p}(t)\), \(A(t)=3^{p-1}a^{p}(t)\), \(B_{1}(t)=3^{p-1}(\frac{b_{1}(t)t^{\gamma_{1}-1+\frac {1}{q}}}{(q(\gamma _{1}-1)+1)^{\frac{1}{q}}})^{p}\), and \(B_{2}(t)=3^{p-1}(\frac{b_{2}(t)t^{\gamma_{2}-1+\frac {1}{q}}}{(q(\gamma _{2}-1)+1)^{\frac{1}{q}}})^{p}\). Fix any \(T_{0}\in[0,T_{1}]\), then for \(t\in[0,T_{0}]\) and (2.13) we have
Let \(V(t)=A(T_{0})+B_{1}(T_{0})\int_{0}^{t}l_{1}^{p}(s)\mu_{1}(w(s))\,ds +B_{2}(T_{0})\int_{0}^{t}l_{2}^{p}(s)\mu_{2}(w(s))\,ds\), then we get
This yields
or
Integrating this inequality from 0 to \(t\in[0,T_{0}]\), we obtain
then
and
So
Now replacing \(T_{0}\) by t in inequality (2.21), we obtain the result (2.11) valid for \(t\in[0,T_{1}]\) provided
for all \(t\in [0,T_{1}]\). □
Lemma 2.7
Let E be a Banach space X, C be a closed, convex subset of E, U be an open subset of C, and \(P\in U\). Suppose that \(F: \overline {U}\rightarrow C\) is a continuous, compact map. Then either
-
(a)
F has a fixed point in U̅; or
-
(b)
there are \(u\in\partial U\) (the boundary of U in C) and \(\lambda\in(0,1)\) with \(u=\lambda F(u)+(1-\lambda)P\).
Lemma 2.8
Let E be a Hausdorff locally convex linear topological space, C be a convex subset of E, U be an open subset of C, and \(P\in U\). Suppose that \(F: \overline{U}\rightarrow C\) is a continuous, compact map. Then either
-
(a)
F has a fixed point in U̅; or
-
(b)
there are \(u\in\partial U\) (the boundary of U in C) and \(\lambda\in(0,1)\) with \(u=\lambda F(u)+(1-\lambda)P\).
3 Main results
In this section, we give the existence and uniqueness results of the fractional differential equations (1.1).
Theorem 3.1
\(f: \mathbb{\Re}^{+}\times\mathbb{\Re}\rightarrow\mathbb{\Re}\) is a continuous function. If \(x(\cdot)\in C[0,a]\) is the solution of the following integral equation
then \(x(t)\) is the solution of the fractional integral equation (1.1).
Proof
If \(x(t)\in C[0,a]\) is the solution of the integral equation (3.1), we know \(x(0)=x_{0}\) and
where \(\mu(t)=x(t)-x_{0}+I^{\beta}f(t,x(t))\). By (3.1) and (3.2), we obtain
If \(2(\alpha-\beta)<\alpha\), then \(x(t)-x_{0}=I^{2(\alpha-\beta)}\mu_{1}(t)\), where \(\mu_{1}(t)=\mu(t)+I^{2\beta-\alpha}f(t,x(t))\). By the same step, we obtain \(x(t)-x_{0}\in I^{\alpha}\phi_{1}(t)\) and \(x(t)-x_{0}\in I^{\beta}\phi_{2}(t)\), where \(\phi_{1}(t), \phi_{2}(t)\in C[0,a]\).
By Lemma 2.1 and Theorem 2.4, we get
□
Theorem 3.2
Let \(x_{0}>0\), \(f: \mathbb{\Re}^{+}\times\mathbb{\Re}^{+}\rightarrow\mathbb{\Re}^{+}\) be a continuous function, and there exist nonnegative continuous functions \(l(t)\) and \(k(t)\) such that
for all \(x\in\mathbb{\Re}^{+}\), \(t\in[0, \infty)\). Then equation (1.1) has at least one positive solution on \([0,\infty)\).
Proof
Consider the operator \(G: W \rightarrow W\) defined by
where \(W=\{x(t)\in C[0,+\infty)| x(t)\geq x_{0}\}\).
By Theorem 3.1, we know that the fixed points of operator G are solutions of equation (1.1). We can show that \(G: W \rightarrow W\) is continuous and compact by the usual techniques (see [12, 13]).
Let \(U=\{x\in W: |x(t)|< (3^{p-1}a^{p}(t)+3^{p-1}b^{p}(t)(A(t)+\int_{0}^{t}L(s)A(s)\exp(\int _{s}^{t}L(\tau)\,d\tau)\,ds))^{\frac{1}{p}}+1, t\in[0,\infty) \}\), where \(a(t)=|x_{0}|+|\frac{x_{0}t^{\alpha-\beta}}{(\alpha-\beta )\varGamma (\alpha-\beta)}|+\frac{1}{\varGamma(\alpha)}\frac{t^{\alpha -1+\frac {1}{q}}}{((\alpha-1)q+1)^{\frac{1}{q}}}(\int_{0}^{t}k^{p}(s)\, ds)^{\frac{1}{p}} \), \(b(t)= \max\{\frac{\frac{1}{\varGamma(\alpha-\beta)}t^{\alpha -\beta -1+\frac{1}{q}}}{(q(\alpha-\beta-1)+1)^{\frac{1}{q}}}, \frac{\frac{1}{\varGamma(\alpha)}t^{\alpha-1+\frac {1}{q}}}{(q(\alpha -1)+1)^{\frac{1}{q}}} \}\), \(A(t)=\int_{0}^{t}3^{p-1}(1+l^{p}(s))a^{p}(s)\,ds\), \(L(t)=3^{p-1}b^{p}(t)(1+l^{p}(t))\), and \(p, q\in(1,+\infty)\) such that \(\alpha-\beta+\frac{1}{q}> 1\) and \(\frac{1}{q}+\frac{1}{p}=1\).
If \(x\in W\) is any solution of
for \(\lambda\in(0,1)\).
Then
Consequently, by Theorem 2.5, we can get
Applying Lemma 2.8, we can obtain that G has at least one fixed point in W. Thus, the proof is completed. □
Theorem 3.3
If \(f: \mathbb{\Re}^{+}\times\mathbb{\Re}^{+}\rightarrow\mathbb{\Re}^{+}\) is a continuous function and
for all \(x, y\in\mathbb{\Re}^{+}\) and \(t\in[0,+\infty)\), where nonnegative function \(l(t)\in C[0,+\infty)\), then equation (1.1) has a unique positive solution on \([0,+\infty)\).
Proof
By Theorem 3.2, we suppose that \(x_{1}(t)\), \(x_{2}(t)\) are two positive solutions of equation (1.1). Then
By Theorem 2.5, we can get \(x_{1}(t)=x_{2}(t)\). □
Theorem 3.4
Let \(x_{0}>0\), \(f: [0,T]\times\mathbb{\Re}^{+}\rightarrow\mathbb{\Re}^{+}\) be a continuous function, and there exist a nonnegative function \(l(t)\in C[0,T]\) and a nonnegative nondecreasing function \(\omega\in C[0,+\infty)\) such that
Then the initial value problem (1.1) has at least a continuous positive solution on \([0,T]\) provided that
for all \(t\in[0,T]\), where \(A(t)=3^{p-1}(|x_{0}|+\frac{|x_{0}t^{\alpha-\beta}|}{(\alpha-\beta )\varGamma(\alpha-\beta)})^{p}\), \(B_{1}(t)=3^{p-1}(\frac{\frac{1}{\varGamma(\alpha-\beta)}t^{\alpha -\beta -1+\frac{1}{q}}}{(q(\alpha-\beta-1)+1)^{\frac{1}{q}}})^{p}\), \(B_{2}(t)=3^{p-1}(\frac{\frac{1}{\varGamma(\alpha)}t^{\alpha -1+\frac {1}{q}}}{(q(\alpha-1)+1)^{\frac{1}{q}}})^{p}\), \(\varOmega(x)=\int_{t_{0}}^{x}\frac{1}{\mu_{1}(t)+\mu_{2}(t)}\,dt\), \(\mu_{1}(t)=t\), \(\mu_{2}(t)=\omega^{p}(t^{\frac{1}{p}})\), \(t_{0}>0\), \(\varOmega^{-1}\) is the inverse of Ω, and \(p, q\in(1,+\infty)\) such that \(\alpha-\beta+\frac{1}{q}> 1\) and \(\frac{1}{q}+\frac{1}{p}=1\).
Proof
Consider the operator \(G: W \rightarrow W\) defined by
where \(W=\{x\in C[0,T] | x(t)\geq x_{0}\}\).
Similarly with the proof of Theorem 3.2, we can show that \(G: W \rightarrow W\) is continuous and compact.
Let \(U=\{x \in W: |x(t)|< (\varOmega^{-1}(\varOmega(A(t))+tB_{1}(t)+B_{2}(t)\int _{0}^{t}l^{p}(s)\, ds))^{\frac{1}{p}}+1, t\in[0,T] \}\).
If \(x\in W\) is any solution of
for \(\lambda\in(0,1)\).
Then
By Theorem 2.6, we can get
By Lemma 2.7, G has at least one fixed point in W. Thus, the proof is completed. □
4 Examples
Example 4.1
We know \(|t^{2}x^{\frac{1}{2}}(t)|\leq\frac{t^{2}(|x(t)|+1)}{2}\), all assumptions of Theorem 3.2 are satisfied. Hence equation (4.1) has at least one positive solution on \([0,+\infty)\).
Example 4.2
We know \(|\ln(1+x)-\ln(1+y)|\leq|x-y|\) for all \(x,y\in(0,+\infty)\). From Theorem 3.3, equation (4.2) has a unique positive solution on \([0,+\infty)\).
Example 4.3
Let \(q=\frac{5}{4}\) and \(p=5\), from Theorem 3.4, equation (4.3) has at least one positive solution on \([0,T]\) provided that
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This work was supported by the funding (No: CKJB201508; CKJB201709) from Nanjing Institute of Technology.
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Zhu, T. Existence and uniqueness of positive solutions for fractional differential equations. Bound Value Probl 2019, 22 (2019). https://doi.org/10.1186/s13661-019-1141-0
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DOI: https://doi.org/10.1186/s13661-019-1141-0