We prove existence and uniqueness of solutions to discrete fractional equations that involve Riemann-Liouville and Caputo fractional derivatives with three-point boundary conditions. The results are obtained by conducting an analysis via the Banach principle and the Brouwer fixed point criterion. Moreover, we prove stability, including Hyers-Ulam and Hyers-Ulam-Rassias type results. Finally, some numerical models are provided to illustrate and validate the theoretical results.
Citation: Omar Choucha, Abdelkader Amara, Sina Etemad, Shahram Rezapour, Delfim F. M. Torres, Thongchai Botmart. On the Ulam-Hyers-Rassias stability of two structures of discrete fractional three-point boundary value problems: Existence theory[J]. AIMS Mathematics, 2023, 8(1): 1455-1474. doi: 10.3934/math.2023073
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We prove existence and uniqueness of solutions to discrete fractional equations that involve Riemann-Liouville and Caputo fractional derivatives with three-point boundary conditions. The results are obtained by conducting an analysis via the Banach principle and the Brouwer fixed point criterion. Moreover, we prove stability, including Hyers-Ulam and Hyers-Ulam-Rassias type results. Finally, some numerical models are provided to illustrate and validate the theoretical results.
Fractional difference calculus is a tool used to explain many phenomena in physics, control problems, modeling, chaotic dynamical systems, and various fields of engineering and applied mathematics. In this direction, different kinds of methods and techniques, including numerical and analytical methods, have been utilized by researchers to discuss given fractional discrete and continuous mathematical models and boundary value problems (BVPs) [1,2,3,4]. For some recent developments on the existence, uniqueness, and stability of solutions for fractional differential equations, see, for example, [5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23] and the references therein.
Discrete fractional calculus and difference equations open a new study context for mathematicians. For this reason, they have received increasing attention in recent years. Some real-world processes and phenomena are analyzed with the aid of discrete fractional operators, since such operators provide an accurate tool to describe memory. A large number of research articles dealing with difference equations and discrete fractional boundary value problems (FBVPs) can be found in [24,25,26,27,28,29,30,31,32].
In 2020, Selvam et al. [33] proved the existence of a solution to a discrete fractional difference equation formulated as
{cΔϱξχ(ξ)=Φ(ξ+ϱ−1,χ(ξ+ϱ−1)),1<ϱ≤2,Δχ(ϱ−2)=M1,χ(ϱ+T)=M2, | (1.1) |
for ξ∈[0,T]N0=[0,1,2,…,T], T∈N, η∈[ϱ−1,T+ϱ−1]Nϱ−1, M1 and M2 constants, Φ:[ϱ−2,ϱ+T]Nϱ−2×R⟶R continuous, and where cΔϱξ denotes the ϱth-Caputo difference. Here, motivated by the discrete model (1.1), we shall consider two generalized discrete problems.
Our first goal consists to study existence and uniqueness of solutions to the following discrete fractional equation that involves Caputo discrete derivatives:
{cΔϱξχ(ξ)=Φ(ξ+ϱ−1,χ(ξ+ϱ−1)),2<ϱ≤3,Δχ(ϱ−3)=A1,χ(ϱ+T)=λΔ−βχ(η+β),Δ2χ(ϱ−3)=A2, | (1.2) |
for 0<β≤1, ξ∈[0,T]N0=[0,1,2,…,T], T∈N, η∈[ϱ−1,T+ϱ−1]Nϱ−1, λ, A1 and A2 constants, and where Φ:[ϱ−3,ϱ+T]Nϱ−3×R⟶R is continuous.
The second goal is to study the stability of solutions to the discrete Riemann-Liouville fractional problem
{RLΔϱξχ(ξ)=Φ(ξ+ϱ−1,χ(ξ+ϱ−1)),2<ϱ≤3,Δχ(ϱ−3)=A1,χ(ϱ+T)=λΔ−βχ(η+β),Δ2χ(ϱ−3)=A2, | (1.3) |
for 0<β≤1, ξ∈[0,T]N0=[0,1,2,…,T] and η∈[ϱ−1,T+ϱ−1]Nϱ−1, where RLΔϱξ is the Riemann-Liouville difference operator.
The organization of the paper is as follows. In Section 2, we collect some fundamental definitions available from the literature. In Section 3, we prove the existence and uniqueness results for the discrete FBVP (1.2). Hyers-Ulam and Hyers-Ulam-Rassias stability of the solution for the FBVP (1.3) is established in Section 4. In Section 5, two examples are given to illustrate the obtained results. We end with Section 6 of conclusions.
We begin by recalling some necessary definitions and essential lemmas that will be used throughout the paper.
Definition 2.1. (See [27]) Let ϱ>0. The ϱ-order fractional sum of Φ is defined by
Δ−ϱξΦ(ξ)=1Γ(ϱ)ξ−ϱ∑l=a(ξ−l−1)(ϱ−1)Φ(l), | (2.1) |
where ξ∈Na+ϱ:={a+ϱ,a+ϱ+1,…} and ξ(ϱ):=Γ(ξ+1)Γ(ξ+1−ϱ).
Definition 2.2. (See [27]) Let ϱ>0 and Φ be defined on Na. The ϱ-order Caputo fractional difference of Φ is defined by
CaΔϱξΦ(ξ)=Δ−(n−ϱ)(ΔnΦ(ξ))=1Γ(n−ϱ)ξ−(n−ϱ)∑l=a(ξ−l−1)(n−ϱ−1)ΔnΦ(l), | (2.2) |
while the Riemann-Liouville fractional difference of Φ is defined by
RLaΔϱξΦ(ξ)=ΔnΔ−(n−ϱ)Φ(ξ), | (2.3) |
where ξ∈Na+n−ϱ and n−1<ϱ≤n.
Lemma 2.1. (See [24,27]) For ϱ>0,
Δ−ϱCaΔϱξΦ(ξ)=Φ(ξ)+C0+C1ξ+⋯+CN−1ξ(N−1), | (2.4) |
where Ci∈R, i=1,2,…,N−1, Φ is defined on Na, and 0≤N−1<ϱ≤N.
Lemma 2.2. (See ([32]) Let 0≤N−1<ϱ≤N and Φ be defined on Na. Then,
Δ−ϱRL0ΔϱξΦ(ξ)=Φ(ξ)+B1ξϱ−1+B2ξ(ϱ−2)+⋯+BNξ(ϱ−N), | (2.5) |
for B1,…,BN∈R.
Lemma 2.3. (See [31]) Let ϱ and ξ be any arbitrary real numbers. Then,
(1)ξ−ϱ∑l=0(ξ−l−1)(ϱ−1)=Γ(ξ+1)ϱΓ(ξ−ϱ+1),(2)L∑l=0(ξ−L−l−1)(ϱ−1)=Γ(ϱ+L+1)ϱΓ(L+1). |
Lemma 2.4. (See [31]) For ζ∈R∖(Z−∖{0}), we have
Δ−ϱξ(ζ)=Γ(ζ+1)Γ(ζ+ϱ+1)ξ(ζ+ϱ). |
In this section, we prove the existence and uniqueness of solution for the Caputo three-point discrete fractional problem (1.2). To accomplish this, we denote by C(Nϱ−3,ϱ+T,R) the collection of all continuous functions χ with the norm
‖χ‖=max{|χ(ξ)|:ξ∈Nϱ−3,ϱ+T}. |
Lemma 3.1. Let 2<ϱ≤3 and Φ:[ϱ−3,ϱ+T]Nϱ−3⟶R. A function χ(ξ) (ξ∈[ϱ−3,ϱ+T]Nϱ−3) that satisfies the discrete FBVP
{cΔϱξχ(ξ)=Φ(ξ+ϱ−1),2<ϱ≤3,Δχ(ϱ−3)=A1,χ(ϱ+T)=λΔ−βχ(η+β),Δ2χ(ϱ−3)=A2,0<β≤1, | (3.1) |
is given by
χ(ξ)=1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1)−λK1Γ(ϱ)η∑l=ϱl−ϱ∑ξ=0(η+β−ρ(l))(β−1)(l−ρ(ξ))(ϱ−1)Φ(ξ+ϱ−1)+Γ(β)K1Γ(ϱ)T∑l=0(ϱ+T−ρ(l))(ϱ−1)Φ(l+ϱ−1)+K2+[A1−A2(ϱ−3)]ξ(1)+A22ξ(2), | (3.2) |
with
K1=λη∑l=ϱ−3(η+β−ρ(l))(β−1)−Γ(β), | (3.3) |
and
K2=λK1[A2(ϱ−3)−A1]η∑l=ϱ−3(η+β−ρ(l))(β−1)l(1)−A22K1λη∑l=ϱ−3(η+β−ρ(l))(β−1)l(2)+Γ(β)K1(ϱ+T)[A1−A2(ϱ−3)+A22(ϱ+T−1)]. | (3.4) |
Proof. Let χ(ξ) be a solution to (3.1). Applying Lemma 2.1 and Definition 2.1, we find that
χ(ξ)=1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1)+C0+C1ξ(1)+C2ξ(2), | (3.5) |
for ξ∈[ϱ−3,ϱ+T]Nϱ−3, where C0,C1,C2∈R. By using the difference of order 1 for (3.5), we have
Δχ(ξ)=1Γ(ϱ−1)ξ−ϱ+1∑l=0(ξ−ρ(l))(ϱ−2)Φ(l+ϱ−1)+C1+2C2ξ(1), |
and
Δ2χ(ξ)=1Γ(ϱ−2)ξ−ϱ+2∑l=0(ξ−ρ(l))(ϱ−3)Φ(l+ϱ−1)+2C2. |
Now, from conditions Δχ(ϱ−3)=A1 and Δ2χ(ϱ−3)=A2, we obtain that
C1=A1−A2(ϱ−3),C2=A22. |
Therefore,
χ(ξ)=1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1)+C0+[A1−A2(ϱ−3)]ξ(1)+A22ξ(2), | (3.6) |
for ξ∈[ϱ−3,ϱ+T]Nϱ−3. By using formula (3.5), one has
Δ−βχ(ξ)=C1Γ(β)ξ−β∑l=ϱ−3(ξ−ρ(l))(β−1)l(1)+C2Γ(β)ξ−β∑l=ϱ−3(ξ−ρ(l))(β−1)l(2)+C0Γ(β)ξ−β∑l=ϱ−3(ξ−ρ(l))(β−1)+1Γ(ϱ)Γ(β)ξ−β∑l=ϱl−ϱ∑ξ=0(ξ−ρ(l))(β−1)(l−ρ(ξ))(ϱ−1)Φ(ξ+ϱ−1). | (3.7) |
The other condition of (3.1) gives
λΔ−βχ(η+β)=λ[A1−A2(ϱ−3)]Γ(β)η∑l=ϱ−3(η+β−ρ(l))(β−1)l(1) +λA22Γ(β)η∑l=ϱ−3(η+β−ρ(l))(β−1)l(2)+λC0Γ(β)η∑l=ϱ−3(η+β−ρ(l))(β−1) +λΓ(ϱ)Γ(β)η∑l=ϱl−ϱ∑ξ=0(η+β−ρ(l))(β−1)(l−ρ(ξ))(ϱ−1)Φ(ξ+ϱ−1)=1Γ(ϱ)T∑l=0(ϱ+T−ρ(l))(ϱ−1)Φ(l+ϱ−1)+C0 +[A1−A2(ϱ−3)](ϱ+T)(1)+A22(ϱ+T)(2). |
We have
C0=Γ(β)K1Γ(ϱ)T∑l=0(ϱ+T−ρ(l))(ϱ−1)Φ(l+ϱ−1)+K2−λK1Γ(ϱ)η∑l=ϱl−ϱ∑ξ=0(η+β−ρ(l))(β−1)(l−ρ(ξ))(ϱ−1)Φ(ξ+ϱ−1), |
where K1 and K2 are defined by (3.3) and (3.4), and one obtains (3.2) by substituting the value of C0 into (3.6).
Now, let us consider the operator H:C(Nϱ−3,ϱ+T,R)→C(Nϱ−3,ϱ+T,R) defined by
(Hχ)(ξ)=1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1,χ(l+ϱ−1))−λK1Γ(ϱ)η∑l=ϱl−ϱ∑τ=0(η+β−ρ(l))(β−1)(l−ρ(τ))(ϱ−1)Φ(l+ϱ−1,χ(l+ϱ−1))+Γ(β)K1Γ(ϱ)T∑l=0(ϱ+T−ρ(l))(ϱ−1)Φ(l+ϱ−1,χ(l+ϱ−1))+K2+[A1−A2(ϱ−3)]ξ(1)+A22ξ(2). |
Theorem 3.1. Assume that:
(H1) Function Φ satisfies |Φ(ξ,χ1)−Φ(ξ,χ2)|≤K|χ1−χ2|, where K>0, ∀ξ∈Nϱ−3,ϱ+T and χ1,χ2∈C(Nϱ−3,ϱ+T,R). The discrete FBVP (3.1) has a unique solution on C(Nϱ−3,ϱ+T,R) provided
Γ(ϱ+T+1)Γ(ϱ+1)Γ(T+1)(1+Γ(β)K1)+MλK1Γ(ϱ)≤1K | (3.8) |
with
M=|η∑l=ϱl−ϱ∑τ=0(η+β−ρ(l))(β−1)(l−ρ(τ))(ϱ−1)|. | (3.9) |
Proof. Let χ1,χ2∈C(Nϱ−3,ϱ+T,R). Then, for each ξ∈Nϱ−3,ϱ+T, we have
|(Hχ1)(ξ)−(Hχ2)(ξ)|≤1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)×|Φ(l+ϱ−1,χ1(l+ϱ−1))−Φ(l+ϱ−1,χ2(l+ϱ−1))|+λK1Γ(ϱ)η∑l=ϱl−ϱ∑τ=0(η+β−ρ(l))(β−1)(l−ρ(τ))(ϱ−1)×|Φ(l+ϱ−1,χ1(l+ϱ−1))−Φ(l+ϱ−1,χ2(l+ϱ−1))|+Γ(β)K1Γ(ϱ)T∑l=0(ϱ+T−ρ(l))(ϱ−1)×|Φ(l+ϱ−1,χ1(l+ϱ−1))−Φ(l+ϱ−1,χ2(l+ϱ−1))|. |
It follows that
‖(Hχ1)(ξ)−(Hχ2)(ξ)‖≤K‖χ1−χ2‖Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)+λK‖χ1−χ2‖K1Γ(ϱ)η∑l=ϱl−ϱ∑τ=0(η+β−ρ(l))(β−1)(l−ρ(τ))(ϱ−1)+Γ(β)K‖χ1−χ2‖K1Γ(ϱ)T∑l=0(ϱ+T−ρ(l))(ϱ−1)≤K‖χ1−χ2‖Γ(ϱ)Γ(ϱ+T+1)ϱΓ(T+1)+λK‖χ1−χ2‖K1Γ(ϱ)η∑l=ϱl−ϱ∑τ=0(η+β−ρ(l))(β−1)(l−ρ(τ))(ϱ−1)+Γ(β)K‖χ1−χ2‖K1Γ(ϱ)Γ(ϱ+T+1)ϱΓ(T+1)≤K‖χ1−χ2‖[Γ(ϱ+T+1)Γ(ϱ+1)Γ(T+1)+MλK1Γ(ϱ)+Γ(β)Γ(ϱ+T+1)K1Γ(ϱ+1)Γ(T+1)]. |
From (3.8), we conclude that H is a contraction. Then, by the Banach contraction principle, the discrete problem (3.1) has a unique solution on C(Nϱ−3,ϱ+T,R).
Theorem 3.2. Suppose that Φ:[ϱ−3,ϱ+T]Nϱ−3×R⟶R is a continuous function and
R=max{|Φ(l+ϱ−1,χ(l+ϱ−1))|,ξ∈Nϱ−3,ϱ+T,χ∈C(Nϱ−3,ϱ+T,R);‖χ‖≤2|K2|}. |
The discrete problem (3.1) has a solution provided that
R≤|K2|−|[A1−A2(ϱ−3)]|(ϱ+T)(1)−|A22|(ϱ+T)(2)Γ(ϱ+T+1)Γ(ϱ+1)Γ(T+1)(1+Γ(β)|K1|)+M|λ|Γ(ϱ)|K1|. | (3.10) |
Proof. Let G={χ∈C(Nϱ−3,ϱ+T,R);‖χ‖≤2|K2|}. For χ(ξ)∈G, we get
|(Hχ)(ξ)|=|1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1,χ(l+ϱ−1))+λK1Γ(ϱ)η∑l=ϱl−ϱ∑τ=0(η+β−ρ(l))(β−1)(l−ρ(τ))(ϱ−1)Φ(l+ϱ−1,χ(l+ϱ−1))−Γ(β)K1Γ(ϱ)T∑l=0(ϱ+T−ρ(l))(ϱ−1)Φ(l+ϱ−1,χ(l+ϱ−1))+K2+[A1−A2(ϱ−3)]ξ(1)+A22ξ(2)|≤1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)|Φ(l+ϱ−1,χ(l+ϱ−1))|+|λ||K1|Γ(ϱ)η∑l=ϱl−ϱ∑τ=0(η+β−ρ(l))(β−1)(l−ρ(τ))(ϱ−1)|Φ(l+ϱ−1,χ(l+ϱ−1))|+Γ(β)|K1|Γ(ϱ)T∑l=0(ϱ+T−ρ(l))(ϱ−1)|Φ(l+ϱ−1,χ(l+ϱ−1))|+|K2|+|[A1−A2(ϱ−3)]ξ(1)|+|A22ξ(2)|≤RΓ(ϱ)[ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)+|λ||K1|η∑l=ϱl−ϱ∑τ=0(η+β−ρ(l))(β−1)(l−ρ(τ))(ϱ−1)+Γ(β)|K1|T∑l=0(ϱ+T−ρ(l))(ϱ−1)]+|K2|+|[A1−A2(ϱ−3)]|(ϱ+T)(1)+|A22|(ϱ+T)(2)≤RΓ(ϱ)[Γ(ϱ+T+1)ϱΓ(T+1)(1+Γ(β)|K1|)+M|λ||K1|]+|K2|+|[A1−A2(ϱ−3)]|(ϱ+T)(1)+|A22|(ϱ+T)(2). |
From (3.10), we have ‖Hχ‖≤2|K2|, which implies that H:G→G. By Brouwer's fixed point theorem, we know that the discrete problem (3.1) has a solution.
In this section, we study the Hyers-Ulam and Hyers-Ulam-Rassias stability for the solutions of the discrete Riemann-Liouville (RL) FBVP
{RLΔϱξχ(ξ)=Φ(ξ+ϱ−1,χ(ξ+ϱ−1)),2<ϱ≤3,Δχ(ϱ−3)=A1,χ(ϱ+T)=λΔ−βχ(η+β),Δ2χ(ϱ−3)=A2,0<β≤1, | (4.1) |
for ξ∈[0,1,2,…,T]=[0,T]N0 and η∈[ϱ−1,T+ϱ−1]Nϱ−1, where RLΔϱξ is the RL fractional difference operator. We begin by proving the following lemma.
Lemma 4.1. Suppose that 2<ϱ≤3 and Φ:[ϱ−3,ϱ+T]Nϱ−3⟶R. A function χ satisfies the discrete problem
{RLΔϱξχ(ξ)=Φ(ξ+ϱ−1),2<ϱ≤3,Δχ(ϱ−3)=A1,χ(ϱ+T)=λΔ−βχ(η+β),Δ2χ(ϱ−3)=A2,0<β≤1, | (4.2) |
if, and only if, χ(ξ), ξ∈[ϱ−3,ϱ+T]Nϱ−3, has the form
χ(ξ)=1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1)+{[A2−A1(ϱ−3)]Γ(ϱ)+fΦ+dϱhϱ(ϱ−3)2(ϱ−1)}ξ(ϱ−1)+[A1−1hϱ[fΦ+dϱ](ϱ−3)Γ(ϱ−2)]Γ(ϱ−1)ξ(ϱ−2)+fΦ+dϱhϱξ(ϱ−3), | (4.3) |
where
hϱ=ϱ−3ϱ−2(ϱ+T)(ϱ−2)−ϱ−32(ϱ−1)(ϱ+T)(ϱ−1)−(ϱ+T)(ϱ−3) | (4.4) |
+λ(ϱ−3)Γ(β)2(ϱ−1)η∑l=ϱ−3(η+β−ρ(l))(β−1)l(ϱ−1)−λ(ϱ−3)Γ(β)(ϱ−2)η∑l=ϱ−3(η+β−ρ(l))(β−1)l(ϱ−2)+λΓ(β)η∑l=ϱ−3(η+β−ρ(l))(β−1)l(ϱ−3),dϱ=A1(ϱ+T)(ϱ−2)Γ(ϱ−1)−[A2−A1(ϱ−3)]λΓ(ϱ)Γ(β)η∑l=ϱ−3(η+β−ρ(l))(β−1)l(ϱ−1) | (4.5) |
−A1λΓ(ϱ−1)Γ(β)η∑l=ϱ−3(η+β−ρ(l))(β−1)l(ϱ−2)+[A2−A1(ϱ−3)]Γ(ϱ)(ϱ+T)(ϱ−1),fΦ=−λΓ(ϱ)Γ(β)η∑l=ϱl−ϱ∑τ=0(η+β−ρ(l))(β−1)(l−ρ(τ))(ϱ−1)Φ(τ+ϱ−1) | (4.6) |
+1Γ(ϱ)T∑l=0(ϱ+T−ρ(l))(ϱ−1)Φ(l+ϱ−1). |
Proof. Let χ(ξ) be a solution to (4.2). Applying Lemma 2.2 and Definition 2.1, we obtain that the general solution of (4.2) is given by
χ(ξ)=1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1)+B1ξ(ϱ−1)+B2ξ(ϱ−2)+B3ξ(ϱ−3), | (4.7) |
ξ∈[ϱ−3,ϱ+T]Nϱ−3, where B1,B2,B3∈R. The first order difference of (4.7) is
Δχ(ξ)=1Γ(ϱ−1)ξ−ϱ+1∑l=0(ξ−ρ(l))(ϱ−2)Φ(l+ϱ−1)+B1(ϱ−1)ξ(ϱ−2)+B2(ϱ−2)ξ(ϱ−3)+B3(ϱ−3)ξ(ϱ−4), |
while
Δ2χ(ξ)=1Γ(ϱ−2)ξ−ϱ+2∑l=0(ξ−ρ(l))(ϱ−3)Φ(l+ϱ−1)+B1(ϱ−1)(ϱ−2)ξ(ϱ−3)+B2(ϱ−2)(ϱ−3)ξ(ϱ−4)+B3(ϱ−3)(ϱ−4)ξ(ϱ−5). |
From the conditions Δχ(ϱ−3)=A1 and Δ2χ(ϱ−3)=A2, we obtain that
B2=1Γ(ϱ−1)[A1−B3(ϱ−3)Γ(ϱ−2)], |
and
B1=1Γ(ϱ)[A2−A1(ϱ−3)]+B3(ϱ−3)2(ϱ−1). |
Now, by using the difference of order β for (4.7), it follows that
Δ−βχ(ξ)=1Γ(β){1Γ(ϱ)[A2−A1(ϱ−3)]+B3(ϱ−3)2(ϱ−1)}ξ−β∑l=ϱ−3(ξ−ρ(l))(β−1)l(ϱ−1)+B3Γ(β)ξ−β∑l=ϱ−3(ξ−ρ(l))(β−1)l(ϱ−3)+[A1−B3(ϱ−3)Γ(ϱ−2)]Γ(ϱ−1)Γ(β)ξ−β∑l=ϱ−3(ξ−ρ(l))(β−1)l(ϱ−2)+1Γ(ϱ)Γ(β)ξ−β∑l=ϱl−ϱ∑τ=0(ξ−ρ(l))(β−1)(l−ρ(τ))(ϱ−1)Φ(τ+ϱ−1). | (4.8) |
Based on condition (4.1), we have
λΔ−βχ(η+β)=λΓ(β){1Γ(ϱ)[A2−A1(ϱ−3)]+B3(ϱ−3)2(ϱ−1)}η∑l=ϱ−3(η+β−ρ(l))(β−1)l(ϱ−1)+λΓ(ϱ−1)Γ(β)[A1−B3(ϱ−3)Γ(ϱ−2)]η∑l=ϱ−3(η+β−ρ(l))(β−1)l(ϱ−2)+B3λΓ(β)η∑l=ϱ−3(η+β−ρ(l))(β−1)l(ϱ−3)+λΓ(ϱ)Γ(β)η∑l=ϱl−ϱ∑τ=0(η+β−ρ(l))(β−1)(l−ρ(τ))(ϱ−1)Φ(τ+ϱ−1)=1Γ(ϱ)T∑l=0(ϱ+T−ρ(l))(ϱ−1)Φ(l+ϱ−1)+1Γ(ϱ−1)[A1−B3(ϱ−3)Γ(ϱ−2)](ϱ+T)(ϱ−2)+{1Γ(ϱ)[A2−A1(ϱ−3)]+B3(ϱ−3)2(ϱ−1)}(ϱ+T)(ϱ−1)+B3(ϱ+T)(ϱ−3). |
Then,
B3=1hϱ[fΦ+dϱ],B2=1Γ(ϱ−1)[A1−1hϱ[fΦ+dϱ](ϱ−3)Γ(ϱ−2)],B1=1Γ(ϱ)[A2−A1(ϱ−3)]+1hϱ[fΦ+dϱ](ϱ−3)2(ϱ−1), |
where hϱ, dϱ and fΦ are defined by (4.4)-(4.6), respectively. Substituting the values of the constants B1, B2 and B3 into (4.7), we obtain (4.3) and our proof is complete.
From Lemma 4.1, the solution of the discrete RL problem (4.1) is given by the formula
χ(ξ)=1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1,χ(l+ϱ−1))+{[A2−A1(ϱ−3)]Γ(ϱ)+fχ+dϱhϱ(ϱ−3)2(ϱ−1)}ξ(ϱ−1)+[A1−1hϱ[fΦ+dϱ](ϱ−3)Γ(ϱ−2)]Γ(ϱ−1)ξ(ϱ−2)+fχ+dϱhϱξ(ϱ−3), | (4.9) |
where dρ and hρ are defined by (4.4) and (4.5), respectively, and
fχ=−λΓ(ϱ)Γ(β)η∑l=ϱl−ϱ∑τ=0(η+β−ρ(l))(β−1)(l−ρ(τ))(ϱ−1)Φ(τ+ϱ−1,χ(τ+ϱ−1))+1Γ(ϱ)T∑l=0(ϱ+T−ρ(l))(ϱ−1)Φ(l+ϱ−1,χ(l+ϱ−1)). | (4.10) |
Lemma 4.2. If χ is a solution of (4.3), then
χ(ξ)=1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1)+Q(ξ)+fχK(ξ), | (4.11) |
where
Q(ξ)={[A2−A1(ϱ−3)]Γ(ϱ)+dϱ(ϱ−3)2hϱ(ϱ−1)}ξ(ϱ−1)+[A1−dϱhϱ(ϱ−3)Γ(ϱ−2)]Γ(ϱ−1)ξ(ϱ−2)+dϱhϱξ(ϱ−3), |
and
K(ξ)=(ϱ−3)2hϱ(ϱ−1)ξ(ϱ−1)−(ϱ−3)hϱ(ϱ−2)ξ(ϱ−2)+ξ(ϱ−3)hϱ. |
Proof. Take χ as a solution of (4.2). For ξ∈[ϱ−3,ϱ+T]Nϱ−3, then
χ(ξ)=1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1)+{[A2−A1(ϱ−3)]Γ(ϱ)+fχ+dϱhϱ(ϱ−3)2(ϱ−1)}ξ(ϱ−1)+[A1−1hϱ[fχ+dϱ](ϱ−3)Γ(ϱ−2)]Γ(ϱ−1)ξ(ϱ−2)+fχ+dϱhϱξ(ϱ−3)=1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1)+{[A2−A1(ϱ−3)]Γ(ϱ)+dϱ(ϱ−3)2hϱ(ϱ−1)}ξ(ϱ−1)+[A1−dϱhϱ(ϱ−3)Γ(ϱ−2)]Γ(ϱ−1)ξ(ϱ−2)+dϱhϱξ(ϱ−3)+fχ[(ϱ−3)2hϱ(ϱ−1)ξ(ϱ−1)−(ϱ−3)hϱ(ϱ−2)ξ(ϱ−2)+ξ(ϱ−1)hϱ]=1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1)+Q(ξ)+fχK(ξ). |
The proof is complete.
Definition 4.1. We say that the discrete RL problem (4.1) is Hyers-Ulam stable if for each function ν∈C(Nϱ−3,ϱ+T,R) of
|RLΔϱξν(ξ)−Φ(ξ+ϱ−1,ν(ξ+ϱ−1))|≤ϵ,ξ∈[0,T]N0, | (4.12) |
and ϵ>0, there exists χ∈C(Nϱ−3,ϱ+T,R) solution of (4.1) and δ>0 such that
|ν(ξ)−χ(ξ)|≤δϵ,ξ∈[ϱ−3,ϱ+T]Nϱ−3. | (4.13) |
Definition 4.2. We say that the discrete RL problem (4.1) is Hyers-Ulam–Rassias stable if for each function ν∈C(Nϱ−3,ϱ+T,R) of
|RLΔϱξν(ξ)−Φ(ξ+ϱ−1,ν(ξ+ϱ−1))|≤ϵθ(ξ+ϱ−1),ξ∈[0,T]N0, | (4.14) |
and ϵ>0, there exists χ∈C(Nϱ−3,ϱ+T,R) solution of (4.1) and δ2>0 such that
|ν(ξ)−χ(ξ)|≤δ2ϵθ(ξ+ϱ−1),ξ∈[ϱ−3,ϱ+T]Nϱ−3. | (4.15) |
Remark 4.1. A function χ(ξ)∈C(Nϱ−3,ϱ+T,R) is a solution of (4.12) if, and only if, there exists μ:[ϱ−3,ϱ+T]Nϱ−3⟶R satisfying:
(H2) |μ(ξ+ϱ−1)|≤ϵ,ξ∈[0,T]N0;
(H3) RLΔϱξν(ξ)=Φ(ξ+ϱ−1,ν(ξ+ϱ−1))+μ(ξ+ϱ−1),ξ∈[0,T]N0.
Remark 4.2. A function χ(ξ)∈C(Nϱ−3,ϱ+T,R) is a solution of (4.14) if, and only if, there exists μ:[ϱ−3,ϱ+T]Nϱ−3⟶R satisfying
(H4) |μ(ξ+ϱ−1)|≤ϵθ(ξ+ϱ−1),ξ∈[0,T]N0,
(H5) RLΔϱξν(ξ)=Φ(ξ+ϱ−1,ν(ξ+ϱ−1))+μ(ξ+ϱ−1),ξ∈[0,T]N0.
Lemma 4.3. If ν satisfies (4.12), then
|ν(ξ)−1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1,ν(l+ϱ−1))−Q(ξ)−fνK(ξ)|≤ϵΓ(ϱ+T+1)Γ(ϱ+1)Γ(T+1). |
Proof. Using our hypothesis, and based on Remark 4.1, the solution to (H3) satisfies
ν(ξ)=1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1,ν(l+ϱ−1))+Q(ξ)+fνK(ξ)+1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)μ(l+ϱ−1). |
Hence,
|ν(ξ)−1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1,ν(l+ϱ−1))−Q(ξ)−fνK(ξ)|=|1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)μ(l+ϱ−1)|≤1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)|μ(l+ϱ−1)|≤ϵΓ(ϱ+T+1)Γ(ϱ+1)Γ(T+1), |
and the desired inequality is derived.
Theorem 4.1. If condition (H1) holds and (4.13) is satisfied, then the discrete RL problem (4.1) is Hyers-Ulam stable under the condition
K≤Γ(β)Γ(T+1)Γ(ϱ+1)2M1Γ(ϱ+T+1)Γ(β)+MM1λϱΓ(T+1), | (4.16) |
where M1=max(1,|K(ξ)|).
Proof. Let ξ∈[ϱ−3,ϱ+T]Nϱ−3. From Lemma 8, we have
|ν(ξ)−χ(ξ)|≤|ν(ξ)−1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1,χ(l+ϱ−1))−Q(ξ)−fχK(ξ)|≤|ν(ξ)−1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1,ν(l+ϱ−1))−Q(ξ)−fνK(ξ)|+1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)|Φ(l+ϱ−1,χ(l+ϱ−1))−Φ(l+ϱ−1,ν(l+ϱ−1))|+|K(ξ)||fν−fχ|. |
It follows that
|ν(ξ)−χ(ξ)|≤ϵΓ(ϱ+T+1)Γ(ϱ+1)Γ(T+1)+KΓ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)|χ(l+ϱ−1)−ν(l+ϱ−1)|+|K(ξ)|K{λΓ(ϱ)Γ(β)η∑l=ϱl−ϱ∑τ=0(η+β−ρ(l))(β−1)(l−ρ(τ))(ϱ−1)|ν(l+ϱ−1)−χ(l+ϱ−1)|+1Γ(ϱ)T∑l=0(ϱ+T−ρ(l))(ϱ−1)|χ(l+ϱ−1)−ν(l+ϱ−1)|}≤ϵΓ(ϱ+T+1)Γ(ϱ+1)Γ(T+1)+K‖χ−ν‖Γ(ϱ)Γ(ξ+1)ϱΓ(ξ+1−ϱ)+|K(ξ)|K‖χ−ν‖[MλΓ(ϱ)Γ(β)+1Γ(ϱ)Γ(ϱ+T+1)ϱΓ(T+1)]≤ϵΓ(ϱ+T+1)Γ(ϱ+1)Γ(T+1)+2KM1Γ(ϱ+1)Γ(ϱ+T+1)Γ(T+1)‖χ−ν‖+‖χ−ν‖KMM1λΓ(ϱ)Γ(β). |
Therefore,
‖ν−χ‖≤ϵΓ(ϱ+T+1)Γ(ϱ+1)Γ(T+1)+‖ν−χ‖[2KM1Γ(ϱ+1)Γ(ϱ+T+1)Γ(T+1)+KMM1λΓ(ϱ)Γ(β)]. |
Moreover, ‖ν−χ‖≤ϵδ, where
δ=Γ(β)Γ(ϱ+T+1)Γ(β)Γ(T+1)Γ(ϱ+1)−2KM1Γ(ϱ+T+1)Γ(β)−KMM1λϱΓ(T+1)>0. |
Thus, the discrete RL problem (4.1) is Hyers-Ulam stable.
Lemma 4.4. If v solves (4.14) under the condition:
(H6) The function θ:[ϱ−3,ϱ+T]Nϱ−3⟶R is increasing and there exists a constant γ>0 such that
1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)θ(l+ϱ−1)≤γθ(ξ+ϱ−1),ξ∈[0,T]N0, |
then
|ν(ξ)−1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1,ν(l+ϱ−1))−Q(ξ)−fϕνK(ξ)|≤γθ(ξ+ϱ−1). |
Proof. Let ν satisfy (4.14). From Remark 4.2, the solution to (H5) satisfies
ν(ξ)=1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1,ν(l+ϱ−1))+Q(ξ)+fνK(ξ)+1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)μ(l+ϱ−1). |
Hence,
|ν(ξ)−1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)Φ(l+ϱ−1,ν(l+ϱ−1))−Q(ξ)−fνK(ξ)|=|1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)μ(l+ϱ−1)|≤1Γ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)|μ(l+ϱ−1)|≤ϵΓ(ϱ)ξ−ϱ∑l=0(ξ−ρ(l))(ϱ−1)|θ(l+ϱ−1)|≤γϵθ(l+ϱ−1), |
and the desired inequality is derived.
Remark 4.3. About the restrictiveness of hypotheses (H1)–(H6), and the bounds imposed on the family of discrete systems that satisfy them, one should note that such hypotheses are the usual conditions for proving existence, uniqueness or stability of solutions. In fact, the conditions (H2)–(H6) are considered as the fundamental conditions in the definition of Hyers-Ulam-Rassias stability. Condition (H1) is a standard Lipschitz condition while other constants are computed based on the given fractional system. Therefore, these conditions are natural and, in real systems, with specified numerical data, their expressions are reduced to numerical bounds.
Theorem 4.2. If the inequality (4.16) and the hypotheses (H1) and (H6) are satisfied, then the discrete RL problem (4.1) is Hyers-Ulam-Rassias stable.
Proof. From Lemmas 4.4 and 2.3, we obtain that
‖ν−χ‖≤δ2ϵθ(ξ+ϱ−1), |
where
δ2=Γ(ϱ+1)Γ(β)Γ(T+1)Γ(ϱ+1)Γ(β)Γ(T+1)−2KM1Γ(β)Γ(ϱ+T+1)−KMM1λϱΓ(T+1)>0. |
Thus, the discrete RL problem (4.1) is Hyers-Ulam-Rassias stable.
In this section, we consider two examples to illustrate the obtained results.
Example 5.1. Let
{∗Δϱξχ(ξ)=Φ(ξ+ϱ−1,χ(ξ+ϱ−1)),ξ∈N0,4,Δχ(ϱ−3)=1,χ(ϱ+4)=0.3Δ−0.5χ(η+0.5),Δ2χ(ϱ−3)=0, | (5.1) |
where ∗Δϱξ denotes the operator cΔϱξ or RLΔϱξ. Set β=0.5, T=4, λ=0.7, A1=1, A2=0, and
Φ(ξ+1.5,χ(ξ+1.5))=137105sin(χ(ξ+1.5)). |
∙ If ∗Δϱξχ(ξ)=cΔ52ξχ(ξ) and η=52, then we obtain that
K1=λη∑l=ϱ−3(η+β−ρ(l))(β−1)−Γ(β)=λΓ(η+β−ϱ+4)βΓ(η−ϱ+4)−Γ(β)=0.9416, |
M=|η∑l=ϱl−ϱ∑ξ=0(η+β−ρ(l))(β−1)(l−ρ(ξ))(ϱ−1)|=2.3562. | (5.2) |
Hence, the inequality (3.8) takes the form
Γ(ϱ+T+1)Γ(ϱ+1)Γ(T+1)(1+Γ(β)K1)+MλK1Γ(ϱ)≈68.9403≤1K≈729.9270, |
such that
K=137105. |
From Theorem 3.1, the discrete problem (5.1) has a unique solution.
∙ In the case ∗Δϱξχ(ξ)=RLΔ3ξχ(ξ) and η=3, we obtain
M=3.5449,M1=1.8824,hϱ=λΓ(η+β−ϱ+4)Γ(β+1)Γ(η−ϱ+4)−1=0.5313,K1=λΓ(η+β−ϱ+4)βΓ(η−ϱ+4)−Γ(β)=0.9416. |
Also,
Γ(β)Γ(T+1)Γ(ϱ+1)2M1Γ(ϱ+T+1)Γ(β)+MM1λϱΓ(T+1)≈0.0075. | (5.3) |
If K=0.0014<0.0075 and
|RLΔ3ξν(ξ)−Φ(ξ+2,v(ξ+2))|≤ϵ,ξ∈[0,4]N0, | (5.4) |
holds, then, by Theorem 4.1, the discrete RL problem (5.1) is Hyers-Ulam stable.
Example 5.2. Let
{cΔ2.4ξχ(ξ)=1107χ4(ξ+1.4),ξ∈N0,2,Δχ(−0.4)=2,χ(3.4)=0.8Δ−1/3χ(1/3+2.4),Δ2χ(−0.6)=0. | (5.5) |
After some calculations, we find that
M=3.277,K1=λη∑l=ϱ−3(η+β−ρ(l))(β−1)−Γ(β)=1.0253,K2=A1Γ(β)K1(ϱ+T)−λA1K1(η−β(3−ϱ))Γ(η−ϱ+β+4)β(β+1)Γ(η−ϱ+4)=16.2963. |
We define the following Banach space:
C(Nϱ−3,ϱ+T,R)={χ(t)|[−0.5,6.5]N−0.5→R,‖χ‖≤2|K2|=32.5927}. |
Note that
|K2|−|[A1−A2(ϱ−3)]|(ϱ+T)(1)−|A22|(ϱ+T)(2)Γ(ϱ+T+1)Γ(ϱ+1)Γ(T+1)(1+Γ(β)|K1|)+M|λ|Γ(ϱ)|K1|=0.3262. |
It is clear that |Φ(t,χ)|≤0.0586≤0.3262 whenever χ∈[−32.5927,32.5927]. Therefore, by Theorem 3.2, we find out that the discrete FBVP (5.5) has a solution.
We proved existence and uniqueness of solution to discrete fractional boundary value problems (FBVPs) involving fractional difference operators via the Brouwer fixed point theorem and the Banach contraction principle. Different versions of stability criteria were obtained for a discrete FBVP involving Riemann-Liouville difference operators. The results were illustrated by suitable examples. The approach of this paper is new and can be a beginning method for discussing different real-world models in the context of discrete behavior structures. In particular, our results can contribute for the development of discrete fractional boundary value problems describing discrete dynamics of some physical applications. In future works, we plan to extend our approach to other types of discrete differential inclusions or fully-hybrid discrete fractional differential equations.
The third and fourth authors would like to thank Azarbaijan Shahid Madani University. The fifth author was supported by the Portuguese Foundation for Science and Technology (FCT) and CIDMA through project UIDB/04106/2020. This research received funding support from the NSRF via the Program Management Unit for Human Resources & Institutional Development, Research and Innovation (Grant number B05F650018).
The authors declare no conflict of interest.
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