#### Abstract

Consider the second-order linear delay differential equation , , where , , is nondecreasing, for and , the (discrete analogue) second-order difference equation , where , , , , , and , and the second-order functional equation , , where the functions , , , for , , and denotes the 2th iterate of the function , that is, , , . The most interesting oscillation criteria for the second-order linear delay differential equation, the second-order difference equation and the second-order functional equation, especially in the case where and for the second-order linear delay differential equation, and and , for the second-order functional equation, are presented.

#### 1. Introduction

The problem of establishing sufficient conditions for the oscillation of all solutions to the second-order delay differential equation where (here ), , is nondecreasing, for , and , has been the subject of many investigations; see, for example,  and the references cited therein.

By a solution of (1) we understand a continuously differentiable function defined on for some and such that (1) is satisfied for . Such a solution is called oscillatory if it has arbitrarily large zeros, and otherwise it is called nonoscillatory.

The oscillation theory of the (discrete analogue) second-order difference equation where , , , , and , has also attracted growing attention in the recent few years. The reader is referred to  and the references cited therein.

By a solution of (1) we mean a sequence which is defined for and which satisfies () for all . A solution of () is said to be oscillatory if the terms of the solution are neither eventually positive nor eventually negative. Otherwise the solution is called nonoscillatory.

The oscillation theory of second-order functional equations of the form where , are given real-valued functions, is an unknown real-valued function, for , , and denotes the 2nd iterate of the function , that is, has also been developed during the last decade. We refer to  and the references cited therein.

By a solution of () we mean a real-valued function such that for any and satisfies () on .

In this paper our purpose is to present the state of the art on the oscillation of all solutions to (1), (), and (), especially in the case where for (1), and for ().

#### 2. Oscillation Criteria for (1)

In this section we study the second-order delay equation (1). For the case of ordinary differential equations, that is, when , the history of the problem began as early as in 1836 by the work of Sturm  and was continued in 1893 by Kneser . Essential contribution to the subject was made by E. Hille, A. Wintner, Ph. Hartman, W. Leighton, Z. Nehari, and others (see the monograph by Swanson  and the references cited therein). In particular, in 1948 Hille  obtained the following well-known oscillation criteria. Let or the conditions being assumed to be satisfied if the integral diverges. Then (1) with is oscillatory.

For the delay differential equation (1), earlier oscillation results can be found in the monographs by Myshkis  and Norkin . In 1968 Waltman  and in 1970 Bradley  proved that (1) is oscillatory if Proceeding in the direction of generalization of Hille's criteria, in 1971 Wong  showed that if for with , then the condition is sufficient for the oscillation of (1). In 1973 Erbe  generalized this condition to without any additional restriction on . In 1987 Yan  obtained some general criteria improving the previous ones.

An oscillation criterion of different type is given in 1986 by Koplatadze  and in 1988 by Wei , where it is proved that (1) is oscillatory if or The conditions () and () are analogous to the oscillation conditions (see ) respectively, for the first-order delay equation

The essential difference between (2.1), (2.2), and (), () is that the first two can guarantee oscillation for ordinary differential equations as well, while the last two work only for delay equations. Unlike first-order differential equations, where the oscillatory character is due to the delay only (see ), equation (1) can be oscillatory without any delay at all, that is, in the case . Figuratively speaking, two factors contribute to the oscillatory character of (1): the presence of the delay and the second-order nature of the equation. The conditions (2.1), (2.2), and (), () illustrate the role of these factors taken separately.

In 1999 Koplatadze et al.  derived the following.

Theorem 2.1 (see ). Let Then (1) is oscillatory.

The following corollaries being more convenient for applications can be deduced from this theorem.

Corollary 2.2 (see ). Let or Then (1) is oscillatory.

Corollary 2.3 (see ). Let or Then (1) is oscillatory.

In the case of ordinary differential equations () the following theorem was given in .

Theorem 2.4 (see ). Let and Then (1) is oscillatory.

In what follows it will be assumed that the condition is fulfilled. As it follows from Lemma in , this condition is necessary for (1) to be oscillatory. The study being devoted to the problem of oscillation of (1), the condition (2.13) does not affect the generality.

Here oscillation results are obtained for (1) by reducing it to a first-order equation. Since for the latter the oscillation is due solely to the delay, the criteria hold for delay equations only and do not work in the ordinary case.

Theorem 2.5 (see ). Let (2.13) be fulfilled and let the differential inequality have no eventually positive solution. Then (1) is oscillatory.

Theorem 2.5 reduces the question of oscillation of (1) to that of the absence of eventually positive solutions of the differential inequality So oscillation results for first-order delay differential equations can be applied since the oscillation of the equation is equivalent to the absence of eventually positive solutions of the inequality This fact is a simple consequence of the following comparison theorem deriving the oscillation of (2.16) from the oscillation of the equation

We assume that are locally integrable, are continuous, , for , and , as .

Theorem 2.6 (see ). Let and let (2.18) be oscillatory. Then (2.16) is also oscillatory.

Corollary 2.7 (see ). Let (2.16) be oscillatory. Then the inequality (2.17) has no eventually positive solution.

Turning to applications of Theorem 2.5, we will use it together with the criteria () and () to get the following.

Theorem 2.8 (see ). Let or Then (1) is oscillatory.

To apply Theorem 2.5 it suffices to note that (i) equation (2.13) is fulfilled since otherwise ; (ii) since as , the relations () and () imply the same relations with changed by any .

Remark 2.9 (see ). Theorem 2.8 improves the criteria (), () by Koplatadze  and Wei  mentioned above. This is directly seen from (), () and can be easily checked if we take and for , where the constants and satisfy In this case neither of (), () is applicable for (1) while both (), () give the positive conclusion about its oscillation. Note also that this is exactly the case where the oscillation is due to the delay since the corresponding equation without delay is nonoscillatory.

Remark 2.10 (see ). The criteria (), () look like (), () but there is an essential difference between them pointed out in the introduction. The condition () is close to the necessary one, since according to  if , then (2.16) is nonoscillatory. On the other hand, for an oscillatory equation (1) without delay we have . Nevertheless, the constant in Theorem 2.8 is also the best possible in the sense that for any it cannot be replaced by without affecting the validity of the theorem. This is illustrated by the following.

Example 2.11 (see ). Let , , , and , where . Then () is fulfilled with 1/e replaced by . Nevertheless (1) has a nonoscillatory solution, namely, . Indeed, denoting , we see that the expression under the limit sign in () is constant and equals .

Note that there is a gap between the conditions (), () when , . In the case of first-order equations (cf., ), using results in this direction, one can derive various sufficient conditions for the oscillation of (1). According to Remark 2.9, neither of them can be optimal in the above sense but, nevertheless, they are of interest since they cannot be derived from other known results in the literature. We combine Theorem 2.5 with Corollary  to obtain the following theorem.

Theorem 2.12 (see ). Let and be defined by (), (), , and where is the smaller root of the equation . Then (1) is oscillatory.

Note that the condition () is analogous to the condition () in .

#### 3. Oscillation Criteria for (1)

In this section we study the second-order difference equation (), where , , , , , and .

In 1994, Wyrwińska  proved that all solutions of () are oscillatory if while, in 1997, Agarwal et al.  proved that, in the special case of the second-order difference equation with constant delay all solutions are oscillatory if

In 2001, Grzegorczyk and Werbowski  studied () and proved that under the following conditions

or all solutions of () are oscillatory. Observe that the last condition () may be seen as the discrete analogue of the condition ().

In 2001 Koplatadze  studied the oscillatory behaviour of all solutions to () with variable delay and established the following.

Theorem 3.1 (see ). Assume that Then all solutions of () oscillate.

Corollary 3.2 (see ). Let and Then all solutions of the equation oscillate.

Corollary 3.3 (see ). Let be an integer and Then all solutions of the equation oscillate.

In 2002 Koplatadze et al.  studied () and established the following.

Theorem 3.4 (see ). Assume that Then all solutions of () oscillate.

In the case where , the following discrete analogue of Hille's well-known oscillation theorem for ordinary differential equations (see (2.2)) is derived.

Theorem 3.5 (see ). Let be an integer and Then all solutions of the equation oscillate.

Remark 3.6 (see ). As in case of ordinary differential equations, the constant in (3.10) is optimal in the sense that the strict inequality cannot be replaced by the nonstrict one. More than that, the same is true for the condition (3.9) as well. To ascertain this, denote by the right-hand side of (3.9) and by the point where the maximum is achieved. The sequence obviously is a nonoscillatory solution of the equation where and denotes the integer part of . It can be easily calculated that Hence for arbitrary , for with sufficiently large. Using the inequality and the arbitrariness of , we obtain This limit cannot be greater than by Theorem 3.4. Therefore it equals and (3.9) is violated.

#### 4. Oscillation Criteria for (1)

In this section we study the functional equation ().

In 1993 Domshlak  studied the oscillatory behaviour of equations of this type. In 1994, Golda and Werbowski  proved that all solutions of () oscillate if or In the same paper they also improved condition () to where is some integer.

In 1995, Nowakowska and Werbowski  extended condition () to higher-order linear functional equations. In 1996, Shen , in 1997, Zhang et al. , and, in 1998, Zhang et al.  studied functional equations with variable coefficients and constant delay, while in 1999, Yan and Zhang  considered a system with constant coefficients.

It should be noted that conditions () and () may be seen as the analogues of the oscillation conditions () and () for (1) and () and () for ().

As far as the lower bound in the condition () is concerned, as it was pointed out in , it cannot be replaced by a smaller number. Recently, in , this fact was generalized by proving that implies that () has a nonoscillatory solution.

It is obvious that there is a gap between the conditions () and () when the limit does not exist. How to fill this gap is an interesting problem. Here we should mention that condition (4.1) is an attempt in this direction. In fact, from condition (4.1) we can obtain (see ) that all solutions of () oscillate if and

In 2002, Shen and Stavroulakis  proved the following.

Theorem 4.1 (see ). Assume that and that for some integer where Then all solutions of () oscillate.

Corollary 4.2 (see ). Assume that and Then all solutions of () oscillate.

Remark 4.3 (see ). It is to be noted that as the condition (4.3) reduces to the condition (4.1) and the conditions (4.4) and (4.2) reduce to the condition (). However the improvement is clear as because It is interesting to observe that when condition (4.4) reduces to which cannot be improved in the sense that the lower bound cannot be replaced by a smaller number.

Example 4.4 (see ). Consider the equation where , , , and is a constant. It is easy to see that Thus, by Corollary 4.2 all solutions of (4.7) oscillate. However, the condition () is satisfied only for , while the condition (4.2) is satisfied for (much smaller) .

#### Acknowledgment

This work was done while L.K. Kikina was visiting the Department of Mathematics, University of Ioannina, in the framework of a postdoctoral scholarship awarded by IKY (State Scholarships Foundation), Athens, Greece.