Abstract and Applied Analysis

Volume 2014, Article ID 392097, 9 pages

http://dx.doi.org/10.1155/2014/392097

## Oscillations of Difference Equations with Several Oscillating Coefficients

^{1}Department of Mathematics, Ben-Gurion University of Negev, 84105 Beer-Sheva, Israel^{2}Department of Electrical and Electronic Engineering Educators, School of Pedagogical and Technological Education (ASPETE), N. Heraklion, 14121 Athens, Greece^{3}Department of Mathematics and Computer Sciences, Ariel University of Samaria, 40700 Ariel, Israel^{4}Department of Mathematics, University of Ioannina, 451 10 Ioannina, Greece

Received 18 April 2014; Accepted 10 May 2014; Published 1 June 2014

Academic Editor: Tongxing Li

Copyright © 2014 L. Berezansky et al. 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 study the oscillatory behavior of the solutions of the difference equation where , are real sequences with oscillating terms, , are general retarded (advanced) arguments, and denotes the forward (backward) difference operator . Examples illustrating the results are also given.

#### 1. Introduction

In the present paper, we study the oscillatory behavior of the solutions of the difference equation where , , are real sequences with oscillating terms, and , are sequences of integers such that and the (dual) advanced difference equation where , , are real sequences with oscillating terms and , , are sequences of integers such that Here, and . Also, as usual, denotes the forward difference operator and denotes the backward difference operator .

Strong interest in is motivated by the fact that it represents a discrete analogue of the differential equation (see [1] and the references cited therein) where, for every , is an oscillating continuous real-valued function in the interval , and is a continuous real-valued function on such that while, represents a discrete analogue of the advanced differential equation (see [1] and the references cited therein) where, for every , is an oscillating continuous real-valued function in the interval and is a continuous real-valued function on such that

By a* solution* of , we mean a sequence of real numbers which satisfies for all . Here,
It is clear that, for each choice of real numbers , there exists a unique solution of which satisfies the initial conditions , , and .

By a* solution* of the advanced difference equation , we mean a sequence of real numbers which satisfies for all .

A solution of [] is called* oscillatory*, if the terms of the sequence are neither eventually positive nor eventually negative. Otherwise, the solution is said to be* nonoscillatory*.

In the last few decades, the oscillatory behavior of all solutions of difference equations has been extensively studied when the coefficients are nonnegative. See, for example, [2–20] and the references cited therein. However, for the general case when are allowed to oscillate, it is difficult to study the oscillation of [], since the difference of any nonoscillatory solution of [] is always oscillatory. Thus, a small number of papers are dealing with this case. See, for example, [1, 21–32] and the references cited therein.

For (3) and (5) with oscillating coefficients, Fukagai and Kusano [1] established the following theorems.

Theorem 1 (see [1, Theorem ]). *Assume (4) and that there is a continuous nondecreasing function such that for , . Suppose moreover that there is a sequence such that , the intervals are disjoint, and
**
If there is a constant such that
**
then all solutions of (3) oscillate.*

Theorem 2 (see [1, Theorem ii]). *Assume (6) and that there is a continuous nondecreasing function such that for , . Suppose moreover that there is a sequence such that , the intervals are disjoint, and
**
If there is a constant such that
**
then all solutions of (5) oscillate.*

For and with oscillating coefficients, recently, Bohner et al. [21, 23] established the following theorems.

Theorem 3 (see [23, Theorem 2.4]). *Assume (1) and that the sequences are increasing for all . Suppose also that for each there exists a sequence such that and
**
where
**
If, moreover,
**
where , then all solutions of oscillate.*

Theorem 4 (see [23, Theorem 3.4]). *Assume (2) and that the sequences are increasing for all . Suppose also that for each there exists a sequence such that and
**
where
**
If, moreover,
**
where , then all solutions of oscillate.*

Theorem 5 (see [21, Theorem 2.1]). *Assume (1) and that the sequences are increasing for all . Suppose also that for each there exists a sequence such that ,
**
If, moreover,
**
then all solutions of oscillate.*

Theorem 6 (see [21, Theorem 3.1]). *Assume (2) and that the sequences are increasing for all . Suppose also that for each there exists a sequence such that ,
**
If, moreover,
**
then all solutions of oscillate.*

In the present paper, the authors study further [] and derive new sufficient oscillation conditions when neither (14) [(17)] nor (20) [(23)] is satisfied (cf. [6–8] and the references cited therein in the case of the equations [] with nonnegative coefficients , ). Examples illustrating the results are also given.

#### 2. Retarded Equations

In this section, we present new sufficient conditions for the oscillation of all solutions of when the conditions (14) and (20) are not satisfied, under the assumption that the sequences are increasing for all . To that end, the following lemma provides a useful tool.

Lemma 7. *Assume that (1) holds, the sequences are increasing for all and is a nonoscillatory solution of . Suppose also that for each there exists a sequence , such that , and (12) where is defined by (13). Set
**where *.*If , then
*

*Proof. *Since the solution of is nonoscillatory, it is either eventually positive or eventually negative. As is also a solution of , we may restrict ourselves only to the case where eventually.

By (12), it is obvious that there exists such that
Also, by (24) we have
where is an arbitrary real number with .

In view of (26) and (27), gives
for every . This guarantees that the sequence is decreasing on .

Assume that , where is defined by (24). From inequality (28), it is clear that there exists such that
This is because in the case where , there exists such that (30) is satisfied, while in the case where , then , and, therefore,
That is, in both cases (30) is satisfied.

Now, we will show that . Indeed, in the case where , since , it is obvious that . In the case where , then . Assume, for the sake of contradiction, that . Hence, and then
which contradicts (28). Thus, in both cases, we have . Therefore

Summing up from to , and using the fact that the function is decreasing and the function (as defined by (13)) is increasing, we have
or
which, in view of (30), gives

Summing up from to , and using the same arguments, we have
or
which, in view of (34), gives
Combining inequalities (37) and (40), we obtain
or
Thus
In view of (43), inequality (42) gives
which, in view of (40) becomes
Thus
or
Hence,
which, for arbitrarily small values of , implies (25).

The proof of the lemma is complete.

*Theorem 8. Assume that (1) holds, the sequences are increasing for all and is defined by (13). Suppose also that for each there exists a sequence such that , (12) and define by (24), where .If , and
then all solutions of oscillate.*

*Proof. *Assume, for the sake of contradiction, that is an eventually positive solution of . Then there exists such that
Therefore, by we have
for every . This guarantees that the sequence is decreasing on .

Summing up from to , and using the fact that the function is decreasing and the function (as defined by (13)) is increasing, we obtain
Consequently,
which gives
Assume that and (49) holds. Then by Lemma 7, inequality (25) is fulfilled, and so (54) leads to
which contradicts condition (49).

The proof of the theorem is complete.

*3. Advanced Equations*

*Oscillation of all solutions of is described by the theorem below. Note that the proof is an easy modification of the proof of Theorem 8 and hence is omitted.*

*Theorem 9. Assume (2) holds, the sequences are increasing for all and is defined by (16). Suppose also that for each there exists a sequence such that , (15) and
where .*

If and then all solutions of oscillate.

*Remark 10. *When , then the conditions (49) and (57) reduce to the conditions (14) and (17), respectively. However the improvement is clear when . The lower bound in (49) and (57) is 0.946475699. That is, when , our conditions (49) and (57) essentially improve (14) and (17).

*4. Examples*

*The significance of the results is illustrated in the following examples.*

*Example 1. *Consider the retarded difference equation
where , , and are oscillating coefficients, as shown in Figure 1.

In view of (13), it is obvious that . Observe that for
we have for every , where
For
we have for every , where
and, for
we have for every , where
Therefore,
Observe that
Now,
Observe that
that is, condition (49) of Theorem 8 is satisfied and, therefore, all solutions of equation (58) oscillate.

On the other hand,
Observe that for every , for every , where
and for every , where
Therefore,
Also,
Therefore none of the conditions (14) and (20) is satisfied.

*Example 2. *Consider the advanced difference equation
where and are oscillating coefficients, as shown in Figure 2.

*In view of (16), it is obvious that . Observe that for
we have for every , where
Also, for
we have for every , where
Therefore,
Observe that
Now
Also
Observe that
that is, condition (57) of Theorem 9 is satisfied and, therefore, all solutions of equation (74) oscillate.*

*On the other hand,
Observe that for every and for every , where
Therefore,
Also,
Therefore none of the conditions (17) and (23) is satisfied.*

*Conflict of Interests*

*The authors declare that there is no conflict of interests regarding the publication of this paper.*

*Acknowledgment*

*The authors would like to thank the referees for the constructive remarks which improved the presentation of the paper.*

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