#### Abstract

The asymptotic behaviour of a real two-dimensional differential system with unbounded nonconstant delays satisfying is studied under the assumption of instability. Here, , and are supposed to be matrix functions and a vector function. The conditions for the instable properties of solutions and the conditions for the existence of bounded solutions are given. The methods are based on the transformation of the considered real system to one equation with complex-valued coefficients. Asymptotic properties are studied by means of a Lyapunov-Krasovskii functional and the suitable WaΕΌewski topological principle. The results generalize some previous ones, where the asymptotic properties for two-dimensional systems with one constant or nonconstant delay were studied.

#### 1. Introduction

Consider the real two-dimensional system where are real functions, , are real square matrices, and is a real vector function, , . It is supposed that the functions , are locally absolutely continuous on , are locally Lebesgue integrable on , and the function satisfies CarathΓ©odory conditions on .

There are a lot of papers dealing with the stability and asymptotic behaviour of -dimensional real vector equations with delay. Among others we should mention the recent results [1β13]. Since the plane has special topological properties different from those of -dimensional space, where or , it is interesting to study the asymptotic behaviour of two-dimensional systems by using tools that are typical and effective for two-dimensional systems. The convenient tool is the combination of the method of complexification and the method of Lyapunov-Krasovskii functional. For the case of instability, it is useful to add to this combination the version of WaΕΌewski topological principle formulated by Rybakowski in the papers [14, 15]. Using these techniques, we obtain new and easy applicable results on stability, asymptotic stability, instability, or boundedness of solutions of the system (1.1).

The main idea of the investigation, the combination of the method of complexification and the method of Lyapunov-Krasovskii functional, was introduced for ordinary differential equations in the paper by RΓ‘b and Kalas [16] in 1990. The principle was transferred to differential equations with delay by Kalas and BarΓ‘kovΓ‘ [17] in 2002. The results in the case of instability were obtained for ODEs by Kalas and OsiΔka [18] in 1994 and for delayed differential equations by Kalas [19] in 2005.

We extend such type of results to differential equations with a finite number of nonconstant delays. We introduce the transformation of the considered real system to one equation with complex-valued coefficients. We present sufficient conditions for the instability of a solution and for the existence of a bounded solution. The applicability of the results is demonstrated with several examples.

At the end of this introduction we append a brief overview of notation used in the paper and the transformation of the real system to one equation with complex-valued coefficients. is the set of all real numbers, the set of all positive real numbers, the set of all nonnegative real numbers, the set of all negative real numbers, the set of all nonpositive real numbers, the set of all complex numbers, the class of all continuous functions , the class of all locally absolutely continuous functions , the class of all locally Lebesgue integrable functions , the class of all functions satisfying CarathΓ©odory conditions on , the real part of , the imaginary part of , and the complex conjugate of .

Introducing complex variables , ,..., , we can rewrite the system (1.1) into an equivalent equation with complex-valued coefficients where for , , , , .

The relations between the functions are as follows: Conversely, putting the equation (1.2) can be written in the real form (1.1) as well.

#### 2. Preliminaries

We consider (1.2) in the case when and study the behavior of solutions of (1.2) under this assumption. This situation corresponds to the case when the equilibrium 0 of the autonomous homogeneous system where is supposed to be regular constant matrix, is a centre or a focus. See [16] for more details.

Regarding (2.1) and since the delay functions satisfy , there are numbers , , and such that

Denote

Notice that the above-defined function need not be positive.

Since and , the inequality is valid for . It can be easily verified that .

For the rest of this section we will denote

The instability and boundedness of solutions are studied subject to suitable subsets of the following assumptions. (i)The numbers , , and are such that (2.3) holds.(ii)There exist functions such that
for , , where is continuous on .(ii_{n})There exist numbersand functionssuch that
for.()is a function satisfying
where is defined for by (iii_{n}) is a function satisfying
where is defined for by (iv_{n}) is a real locally Lebesgue integrable function satisfying the inequalities , for almost all , where is defined by

Obviously, if , , , and are locally absolutely continuous on and , , the choice is admissible in (iii). Similarly, if , , , and are locally absolutely continuous on and , , the choice is admissible in (iii_{n}).

Denote
From assumption (i) it follows that
therefore the function is locally Lebesgue integrable on , assuming that (i) holds true. If the relations , , and for almost all together with conditions (i) and (ii_{n}) are fulfilled, then we can choose for in (iv_{n}).

#### 3. Results

Theorem 3.1. *Let assumptions (i), (ii _{0}), (iii_{0}), and (iv_{0}) be fulfilled for some . Suppose there exist and such that
*

*If is any solution of (1.2) satisfying*

*where*

*then*

*for all , for which is defined.*

In the proof we use the following Lemma.

Lemma 3.2. *Let , , , , . Then,
**
for , .*

The proof is analogous to that of Lemma 1 in [20, page 101] or to the proof of Lemma in [16, page 131].

*Proof of Theorem 3.1. *Let be any solution of (1.2) satisfying (3.2). Consider the Lyapunov functional
where
For brevity we shall denote and we shall write the function of variable simply without indicating the variable , for example, instead of .

In view of (3.6), we have
for almost all for which is defined and exists. Put exists,. Clearly for . The derivative exists for almost all .

Since is a solution of (1.2), we obtain
for almost all . Taking into account
we get

By the use of Lemma 3.2, we get

The last inequality together with (2.12), taken for , assumption (ii_{0}), and the relation
yields
for almost all .

Consequently,
for almost all . Inequality (3.15) together with relation (3.8) gives

Using (2.11) and (2.13) for , we obtain

Hence, in view of (iv_{0}),
for almost all .

Multiplying (3.18) by and integrating over , we get
on any interval , where the solution exists and satisfies the inequality . Now, with respect to (3.6), (3.7), and , we have
If (3.2) is fulfilled, there is such that . By virtue of (3.1), and (3.2), we can easily see that
for all , for which is defined.

To obtain results on the existence of bounded solutions, we shall suppose that (1.2) satisfies the uniqueness property of solutions. Moreover, we suppose that the delays are bounded, that is, that the functions satisfy the condition where is a constant. Our assumptions imply the existence of numbers , , and such that

In view of this, we replace (2.3) in assumption (i) with (). All other assumptions we keep in validity.

In the proof of the following theorem we shall utilize WaΕΌewski topological principle for retarded functional differential equations of CarathΓ©odory type. Details of this theory can be found in the paper of Rybakowski [15].

Theorem 3.3. *Let conditions (i), (ii), and (iii) be fulfilled, and let , be continuous functions such that the inequality holds a. e. on , where is defined by (2.14). Suppose that is a continuous function such that
**
for and some constant . Then, there exist and a solution of (1.2) satisfying
**
for .*

*Proof. *Write (1.2) in the form
where is defined by
and is the element of defined by the relation , . Let . Put

It can be easily verified that is a polyfacial set generated by the functions , (see Rybakowski [15, page 134]). It holds that . As , we have
for . It holds that
Let and be such that and for all . If , then
Therefore,
provided that the derivatives , exist and that . Thus,
Using (3.10), (3.13), and (ii), similarly to the proof of Theorem 3.1, we obtain
and consequently, with respect to (iii),
for almost all and for sufficiently close to . Replacing and by and , respectively, in the last expression, we get
Therefore, in view of the continuity, holds for sufficiently close to and almost all sufficiently close to . Hence, is a regular polyfacial set with respect to ().

Choose , where is fixed. It can be easily verified that is a retract of , but is not a retract of . Let be such that and for . Define the mapping for by the relation
The mapping is continuous, and it holds that
Since
we have
for and . Clearly, inequality (3.39) holds also for and .

Using a topological principle for retarded functional differential equations (see Rybakowski [15, Theorem 2.1]), we see that there is a solution of (1.2) such that for all for which the solution exists. Obviously, exists for all and
Hence

Theorem 3.4. *Suppose that hypotheses (i), (ii), (ii _{n}), (iii), (iii_{n}), and (iv_{n}) are fulfilled for and , where , . Let , be continuous functions satisfying the inequality a. e. on , where is defined by (2.14). Assume that is a continuous function such that
*

*for and some constant . Suppose that*

*for , where and . Then, there exists a solution of (1.2) such that*

*Proof. *By the use of Theorem 3.3 we observe that there is a and a solution of (1.2) with property
for . Suppose that (3.46) is not satisfied. Then, there is such that
Choose such that
It holds that
for some . In view of (3.44), we can suppose that