Research

# Asymptotic problems for fourth-order nonlinear differential equations

Miroslav Bartušek and Zuzana Došlá*

Author Affiliations

Faculty of Science, Masaryk University Brno, Kotlářská 2, Brno, 611 37, The Czech Republic

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Boundary Value Problems 2013, 2013:89  doi:10.1186/1687-2770-2013-89

 Received: 13 November 2012 Accepted: 16 March 2013 Published: 12 April 2013

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

We study vanishing at infinity solutions of a fourth-order nonlinear differential equation. We state sufficient and/or necessary conditions for the existence of the positive solution on the half-line which is vanishing at infinity and sufficient conditions ensuring that all eventually positive solutions are vanishing at infinity. We also discuss an oscillation problem.

### Dedication

Dedicated to Jean Mawhin on occasion of his seventieth birthday.

### 1 Introduction

In this paper we study the fourth-order nonlinear differential equation

(1)

where , , for large t, such that for large t and .

Jointly with (1), we consider a more general equation

where satisfies for , and the associated linear second-order equation

(2)

By a solution of (1) we mean a function , , which satisfies (1) on . A solution is said to be nonoscillatory if for large t; otherwise, it is said to be oscillatory. Observe that if , according to [[1], Theorem 11.5], all nontrivial solutions of (1) satisfy for , on the contrary to the case , when nontrivial solutions satisfying for large t may exist.

Fourth-order differential equations have been investigated in detail during the last years. The periodic boundary value problem for the superlinear equation has been studied in [2]. In [3], the fourth-order linear eigenvalue problem, together with the nonlinear boundary value problem , has been investigated. Oscillatory properties of solutions for self-adjoint linear differential equations can be found in [4]. Equation (1) with can be viewed as a prototype of even-order two-term differential equations, which are the main object of monographs [1,5,6].

Equation (1′) with for is a special case of higher-order differential equations investigated in [7]. Equation (1′) with q near to a nonzero constant as has been considered in [8] as a perturbation of the linear equation , and the existence of oscillatory solutions of (1′) has been proved. In [9], necessary and sufficient conditions for the existence of asymptotically linear solutions of (1′) have been given.

In the recent paper [10], the equation

where , for , and has been investigated and applications to the biharmonic PDE’s can be found there. In particular, the so called homoclinics solutions, which are defined as nontrivial solutions x such that , are studied.

The goal of this paper is to investigate asymptotic problems associated with (1) and the asymptotic boundary condition

(3)

A solution x of (1) satisfying (3) is said to be vanishing at infinity.

We start with the Kneser problem for (1). The Kneser problem is a problem concerning the existence of solutions of (1) subject to the boundary conditions on the half-line

(4)

We establish necessary and/or sufficient conditions for the solvability of the boundary value problem (1), (3), (4). In the light of these results, as the second problem, we study when all eventually positive solutions x of (1) are vanishing at infinity assuming that and (2) is oscillatory. As a consequence, we give a bound for the set of all nonoscillatory solutions. Finally, we discuss when problem (1), (3) is not solvable and solutions to (1) are oscillatory.

A systematic analysis of solutions of (1) satisfying (3) is made according to whether (2) is nonoscillatory or oscillatory. If (2) is nonoscillatory, then the following approach will be used. Equation (1) can be rewritten as the two-term equation

(5)

where h is a positive solution of (2). According to [11], a solution h of (2) is said to be a principal solution if , and such a solution is determined uniquely up to a multiple constant. Since , every eventually positive solution of (2) is nondecreasing for large t. Hence there exists a principal solution h of (2) such that for and

(6)

Therefore, we can use the known results [12,13] stated for systems of differential equations or in [14] for fourth order differential equations.

If (2) is oscillatory, then our approach is based on the choice of a suitable transformation. The main idea is based on a transformation of (1) to the fourth-order quasilinear equation and the use of the estimates for positive solutions of such an equation on a compact interval stated in [15]. This, together with an energy function associated with (1), enables us to state an oscillation theorem. In the final section, some extensions of our results to (1′) are given.

### 2 The Kneser problem

In this section we present necessary and/or sufficient conditions for solvability of boundary value problem (1), (3), (4).

#### 2.1 Case

Proposition 1Let, (2) be disconjugate on, andfor. Then boundary value problem (1), (4) is solvable for any.

To prove this theorem, we use Chanturia’s result [[12], Theorem 1] for the system of differential equations

(7)

where we restrict to the case that are continuous functions, , , , and . Then this result reads as follows.

Theorem A ([12])

Let there existsuch that

for, , (). Suppose

for, , (), where functions () are continuous and nondecreasing in the second argument such that

is a continuous function andis a continuous nondecreasing function such that

Then, for any, system (7) has a solution satisfying

(8)

Proof of Proposition 1 Assume for . Since (2) is disconjugate, it has a positive solution h on , and (1) can be written as (5) where on . Let x be a solution of (5) and denote

(9)

Then (5) is equivalent to the system

(10)

Let be from (4). We apply Theorem A choosing

where , and . By this result, system (10) has a solution such that

Since , system (10) has no solutions such that for some and large t; see [16] or [[17], Lemma 2, Theorem 2]. Thus, for any , equation (1) has a solution x such that , , and for . □

Now we state conditions for the existence of a solution for problem (1), (3), (4).

Theorem 1Let, and

(11)

on. If

(12)

then problem (1), (3), (4) is solvable for any.

(13)

then the condition

(14)

is necessary and sufficient for the solvability of problem (1), (3), (4).

For the proof, the following lemma will be needed.

Lemma 1Consider system (10) on (), whereforandhis a principal solution of (2). Letbe a solution of (10) such thatandfor, and. Thenfor, and if

(15)

then, too. Vice versa, ifand, then (15) holds.

Proof In view of the monotonicity of , there exist , . Since h is the principal solution, (6) holds, and integrating the first three equations in (10) from a to t, we get for . Now integrating (10) from t to ∞, we have

Let (15) hold and assume, by contradiction, that . Then

(16)

Letting and using the change of the order of integration, we get a contradiction with the boundedness of . This proves that .

Let the integral in (15) be convergent and assume, by contradiction, that . Then we have

so

Since , then using the change of the order of integration, we get a contradiction for large t. This proves that . □

Proof of Theorem 1 In view of (11), (2) is disconjugate on . By Proposition 1, equation (1) has a solution x satisfying (4). Therefore, system (10) has a solution such that and for . Choose h in (10) as a principal solution of (2). The Euler equation

(17)

is the majorant of (2) on and has the principal solution . By the comparison theorem, for the minimal solution of the Riccati equation related to (2) and (17), we have

for large t; see, e.g., [11]. Thus there exists such that for . Assume (12). Then (15) holds, and by Lemma 1 a solution x satisfies (3).

Assume (13). Then the principal solution h of (2) satisfies for large t (see, e.g., [11]). Hence, condition (15) reads as (14), and by Lemma 1 this condition is equivalent to the property (3). □

As a consequence of Lemma 1, we get the following result.

Corollary 1Let (2) be disconjugate on, andfor. Then any solutionxof (1) satisfying

(18)

is a solution of the Kneser problem, i.e., forand.

Proof Let h be a positive solution on satisfying (6), and let x be a solution of (1) satisfying (18). Then , where are defined by (9), is a solution of system (10). Since for and (6) holds, we have by the Kiguradze lemma (see, e.g., [1]) that either or for and large t, say for . Since x is positive and tends to zero, we have for , so also () for . By Lemma 1, we get () for . Since for , we have for and is positive and decreasing on . Hence, proceeding by the same argument, () is positive and decreasing on . Now the conclusion follows from (9). □

#### 2.2 Case

First we show that the sign condition posed on r is necessary for the solvability of problem (1), (4).

A function g, defined in a neighborhood of infinity, is said to change sign if there exists a sequence such that .

Theorem 2Letfor larget. Then problem (1), (4) has no solution and the following hold:

(a) If (2) is nonoscillatory, then every nonoscillatory solutionxof (1) satisfiesandis of one sign for larget.

(b) If (2) is oscillatory, then every nonoscillatory solutionxof (1) satisfies either, orchanges sign. In addition, if a solutionxsatisfies (3), thenchanges sign.

Proof Claim (a). Let x be a positive solution of (1) on , or, equivalently, of (5) on , where h satisfies (6). Denote

(19)

Then (5) is equivalent to the system

(20)

We have for . Assume by contradiction that for . Let and . Since is nonincreasing, and

Letting , we get a contradiction with the positiveness of . The remaining case can be eliminated in a similar way using (6). Observe that system (20) is a special case of the Emden-Fowler system investigated in [13], and the proof follows also from [[13], Lemma 2.1].

Claim (b). Without loss of generality, suppose that for and there exists a solution x of (1) such that and on , . Borůvka [18] proved that if (2) is oscillatory, then there exists a function , called a phase function, such that and

(21)

Using this result, we can consider the change of variables

(22)

for , , . Thus, and

Substituting into (1), we obtain the second-order equation

From here and (21), we obtain

(23)

Since , (22) yields and so , that is, is decreasing. If there exists such that , X becomes eventually negative, which is a contradiction. Then and is nondecreasing. Let be such that on . Thus, using (23) we obtain

Hence, , which contradicts the nonnegativity of . Finally, the case on cannot occur, because if on , then from (1) and , we have on , which is a contradiction.

Finally, let x be a positive solution of (1) satisfying (3). Then is either oscillatory or for large t. Assume on some , then is decreasing and either or for large t. If for large t, then we get a contradiction with (3). If , then for and x becomes negative for large t. Hence must be oscillatory. □

For , the analogous result to Theorem 1 is the following oscillation result.

Proposition 2Let, for larget. Assume either (11) for larget, (12), or (13), (14). Then all the solutions of (1) are oscillatory.

Proof Let x be a solution of (1) and h be the principal solution of (2). Then , where are given by (19), is a solution of system (20). Proceeding by the similar way as in the proof of Theorem 1, we have that (15) holds. Using the change of the order of integration in (15), we can check that conditions of Theorem 4.3 in [13] applied to system (20) are verified. Hence by this result all the solutions of (20) are oscillatory, which gives the conclusion. □

The following result follows from [[7], Theorem 1.5] and completes Proposition 2 in the case when (2) is oscillatory.

Proposition 3Let, andfor. Then the conditionis necessary and sufficient for every solution of (1) to be oscillatory.

In the light of these results, in the sequel, we study asymptotic and oscillation problems to (1) when (2) is oscillatory.

### 3 Vanishing at infinity solutions

In this section we study when all nonoscillatory solutions of (1) are vanishing at infinity.

Theorem 3Letand (2) be oscillatory. Assume thatfor largetand some, the functions

(24)

and

(25)

Then any eventually positive solution of (1) is vanishing at infinity.

The proof of Theorem 3 is based on the following auxiliary results.

Consider the fourth-order quasi-linear differential equation

(26)

where and R are continuous functions on . In [[15], Theorem 2.4], the following uniform estimate for positive solutions of (26) with a common domain was proved.

Proposition 4 ([[15], Theorem 3.4, Corollary 3.6])

Assume. Letybe a positive solution of (26) defined onand

(27)

onfor some constantsand. Then there exists a positive constantsuch that

(28)

where

Remark 1 In [[15], Theorem 3.4] the constant M is explicitly calculated.

Lemma 2Let. Assume that (27) holds on. Then any positive solution of (26) defined onsatisfies

(29)

whereαandMare constants from Proposition 4.

Proof Let . By Proposition 4, applied on , we have for and

Letting , we get (29). □

The next lemma describes the transformation between solutions of (1) and a certain quasi-linear equation.

Lemma 3Letonbe such that

and consider the transformation

Thenis a solution of equation (1) onif and only ifis a solution of the equation

(30)

whereis the inverse function to.

Proof We have

Substituting into (1), we get the conclusion. □

Proof of Theorem 3 Let x be a positive solution of (1) on (). Suppose, for simplicity, that for . Let

(31)

on for some positive constants , , and

(32)

Denote

(33)

Define the function such that

(34)

for . Then, according to , we have

(35)

Let be nondecreasing on and put . Choose arbitrarily fixed. Since , we can consider the transformation from Lemma 3 with , i.e.,

(36)

Then equation (1) is transformed into equation (30) which is a quasilinear equation of the form (26), where

and Q is defined by (31) and (32). Choose arbitrarily. We apply Lemma 2 to equation (30) with

Hence estimate (29) with reads as

(37)

where . Letting , we have by (35) that and the conclusion follows from (25) and (37). □

From the proof of Theorem 3, we get the estimate for the set of all nonoscillatory solutions of (1) which will be used in the next section.

Corollary 2Let, , (24) and (25) hold. Then, for any, there exists a positive constantandsuch that every nonoscillatory solutionxof (1) satisfies

(38)

Proof Let be fixed and let be such that

where α is given by (33). Let be fixed. Using estimate (37) with , we have

(39)

where is given by (34), i.e.,

Therefore and estimate (38) follows from (25) and (39). □

Example 1 Consider the equation

(40)

Then and by Theorem 3 all eventually positive solutions are vanishing at infinity. One can check that is such a solution of (40).

Open problem It is an open problem to find conditions for the solvability of boundary value problem (1), (3), (4) in case and (2) is oscillatory.

In view of Theorem 2, Corollary 2 and Proposition 1, it is a question whether (1) can have vanishing at infinity solutions in case and (2) is oscillatory.

In the next section, we show that under certain additional assumptions the answer is negative.

### 4 Oscillation

Here we consider (1) in case for large t. When (2) is nonoscillatory, we have established the oscillation criterion in Proposition 2. When (2) is oscillatory, the following oscillation theorem holds.

Theorem 4Let, and assumptions (24), (25) hold. Assume

(41)

and

(42)

for someand. Then problem (1), (3) is not solvable and all the solutions of (1) are oscillatory.

Proof Suppose that (25) holds on . First, observe that the assumption (42) implies that

(43)

Indeed, putting , we have

and thus, in view of (42), we get (43). Consider a solution x of (1) such that for . According to Corollary 2, there exists such that

(44)

and in view of (25) we get . Consider the function

Then

and in view of (41) the function F is increasing for large t. Hence, there exists such that either

(45)

or

(46)

According to Theorem 2(b), oscillates. Define by an increasing sequence of zeros of tending to ∞ with .

Define

(47)

In view of (44) and (43) the function Z is well defined and

(48)

on . Moreover, we have from (42), (44) and (47)

(49)

If (45) holds, then and because

we get for . This is a contradiction with (49), so (45) is impossible.

If (46) holds, then and . This is again a contradiction with (49), so also this case is impossible. □

Example 2 Consider the equation

where . If and , then by Theorem 1 this equation has a solution satisfying (3) and (4). If and , then by Theorem 3 any nonoscillatory solution (if any) satisfies (3).

### 5 Extensions

As it was mentioned in [10], a certain nonlinear PDE leads to the fourth-order equation with the exponential nonlinearity. In the sequel, we show that the results of this paper can be extended to the nonlinear equation

where q, r are as for (1) and for such that

(50)

for some and . The prototype of such an extension is the function for .

Theorems 1-4 read for (1′) as follows.

Theorem 1′Let, and (11) hold for. Assume that either (i) (12), or (ii) (13) and (14) hold. Then problem (1′), (3), (4) has a solution for any.

Proof of Theorem 1′ It is analogous to the proofs of Proposition 1 and Theorem 1 replacing the nonlinearity in system (10) by . Lemma 1 remains to hold as a sufficient condition for (3). □

Theorem 2′Theorem 2 remains to hold for (1′) without assuming (50).

Proof of Theorem 2′ In the proof of claim (a) of Theorem 2, we consider system (20) where the nonlinearity is replaced by . The proof of claim (b) of Theorem 2 is the same for the nonlinearity f. □

Theorem 3′Theorem 3 remains to hold for (1′).

Proof of Theorem 3′ Let x be a positive solution of (1′) on . Then is a solution of the equation

(51)

where

(52)

Now we apply Theorem 3 to (51). □

Theorem 4′Let the assumptions of Theorem 4 hold. Then (1′) has no eventually positive solutions.

Proof of Theorem 4′ It is similar to the one of Theorem 4. In view of (52), the estimate (38) holds and the energy function F is the same. □

### Competing interests

The authors declare that they have no competing interests.

### Authors’ contributions

Both authors contributed equally to the manuscript and read and approved the final draft.

### Acknowledgements

Supported by the grant GAP 201/11/0768 of the Czech Grant Agency.

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