Abstract and Applied Analysis

Volume 2012 (2012), Article ID 210156, 10 pages

http://dx.doi.org/10.1155/2012/210156

## Finite-Time Attractivity for Diagonally Dominant Systems with Off-Diagonal Delays

^{1}Department of Mathematics, Imperial College London, London SW7 2AZ, UK^{2}Institute of Mathematics, Vietnam Academy of Science and Technology, Hanoi 10307, Vietnam^{3}Center for Dynamics, TU Dresden, 01062 Dresden, Germany

Received 23 April 2012; Accepted 6 June 2012

Academic Editor: Agacik Zafer

Copyright © 2012 T. S. Doan and S. Siegmund. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We introduce a notion of attractivity for delay equations which are defined on bounded time intervals. Our main result shows that linear delay equations are finite-time attractive, provided that the delay is only in the coupling terms between different components, and the system is diagonally dominant. We apply this result to a nonlinear Lotka-Volterra system and show that the delay is harmless and does not destroy finite-time attractivity.

#### 1. Introduction

Finite-time dynamical systems generated by nonautonomous differential equations which are defined only on a compact interval of time have recently become an active field of research, see, for example, [1–3] and the references therein.

A key ingredient of a qualitative theory is the notion of hyperbolicity of solutions. Finite-time versions of hyperbolicity are introduced and discussed, for example, in [4–8]. Finite-time attractivity is a special case of finite-time hyperbolicity, in case the unstable direction is missing. So far finite-time attractivity has been discussed only for ordinary differential equations, see [9, 10]. For a closely related notion, namely, finite-time stability, we refer to [11, 12] and the references therein for an overview. In this paper we go one step further to extend and investigate finite-time attractivity for delay equations.

For a nonnegative number , let denote the space of all continuous functions . For , the norm on is defined as follows: where for all . Consider a finite-time delay differential equation where is assumed to be continuous and Lipschitz in the second argument. For each , let denote the solution of (2.1) satisfying the initial condition for all . The evolution operator generated by (1.2) is defined as Motivated by recent results on finite-time hyperbolicity (see, e.g., [4, 6, 7]), we introduce in the following an analog notion of finite-time attractivity for delay equations.

*Definition 1.1 (finite-time attractivity). *The solution is called *finite-time attractive* on with respect to the norm if there exist positive constants and such that for all with the following estimate holds:
for all in the neighborhood of .

*Remark 1.2. *In the case that is a linear function in the second argument, it is easy to see that the generated semigroup is also linear in the second argument. In particular, for linear systems the following statements are equivalent: (i)there exists a finite-time attractive solution for a ,(ii)for all , the solution is finite-time attractive, and(iii)there exists such that for all we have

In this paper, we prove the finite-time attractivity for linear off-diagonal delay systems in Section 2. Section 3 is devoted to show the finite-time attractivity of the equilibrium for a Lotka-Voltera system.

#### 2. Finite-Time Attractivity for Linear Off-Diagonal Delay Systems

In this section, we consider the following finite-time nonautonomous linear differential equation with *off-diagonal delays* (see, e.g., [13] and the reference therein):
where is a given positive constant, , , are continuous functions and for with . Define
Note that (2.1) is a special case of (1.2). More precisely, the right hand side of (2.1) equals , where is defined as follows:
Let denote the evolution operator of (2.1). From (2.3), we see that the function is linear in the second argument. Therefore, the evolution operator is also linear in the second argument. Our aim in this section is to provide a sufficient condition for the finite-time attractivity for the zero solution of (2.1) and thus for all solutions of (2.1), see Remark 1.2.

Before presenting the main result, we recall the notion of row diagonal dominance. We refer the reader to [14, Definition 7.10] for a discussion of this notion. System (2.1) is called *row diagonally dominant* if there exists a positive constant such that

Theorem 2.1 (finite-time attractivity for delay equations). *Consider system (2.1) on a finite-time interval . Suppose that system (2.1) is row diagonally dominant with a positive constant and for all and . Define
**
and let be a positive number satisfying that
**Then for every , the zero solution of (2.1) is finite-time attractive with respect to the norm with exponent , that is,
*

* Proof. * We divide the proof into two steps.*Step *1. We show that for the inequality
holds. Suppose the opposite, that is, assume that there exists such that the set
is not empty. Define . By continuity of the map , we get that
Now, we will show that
Indeed, we consider the following two cases: (i) and (ii) .*Case (i).* If , then, according to (2.10) and (2.11), we obtain that
which proves (2.12) in this case.*Case (ii).* If , then, according to (2.10) and the definition of the norm , we obtain that
Hence, (2.12) is proved. To conclude the proof of this step, we estimate the norm for all in a neighborhood of in order to show a contradiction to the assumption that the set is not empty. To this end, we define the following set:
The continuity of the functions for implies that there exists a neighborhood for some such that
By virtue of (2.1), the derivative of the function is estimated as follows:
which together with (2.12) and the definition of implies that for all
Thus, from the row diagonal dominance (2.4) and bound (2.5), we derive that
Using (2.6), we obtain that
which yields that there exists a neighborhood for an such that for
Thus, for all with , we have
which together with (2.10) implies that
Consequently, , and this is a contradiction to the definition of and . Thus, (2.8) is proved.*Step *2. Using (2.8) from Step 1, we show (2.7) by considering two cases: (i) and (ii) .*Case (i).* If , then by virtue of (2.8) we have
which proves (2.7) in this case. *Case (ii).* If , then we have

Hence, using (2.8), we obtain that

Thus, (2.7) is proved, and the proof is complete.

#### 3. Finite-Time Attractivity of Lotka-Voltera Systems

Consider a Lotka-Voltera system of the following form: where and for all . Suppose that is row diagonal dominant with for all ; that is, there exists such that We assume in the following that there exists a positive vector such that To shorten the notation, the function for all is also denoted by . The function is a fixed point of the evolution operator generated by (3.1), that is, for all . For system (3.1), the result in [13, Theorem 1] showed that the equilibrium is exponentially attractive on the positive real line ; that is, there exist positive constants and such that However, the constant is usually greater than . Using the result developed in the preceding section, we show in the next theorem that the constant in (3.4) can be chosen to be equal to on the state space with norm for some . As a consequence, the equilibrium solution of system (3.1) is finite-time attractive with respect to these norms.

Theorem 3.1 (finite-time attractive equilibrium of Lotka-Voltera equations). *Consider (3.1) on an arbitrary finite-time interval satisfying (3.2) and (3.3). Then, there exists a positive weight factor such that for all the positive equilibrium is finite-time attractive on with respect to the norm . *

*Proof . *The proof is divided into three steps. *Step *1. Construction of the weight factor . Due to compactness of and continuity of solutions of (3.1), there exists such that
Then, we have
which implies that for all and we have
Define
Now let be the solution of the following equation:
*Step *2. In this step, we show that is the solution of the delay equation
with the initial condition for , where
Indeed, we have
which together with the fact that implies that is a solution of (3.10). Furthermore, we have
where denotes the evolution operator generated by (3.10). *Step *3. In this step, we show that for all and we have
Choose and fix , and we show that (3.10) fulfills all assumptions of Theorem 2.1. Indeed, using (3.7) and the definition of , we obtain the following upper bound:
Combining (3.2) and (3.7), we also get that
Therefore, system (3.10) fulfills all assumptions of Theorem 2.1. Then, the zero solution of system (3.10) is finite-time attractive with respect to the norm for all , that is,
In particular, substituting we get that
which together with (3.13) implies (3.14) and the proof is complete.

In the rest of the paper, we discuss a planar Lotka-Voltera system, for which we can explicitly compute its equilibrium. Consequently, applying Theorem 3.1 yields a sufficient condition for finite-time attractivity of this equilibrium.

*Example 3.2. *Consider a planar Lotka-Voltera of the following form:
where and the coefficients for satisfy the following inequalities for some :
Additionally, we assume that the equilibrium point is positive, where
According to Theorem 3.1, for any finite-time interval , there exists a positive weight factor such that for all the positive equilibrium is finite-time attractive on with respect to the norm .

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