Research

# Stability of abstract dynamic equations on time scales

Alaa E Hamza1* and Karima M Oraby2

Author Affiliations

1 Department of Mathematics, Faculty of Science, Cairo University, Cairo, Egypt

2 Department of Mathematics, Faculty of Science, Suez Canal University, Suez, Egypt

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Advances in Difference Equations 2012, 2012:143  doi:10.1186/1687-1847-2012-143

 Received: 28 February 2012 Accepted: 25 July 2012 Published: 12 August 2012

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

In this paper, we investigate many types of stability, like uniform stability, asymptotic stability, uniform asymptotic stability, global stability, global asymptotic stability, exponential stability, uniform exponential stability, of the homogeneous first-order linear dynamic equations of the form

where A is the generator of a -semigroup , the space of all bounded linear operators from a Banach space X into itself. Here, is a time scale which is an additive semigroup with the property that for any such that . Finally, we give an illustrative example for a nonregressive homogeneous first-order linear dynamic equation and we investigate its stability.

### 1 Introduction and preliminaries

The history of asymptotic stability of dynamic equations on a time scale goes back to Aulbach and Hilger [3]. For a real scalar dynamic equation, stability and instability results were obtained by Gard and Hoffacker [12]. Pötzche [20] provides sufficient conditions for the uniform exponential stability in Banach spaces, as well as spectral stability conditions for time-varying systems on time scales. Doan, Kalauch, and Siegmund [10] established a necessary and sufficient condition for the existence of uniform exponential stability and characterized the uniform exponential stability of a system by the spectrum of its matrix. Properties of exponential stability of a time varying dynamic equation on a time scale have been also investigated recently by Bohner and Martynyuk [7], DaCunha [9], Du and Tien [11], Hoffacker and Tisdell [16], Martynyuk [17], and Peterson and Raffoul [19].

The theory of dynamic equations on time scales was introduced by Stefan Hilger in 1988 [14], in order to unify continuous and discrete calculus [4,15]. A time scale is a nonempty closed subset of . The forward jump operator is defined by (supplemented by ) and the backward jump operator is defined by (supplemented by ). The graininess function is given by . A point is said to be right-dense if , right-scattered if , left-dense if , left-scattered if , isolated if , and dense if . A time scale is said to be discrete if t is left-scattered and right-scattered for all , and it is called continuous if t is right-dense and left-dense at the same time for all . Suppose that has the topology inherited from the standard topology on . We define the time scale interval . Open intervals and open neighborhoods are defined similarly. A set we need to consider is which is defined as if has a left-scattered maximum M, and otherwise. A function is called right dense continuous, or just rd-continuous, if

(i) f is continuous at every right-dense point ;

(ii) exists (finite) for every left-dense point .

The set of rd-continuous functions will be denoted by .

A function is called delta differentiable at provided there exists an α such that for every there is a neighborhood U of t with

In this case, we denote the α by ; and if f is differentiable for every , then f is said to be differentiable on . If f is differentiable at , then it is easy to see that

A function is called an antiderivative of if , . The Cauchy integral is defined by

where F is an antiderivative of f. Every rd-continuous function has an antiderivative and is an antiderivative of f, i.e., , . Equations which include Δ-derivatives are called dynamic equations. We refer the reader to the very interesting monographs of Bohner and Peterson [5,6].

Definition 1.1 A mapping is called regressive if is invertible for every , and we say that

is regressive if A is regressive. We say that a real valued function on is regressive (resp. positively regressive) if (resp. ), . The family of all regressive functions (resp. positively regressive functions) is denoted by (resp. ).

It is well known that if , the space of all right dense continuous and regressive bounded functions from to , then the initial value problem (IVP)

(1.1)

has the unique solution

Here, is the exponential operator function. For more details, see [2]. When and is a real valued function, Eq. (1.1) yields

(1.2)

whose solution has the closed form

(1.3)

where

(1.4)

and , is the principal logarithm function. It is evident that when , , then

(1.5)

(1.6)

It can be seen that for with , the following claim is true

(1.7)

Indeed, by taking in Eqs. (1.5) and (1.6), we have

(1.8)

(1.9)

This implies that the claim is true.

In the sequel, we denote by for a time scale which is an additive semigroup with the property that for any such that . In this case, is called a semigroup time scale. We assume X is a Banach space. Finally, we assume that is a -semigroup on , that is, it satisfies

(i) for every (the semigroup property).

(ii) (I is the identity operator on X).

(iii) (i.e., is continuous at 0) for each .

If in addition , then T is called a uniformly continuous semigroup. A linear operator A is called the generator [1] of a -semigroup T if

(1.10)

where the domain of A is the set of all for which the above limit exists uniformly in t. Clearly, when , the concept of the generator defined by relation (1.10) coincides with the classical definition by Hille. See [13].

In Section 2 of this paper we present some results from [1] that we need in our study. One of them is that an abstract Cauchy problem

(1.11)

has the unique solution

when A is the generator of the -semigroup T. When , we get the classical existence and uniqueness theorem of the abstract Cauchy problem (1.11); see [21]. The other results include some properties of T and its generator A, which we use in the subsequent sections. The solution is a function of the variables t, τ and the initial value . Generally, we consider τ and as parameters. Therefore, when we investigate the asymptotic behavior of with respect to , we must investigate whether or not the asymptotic behavior uniformly depends on τ or . Accordingly, there are many types of stability which we give in Section 3.

S. K. Choi, D. M. Im, and N. Koo in [8], Theorem 3.5] proved that the stability of the time variant abstract Cauchy problem

(1.12)

where , and is the family of all real matrices is equivalent to the boundedness of all its solutions. DaCunha in [9] defined the concepts of uniform stability and uniform exponential stability. These two concepts involve the boundedness of the solutions of the regressive time varying linear dynamic Eq. (1.12). He established a characterization of uniform stability and uniform exponential stability in terms of the transition matrix for system (1.12). Also, he illustrated the relationship between the uniform asymptotic stability and the uniform exponential stability.

In Section 4, we extend these results for the case where A is the generator of T and we prove that the concepts of stability and uniform stability are same.

Sections 5 and 6 are devoted to establishing characterizations for many other types of stability, like asymptotic stability, uniform asymptotic stability, global asymptotic stability, exponential stability, and uniform exponential stability for the abstract Cauchy problem (1.11).

We end this paper with a new illustrative example including non-regressive dynamic equation and we investigate its stability.

### 2 The existence and uniqueness of solutions of dynamic equations

Our aim in this section is to prove that the first order initial value problem

(2.1)

has the unique solution

when A is the generator of a -semigroup .

At first, we establish some properties of T and its generator A which we use to arrive at our aim.

Theorem 2.1For, the following statements are true:

1. For,

(2.2)

(2.3)

and

(2.4)

2. For,

and

(2.5)

Proof 1. Set . Then

Also, we have

and

where .

2. Let be a number in . We have

□

Theorem 2.2For, the following statements are true:

1. For, and

(2.6)

2. For, we have

(2.7)

(2.8)

and

(2.9)

Proof 1. Let . It is evident that , .

Now, we show that solves the initial value problem

We have either or . The case implies

On the other hand,

When , we obtain

2. Relations (2.7) and (2.8) can be obtained by integrating both sides of Eq. (2.6) from s to t. Relation (2.9) follows from Eqs. (2.5) and (2.7). □

Corollary 2.3IfAis the generator of a-semigroupTon, thenis dense in X andAis a closed linear operator.

Proof For every and fixed , set

Theorem 2.1 implies that

By the same theorem, as . So , the closure of , is equal to X. The linearity of A is evident.

To prove its closeness, let , and as . In view of equality (2.7), we obtain

(2.10)

The integrand on the right-hand side of (2.10) converges to uniformly on bounded intervals. Consequently, letting in (2.10), we get

(2.11)

Dividing Eq. (2.11) by , and letting , we see, using identity (2.4), that and . □

Theorem 2.4Equation (2.1) has the unique solution

Proof The existence of the solution follows by Theorem 2.2. To prove the uniqueness, assume that is another solution. Consider the function

where . We have

On the other hand, we have

from which we obtain that on . Then , i.e.. □

### 3 Types of stability

In this section, the definitions of the various types of stability for dynamic equations of the form

(3.1)

are presented, where and is the delta derivative of with respect to . See [8,18].

Definition 3.1 Equation (3.1) is said to be stable if, for every and for every , there exists a such that, for any two solutions and of Eq. (3.1), the inequality implies , for all , .

Definition 3.2 Equation (3.1) is said to be uniformly stable if, for each , there exists a independent on any initial point such that, for any two solutions and of Eq. (3.1), the inequality implies , for all , .

Definition 3.3 Equation (3.1) is said to be asymptotically stable if it is stable and for every , there exists a such that, the inequality implies .

Definition 3.4 Equation (3.1) is said to be uniformly asymptotically stable if it is uniformly stable and there exists a such that for every the inequality implies , .

Definition 3.5 Equation (3.1) is said to be globally asymptotically stable if it is stable and for any solution of Eq. (3.1), we have .

Definition 3.6 Equation (3.1) is said to be exponentially stable if there exists with such that for every , there is such that, for any two solutions and of Eq. (3.1), we have , for all , .

Definition 3.7 Equation (3.1) is said to be uniformly exponentially stable if there exists with and there is independent on any initial point such that, for any two solutions and of Eq. (3.1), we have , for all , .

### 4 Characterization of stability and uniformly stability

In this section, we obtain some results concerning characterizations of stability and uniform stability of linear dynamic equations of the form

where A is the generator of T. The initial value problem has the unique solution

(4.1)

In the following two lemmas, by linearity of , we get an equivalent definition of stability and uniform stability of .

Lemma 4.1The following statements are equivalent:

(i) is stable;

(ii) For everyand for every, there existssuch that for any solutionof, we have

Lemma 4.2The following statements are equivalent:

(i) is uniformly stable;

(ii) For everythere existssuch that for any solutionof, we have

S. K. Choi, D. M. Im, and N. Koo in [8], Theorem 3.5] proved that the stability of (1.12) is equivalent to the boundedness of all its solutions when , where is the family of all real matrices. Also, DaCunha in [9] proved that the uniform stability of (1.12) is equivalent to the uniform boundedness of all its solutions with respect to the initial point , when .

In the following theorem, we extend these results for the case where A is the generator of a -semigroup T and we prove that the concepts of stability and uniform stability are the same.

Theorem 4.3The following statements are equivalent:

(i) is stable.

(ii) is bounded.

(iii) is uniformly stable.

Proof (i) ⟹ (ii) Assume is stable. Let . Fix . There exists such that for any solution , where , we have

Let . Take . Since , then

i.e.

The density of in X, by Corollary 2.3, implies that

Thus, for every , is bounded. By the uniform boundedness theorem [22], is bounded.

(ii) ⟹ (iii) Assume that there is such that , . Clearly, condition (ii) of Lemma 4.2 holds, because for , choose . □

### 5 A characterization of global asymptotic stability

In the following result, we establish necessary and sufficient conditions for to be globally asymptotically stable.

Theorem 5.1The following statement are equivalent:

(i) is asymptotically stable;

(ii) , for every;

(iii) is globally asymptotically stable;

(iv) is uniformly asymptotically stable.

Proof (i) ⟹ (ii) Suppose that is asymptotically stable. Let . There exists such that any solution of with initial value , vanishes at ∞ whenever . Fix . Then

Hence,

Consequently, we obtain

By the boundedness of and the density of in X, we deduce that

(ii) ⟹ (iii) Condition (ii) implies that is bounded for every . The uniform boundedness theorem insures the boundedness of . Consequently, is stable, and by our assumption, is globally asymptotically stable.

(iii) ⟹ (iv) Condition (iii) implies that is bounded for every . Again the uniform boundedness theorem guarantees the boundedness of . Consequently, is uniformly stable by Theorem 4.3, and by our assumption, is uniformly asymptotically stable. □

### 6 A characterization of exponential stability and uniform exponential stability

We need the following lemmas to establish a characterization of the exponential stability of . Their proofs are straightforward and will be omitted.

Lemma 6.1is exponentially stable if and only if there existswithsuch that for any, there existssuch that for any solutionofwith initial valuewe have

Lemma 6.2is uniformly exponentially stable if and only if there existswithand there existssuch that for any, and any solutionofwith initial valuewe have

In the following two theorems, we extend the results of DaCunha [9], Theorem 2.2] when to the case where A is the generator of T.

Theorem 6.3The following statements are equivalent:

(i) is exponentially stable;

(ii) There existswithsuch that for any, there existssuch that

Proof (i) ⟹ (ii) Let be exponentially stable. Then there is with such that for any , there exists such that for any solution of with initial value , we have

Fix , and let . Then

Using is dense in X and Corollary 2.3, we obtain

This implies that

(ii) ⟹ (i) Assume there exists with such that for every , there exists such that

Let be any solution of with initial value . Then

□

By same way as in the proof of Theorem 6.3, we can obtain the following result.

Theorem 6.4The following statements are equivalent:

(i) is uniformly exponentially stable;

(ii) There existswithand there existssuch that for any,

From Theorem 5.1 (Theorem 6.4), Lemma 6.1 (Lemma 6.2), and relation (1.7), we get the following result.

Corollary 6.5Ifis (uniformly) exponentially stable, thenis (uniformly) asymptotically stable.

### 7 Example

Choi in [8] gave an example to illustrate many types of stability. He considered the linear dynamic system

(7.1)

where is a time scale and and investigated some types of stability of Eq. (7.1) when A is regressive, i.e., for all . In this case the equation has the unique solution , where is the matrix exponential function. It is given by

(7.2)

We see that the generalized exponential function is given by

(7.3)

and

(7.4)

The following stability results [8] for (7.1) were obtained in different cases of .

(1) If , then (7.1) is uniformly stable, exponentially stable and asymptotically stable, since as .

(2) If , then (7.1) is uniformly stable but not asymptotically stable, since .

(3) If with and , then (7.1) is not asymptotically stable. However, goes to zero as .

(4) If with , then (7.1) is not asymptotically stable.

Now we consider the time scale with the graininess function , . So A is nonregressive and the matrix exponential function does not exist. On the other hand, A is the generator of the -semigroup

Indeed, for , we have

Then

Consequently, , which implies that Eq. (7.1) is uniformly stable but is not asymptotically stable.

### Competing interests

The authors declare that they have no competing interests.

### Authors’ contributions

All authors contributed equally and significantly in writing this article. All authors read and approved the final manuscript.

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