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# Oscillation of Solutions of a Linear Second-Order Discrete-Delayed Equation

*Advances in Difference Equations*
**volume 2010**, Article number: 693867 (2010)

## Abstract

A linear second-order discrete-delayed equation with a positive coefficient is considered for . This equation is known to have a positive solution if fulfils an inequality. The goal of the paper is to show that, in the case of the opposite inequality for , all solutions of the equation considered are oscillating for .

## 1. Introduction

The existence of a positive solution of difference equations is often encountered when analysing mathematical models describing various processes. This is a motivation for an intensive study of the conditions for the existence of positive solutions of discrete or continuous equations. Such analysis is related to an investigation of the case of all solutions being oscillating (for relevant investigation in both directions, we refer, e.g., to [1–15] and to the references therein). In this paper, sharp conditions are derived for all the solutions being oscillating for a class of linear second-order delayed-discrete equations.

We consider the delayed second-order linear discrete equation

where , is fixed, , and . A solution of (1.1) is positive (negative) on if () for every . A solution of (1.1) is oscillating on if it is not positive or negative on for arbitrary .

Definition 1.1.

Let us define the expression , , by , where and , , and instead of , , we will only write and .

In [2] a delayed linear difference equation of higher order is considered and the following result related to (1.1) on the existence of a positive solution is proved.

Theorem 1.2.

Let be sufficiently large and . If the function satisfies

for every , then there exist a positive integer and a solution , of (1.1) such that holds for every .

Our goal is to answer the open question whether all solutions of (1.1) are oscillating if inequality (1.2) is replaced by the opposite inequality

assuming and is sufficiently large. Below we prove that if (1.3) holds and , then all solutions of (1.1) are oscillatory. The proof of our main result will use a consequence of one of Domshlak's results [8, Corollary , page 69].

Lemma 1.3.

Let and be fixed natural numbers such that . Let be a given sequence of positive numbers and a positive number such that there exists a number satisfying

Then, if and for

holds, then any solution of the equation

has at least one change of sign on .

Moreover, we will use an auxiliary result giving the asymptotic decomposition of the iterative logarithm [7]. The symbols "" and "" used below stand for the Landau order symbols.

Lemma 1.4.

For fixed and fixed integer , the asymptotic representation

holds for .

## 2. Main Result

In this part, we give sufficient conditions for all solutions of (1.1) to be oscillatory as .

Theorem 2.1.

Let be sufficiently large, , and . Assuming that the function satisfies inequality (1.3) for every , all solutions of (1.1) are oscillating as .

Proof.

We set

and consider the asymptotic decomposition of when is sufficiently large. Applying Lemma 1.4 (for , , and ), we get

Finally, we obtain

Similarly, applying Lemma 1.4 (for , , and ), we get

Using the previous decompositions, we have

Recalling the asymptotical decomposition of when : , we get (since )

as . Due to (2.3) and (2.4) we have and as . Then it is easy to see that, for the right-hand side of the inequality (1.5), we have

where

Moreover, for , we will get an asymptotical decomposition as . We represent in the form

As the asymptotical decompositions for

have been derived above (see (2.3)–(2.5)), after some computation, we obtain

Thus we have

Finalizing our decompositions, we see that

It is easy to see that inequality (1.5) becomes

and will be valid if (see (1.3))

or

for . If where is sufficiently large, then (2.16) holds for sufficiently small with fixed because . Consequently, (2.14) is satisfied and the assumption (1.5) of Lemma 1.3 holds for . Let in Lemma 1.3 be fixed and let be so large that inequalities (1.4) hold. This is always possible since the series is divergent. Then Lemma 1.3 holds and any solution of (1.1) has at least one change of sign on . Obviously, inequalities (1.4) can be satisfied for another couple of , say with and sufficiently large, and by Lemma 1.3 any solution of (1.1) has at least one change of sign on . Continuing this process, we get a sequence of intervals with such that any solution of (1.1) has at least one change of sign on . This fact concludes the proof.

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## Acknowledgments

The first author was supported by Grants 201/07/0145 and 201/10/1032 of the Czech Grant Agency (Prague) and by the Council of Czech Government MSM 0021630529. The second author was supported by Grants 201/07/0145 and 201/10/1032 of the Czech Grant Agency (Prague) and by the Council of Czech Government MSM 00216 30519. The third author was supported by the Council of Czech Government MSM 00216 30503 and MSM 00216 30529.

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Baštinec, J., Diblík, J. & Šmarda, Z. Oscillation of Solutions of a Linear Second-Order Discrete-Delayed Equation.
*Adv Differ Equ* **2010, **693867 (2010). https://doi.org/10.1155/2010/693867

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### Keywords

- Ordinary Differential Equation
- Functional Equation
- Natural Number
- Difference Equation
- Continuous Equation