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Małgorzata Wyrwas, Dorota Mozyrska, Ewa Girejko, "On Solutions to Fractional Discrete Systems with Sequential -Differences", Abstract and Applied Analysis, vol. 2013, Article ID 475350, 11 pages, 2013. https://doi.org/10.1155/2013/475350
On Solutions to Fractional Discrete Systems with Sequential -Differences
We study the subject of a behaviour of the solutions of systems with sequential fractional -differences. We give formulas for the unique solutions to initial value problems for systems in linear and semilinear cases. Moreover, the sufficient condition that guaranties the positivity of considered systems is presented.
The first definition of the fractional derivative was introduced by Liouville and Riemann at the end of the 19th century. Later on, in the late 1960s, this idea was used by engineers for modeling various processes. Thus the fractional calculus started to be exploited since that time. This calculus is a field of mathematics that grows out of the traditional definitions of integrals, derivatives, and difference operators and deals with fractional integrals, derivatives, and differences of any order. Many authors prove that fractional differential and difference equations are more adequate for modeling physical and chemical processes than integer-order equations. Fractional differential and difference equations describe many phenomena arising in engineering, physics, economics, and science. In fact, several applications can be found in viscoelasticity, electrochemistry, electromagnetic, and so forth. For example, Machado  gave a novel method for the design of fractional order digital controllers. Fractional difference calculus has been investigated by many authors, for example, [2–12] and others. In particular, different delta and nabla type fractional differences have been studied in [13–15], where the authors relate these differences by deriving some dual identities. The calculus of fractional -differences was given, for instance, in [5, 10, 16–19]. The properties of systems defined by the fractional difference equations where studied, for example, in [20–24]. There exist definitions of sequential operators in continuous case with different types of derivatives like Caputo type, Riemann-Liouville type, and Hadamard type, see, for example, [9, 12, 25–27]. In parallel with this paper we developed the theory of fractional -differences with sequential operators in the paper , where the approximation of continuous fractional sequential derivative is considered. We compute different formulas of solutions and then we try to check the system's behaviour, precisely the positivity of solutions. As far as we know the subject of positivity is well developed for fractional linear systems with continuous time; see [8, 29–32]. However, positivity of fractional discrete systems with sequential -differences is still a field to be examined. In the present paper we open our studies in this field. We give formulas for the unique solutions to initial problems for systems in linear and semilinear cases. Moreover, the positivity of systems with sequential -differences is considered. We consider systems with sequential -differences of Caputo type, while in  systems with Grünwald-Letnikov operator are studied. It is worth to add that in  (and references within) the considered systems are not of sequential type. Additionally, we have the exact formulas of the solutions of the systems both with the sequential differences and with the ordinary differences while in  the recurrence form of solution for discrete systems with Grünwald-Letnikov difference is given.
The paper is organized as follows. In Section 2 all preliminary definitions, facts, and notations are gathered. Section 3 presents systems with sequential fractional differences with results on uniqueness of solutions. We include semilinear systems in Section 4. The Section 5 concerns positivity of considered systems. Finally, the illustrative example is presented.
Let us denote by the set of all real valued functions defined on . Let , and put , where . Let Then the operator is defined by . The next two definitions of -difference operator were originally given in [16, 17].
Definition 1. For a function the forward -difference operator is defined as while the -difference sum is given by where , and .
Definition 2. For arbitrary the -factorial function is defined by where is the Euler gamma function, that is, for all such that , , and we use the convention that division at a pole yields zero.
Notice that if we use the general binomial coefficient , then (4) can be rewritten as In the sequel we need the following technical properties.
Proposition 3 (see ). Let . (1)For one has , where , are such that both sides are well defined; (2)For one has ;(3)For one has .
The next definition with another notation was stated in . Here we use more suitable summations.
Definition 4. For a function the fractional -sum of order is given by
where , . Moreover, we define .
It is important to notice that the operator changes the domains of functions.
Remark 6. In  one can find the following form of the fractional -sum of order : that can be useful in implementation.
Definition 7. Let . The Caputo -difference operator of order for a function is defined by
Remark 8. Note that , where .
We need the power rule formulas in the sequel. Firstly, we easily notice that for the well-defined -factorial functions have the following property: More properties of -factorial functions can be found in . In our consideration the crucial role plays the power rule formula presented in , that is, where , , . Note that using the general binomial coefficient one can write (11) as If , then we have for , and Let us define special functions that we use in the next section to write the formula for solutions.
Definition 9. For we define where belongs to the set of integers , and .
Remark 10. It is worthy to notice that for (a);(b) and the values are neglected for ;(c) and as the division by pole gives zero, the formula works also for , ;(d), .
We also need the following proposition.
Proposition 11. Let and . Then for ,
Proposition 12. Let be a real valued function defined on , where , . For the following equalities hold:
The next proposition gives a useful identity of transforming Caputo fractional difference equations into fractional summations for the case when an order is from the interval .
Proposition 13 (see ). Let , , and let be a real valued function defined on . The following formula holds:
The operators presented above can be extended to vectors in a componentwise manner.
3. Solutions of Systems with Sequential Fractional Differences
Let and . Moreover, let us take and . Then we define Note that . Then we apply the next difference operator of order on the new function and consider here an initial value problem stated by the system where , with initial values where are constant vectors from .
Solutions of the state equations (23) and (24) of the sequential fractional discrete-time system can be computed in the recursive way. From Definition 7 and by Proposition 3, (23) can be written as Since by Proposition 3 the following relation holds, one gets Repeating the same computation for (24) one gets Hence, Then using the Chu-Vandermonde identity, that is, , , , one gets Then by Proposition 3, we have Consequently, since , and for Using Proposition 3 we get for
Proof. Notice that for we get . Since and , we get .
For let us define the sequence in the following way: where and with .
We calculate the first step. As , then . Going further, which could be written as and, using Proposition 11, we get
Taking tending to we get formula (41) as the solution of (39) and (40) with initial conditions (25) and (26).
3.1. Semilinear Sequential Systems
Firstly we state a technical lemma and notations.
Lemma 15. Let and . Let and for . Then for one gets where .
Note that Similar to the procedure presented in the proof of Lemma 15 we can prove that for and , :
Moreover, we can also write direct formula for values given in (38) for nonlinear problem. In fact using Definition 4 of fractional summation, formula (38) of functions and Proposition 12 we write (51) as follows: Using the power rule formula for and by Remark 10 we can write the recursive formula for the solution to nonlinear problem given by (23) and (24) and conditions (25) and (26): The given formula (53) also works for as . Then . We can check the next steps: For special semilinear case when we have . Then
Proof. Note that , and for , so we get .
For based on the proof for linear case we can write the solution formula as follows: where . Then taking into account formulas (50) and (51) we get the form (58) as the solution of (56) and (57) with initial conditions (25) and (26).
Let be the set of real matrices with the nonnegative entries and .
Similarly as in  we will use the recursive formula (35) to show the positivity of the considered systems. Observe that the systems considered in this paper are of the sequential type while in  the sequential systems are not studied. Moreover, the Grünwald-Letnikov operator with the step equal to one is used in , whereas we study the systems with the -differences of Caputo type. So in our case the steps are equal to .
From  we have the following lemma.
Lemma 18. If , then , .
Moreover, for we have .
Using the properties of Euler gamma function one can show that if , then for , one has the following inequality:
Proposition 19. Let . If and for all , then for all .
Proof. The proof is by the induction principle. Assume that and both and . Since and , by (35) we have Moreover, where and . Let and denote the th coordinates of the vectors and , respectively. Then since for , Note that for we have , so . Then since and (61) holds for , we get For we have Since , for , and (61) holds for , we have for , .
Assume that for . Using the properties of gamma function one can show that for . Then applying to (35) we get for By inequality (60) and the assumptions, that is, , , and by (61) one gets . Hence using the induction principle we get that for all .
Remark 21. In  the sufficient condition concerning the positivity of the linear discrete systems with Grünwald-Letnikov operator is as follows: . In our case since we have systems with the sequential fractional -difference, in our condition and both orders and appear. Note that taking the sufficient condition (71) has the form .
Let us now consider some examples that illustrate the solution of the considered systems.
Example 22. Let , , and . Then let us take and and consider the linear system with sequential difference in the following form: with initial conditions (25) and (26), that is, and , . Moreover let and . Then the matrix has nonnegative values and the fact of staring from nonnegative initial conditions provides the remaining in the same positive cone. In Figure 1 we present the trajectory for steps on the -plane that start from and with the initial value . Let us notice that in the case one can see that in a few initial steps points are escaping from the positive area (see Figure 2). All calculations were done in the Maple program.