Research Article | Open Access

# Oscillation Theorems for Second-Order Damped Nonlinear Differential Equations

**Academic Editor:**Qingkai Kong

#### Abstract

We present new oscillation criteria for the differential equation of the form , where , . Our research is different from most known ones in the sense that H function is not employed in our results, though Riccati's substitution and its generalized forms are used. Our criteria which are established under quite general assumptions are an extension for previous results. In particular, by taking , the above-mentioned equation can be reduced into the various types of equations concerned by people currently.

#### 1. Introduction

The existence of the oscillatory solutions of the nonlinear differential equation with damping,

has received considerable attention from researchers for a long time.

People previously focused on the cases . In recent years, people concerned that may change sign for , regarding work in this area can be seen in literature [1–4].

Recently, Li [5] has extended (1.1) to more general equations of the form

Yamaoka [6] has studied the following class of particular equation:

Tiryaki and Zafer [1] and other authors [2, 7] have considered the following equation of the form

Zheng [8] has discussed the oscillation problem for the following equation:

It is worth noting that (1.1), (1.2), and (1.3) can transform into an undamping equation. For example, the equation

can transform into the undamping equation

where . Although (1.4) and (1.5) can not be transformed into the undamping equation, but from the conditions given by [1, 8, 9], if is changed into or , the above-mentioned equations consistent with (1.6). This shows that under the above conditions, there is no essential difference between (1.4), (1.5), and the undamping equation. We note that the condition must be used for (1.5); however, at this point the condition cannot be guaranteed.

We have removed the condition , considered the oscillation problem for the following equation: applied the results to the above-mentioned equation, and obtained a very good result.

In this paper, we consider the oscillatory behavior of the following differential equation of the form:

where

Today, Riccati transformation, and its generalized forms are one of the most effective method in the oscillatory theory of nonlinear differential equations. Most obvious merits of Riccati's approach is that may change sign in (1.8). For getting the more general results [10, 11], a lot of authors have introduced to a class of *Y* function

where exists on *E* and is integral with respect to *s*. By using this method, peoples have obtained some general results, but its shortcoming is that the property of can be weakened as . We use the method similar to [4], that is, replace the above-mentioned function with . Perhaps the reason that people like to use this method is that integrating by parts with respect to *s* on can employ .

For (1.9), we make the following assumptions:

(A);(B);(C).In the paper, a solution of (1.9) is called oscillatory if it has zeros unbounded set. If the solutions are oscillatory, (1.9) is called to be oscillatory equation.

#### 2. Main Theorem

We establish some lemmas which are useful in our discussions.

Lemma 2.1. *Let , then
*

Lemma 2.1 can easily be proved by using the extremum of one variable function. For the sake of convenience, we denote

Theorem 2.2. *Assume that holds and there exists , such that
**
If any one of the following two conditions holds, then the solution of (1.9) is oscillatory.**(1)
**(2), and
*

*Proof. *Let be a nonoscillatory solution of (1.9). Then, there exists such that . Without loss of generality, we may assume that

Define the Riccati Transformation by
From conditions (A) and (B), we have
Differentiating and applying (1.9) and (2.10), we have

By (2.1), we have
Integrating the above inequality from to , we have

Condition (2.4) shows that Without loss of generality, we may assume that , by applying (2.11), we have

Integrating the above inequality from to , we obtain
Let
We will discuss in the following two cases.

(1) By (2.1) and (2.5), we see that
Integrating the above inequality from to , we have
But, it is impossible that the above inequality holds.

(2) Observe that and by (2.8), we have so that is monotonic decreasing function for , and if , then Otherwise, if , by (2.15) and (2.9), we have
By condition (B), we have
Integrating the above inequality from to leads to
But, this is impossible. We choose , thus (2.15) has the following form:

When , by considering inequality and (2.10), we have
such that However, this is also impossible.

If , then by (2.8), we see that From (2.15), we obtain
Integrating the above inequality from to , we have
Let ; the above inequality contradicts (2.8); this completes the proof.

Theorem 2.3. *Suppose that and
**
If there exists such that
**
where
**
then every solution of (1.9) is oscillatory.*

*Note*

From (2.27), it is easy to obtain the following equation:

*Proof. *Let be a nonoscillatory solution of (1.9). Then, there exists such that We may assume that

Introduce the Riccati transformation . From conditions (A) and (B), we have
Differentiating , and applying (1.9) and the above inequality, leads to
By (2.1), we see that
The following proof is similar to that in Theorem 2.2, using (2.26), we find that Thus there exists , such that Because is monotonic decreasing function on ; hence By (2.31), we have

By using of weighted mean inequality, we can transform the above inequality into
We need to show that Otherwise, if by the above inequality, we have
By choosing , such that , integrating the above inequality from to and by (2.27), we can get
or
Differentiating the above inequality on the interval , we have
This is a contradiction to (2.29); hence, we have . According to the above discussion, we have
We will discuss in the following two cases.

If there exists such that , by considering inequality and (2.39), we have
By choosing , such that , from the above inequality, we have . Inserting it in (2.40), we can get
This is contradiction to (2.29).

If for , along with (2.39) and (2.30), we have
leading to
Integrating the above inequality from to and by (2.40), we can get
or
Integrating the above inequality on the interval , we have

If the above inequality cannot be satisfied; hence, Inserting it in (2.40), we can get
This is contradiction to (2.26).

Hence, we complete the proof of Theorem 2.3.

#### 3. Some Examples

*Example 3.1. *Let us consider the oscillatory behavior of the following differential equation:

Comparing (3.1) with (1.9), we can find that

Let , (2.4)–(2.7) are transformed into the equations
We will discuss inthe following cases.

(1) choosing , provided that , or is satisfied for , then (3.3)–(3.5) hold, and the solution of (3.1) is oscillatory.

(2) , choosing provided that and or then (3.3)–(3.5) hold, the solution of (3.1) is oscillatory.

(3) , we can see that choosing , and therefore, (3.6) and (3.7) are transformed into the following equation:
provided that and or then (3.3)-(3.4) and (3.6)-(3.7) hold, the solution of (3.1) is oscillatory.

In particular, we chose , thus the conditions of the case (1) can be satisfied. This is the sufficient condition for all solutions of to be oscillatory.

If we choose