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Abstract and Applied Analysis
Volume 2014 (2014), Article ID 168425, 13 pages
On Asymptotic Behavior of Solutions of Generalized Emden-Fowler Differential Equations with Delay Argument
1Department of Mathematics and Computer Sciences, Ariel University, 40700 Ariel, Israel
2Department of Mathematics, Tbilisi State University, University Street 2, 0143 Tbilisi, Georgia
Received 4 July 2013; Accepted 8 November 2013; Published 22 January 2014
Academic Editor: Josef Diblik
Copyright © 2014 Alexander Domoshnitsky and Roman Koplatadze. 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.
The following differential equation is considered. Here , , , , and . We say that the equation is almost linear if the condition is fulfilled, while if or , then the equation is an essentially nonlinear differential equation. In the case of almost linear and essentially nonlinear differential equations with advanced argument, oscillatory properties have been extensively studied, but there are no results on delay equations of this sort. In this paper, new sufficient conditions implying Property A for delay Emden-Fowler equations are obtained.
This work deals with oscillatory properties of solutions of a functional differential equation of the form where
It will always be assumed that the condition is fulfilled.
Let . A function is said to be a proper solution of (1) if it is locally absolutely continuous together with its derivatives up to order inclusive, and there exists a function such that on and the equality holds for . A proper solution of (1) is said to be oscillatory if it has a sequence of zeros tending to . Otherwise the solution is said to be nonoscillatory.
Definition 1. We say that (1) has Property A if any of its proper solutions is oscillatory whenis even and either is oscillatory or satisfies when is odd.
Definition 2. We say that (1) is almost linear if the condition holds, while if or , then we say that the equation is an essentially nonlinear differential equation.
The Emden-Fowler equation originated from theories concerning gaseous dynamics in astrophysics in the middle of the nineteenth century. In the study of stellar structure at that time it was important to investigate the equilibrium configuration of the mass of spherical clouds of gas. Lord Kelvin in 1862 assumed that the gaseous cloud is under convective equilibrium and then Lane  studied the equation
The Emden-Fowler equations were first considered only for second-order equations and written in the form which could be reduced in the case of positive and continuous coefficients to the equation
To avoid difficulties of defining when is negative and is not an integer, the equation was usually considered. The mathematical foundation of the theory of such equations was built by Fowler  and the description of the results can be found in Chapter 7 of .
We see also the Emden-Fowler equation in gas dynamics and fluid mechanics (see Sansone , page 431 and the paper ). Nonoscillation of these equations is important in various applications. Note that the zero of such solutions corresponds to an equilibrium state in a fluid with spherical distribution of density and under mutual attraction of its particles. The Emden-Fowler equations can be either oscillatory (i.e., all proper solutions have a sequence of zeros tending to zero) or nonoscillatory, if solutions are eventually positive or negative, or, in contrast with the case of linear differential equations of second order, may possess both oscillating and nonoscillating solutions. For example, for the equation it was proven in  that for all solutions oscillate, for —all solutions nonoscillate, and for there are both oscillating and nonoscillating solutions.
The Emden-Fowler equation presents one of the classical objects in the theory of differential equations. Tests for oscillation and nonoscillation of all solutions and existence of oscillating solutions were obtained in the works [6–8]. In  for the case it was obtained that all solutions of the equation oscillate if and only if
The latest research results in this area are presented in the book . Behavior of solutions to th order Emden-Fowler equations can be essentially more complicated. Properties A and B defined by Kiguradze are studied in the abovementioned book.
There are essentially less results on oscillation of delay Emden-Fowler equations. Oscillation properties of nonlinear delay differential equations, where Emden-Fowler equations were also included as a particular case, were studied in [10–20]. Results of these papers are discussed in [13, 15], where various examples demonstrating essentialities of conditions are also presented. Note that for delay differential equations there are no results on nonoscillation of all solutions and only existence of nonoscillating solutions is studied. Actually, the results on oscillation of delayed equations are based on the approaches existing for ordinary differential equations with development in the direction of preventing the obstructive influence of delay. In the paper  the following equation and its particular case are considered. It was obtained for the last equation under some standard assumptions on the coefficients  that in the case , all solutions oscillate. We see that the integral depends on deviation of argument and the power of the equation . For the equation where is the ratio of two positive odd integers, for and as each of the following conditions (a), (b), and (c) ensures oscillation of all solutions:(a)(b)(c)
Most proofs of results on oscillation of all solutions to second order equations utilize the fact that if a nonoscillating solution exists, the signs of the solution and its second derivative are opposite to each other for sufficiently large . Then a growth of nonoscillating solution is estimated and the authors come to contradiction with conditions that proves oscillation of all solutions. Note that delays disturb oscillation. Instead of appears. The principle is clear: for oscillation of all solutions we have to achieve a corresponding smallness of the delay . All this is more complicated if we study th order equations. In this case also the fact that and its th derivative have different signs for sufficiently large is used, but the technique is more complicated.
In the papers [21–28] a generalization of Emden-Fowler equations was considered. The powers in these papers can be functions and not constants. In many cases, it leads to essentially new oscillation properties of such equations. Surprisingly, oscillation behavior of equations, with the power and with functional power such that can be quite different. The main purpose of our paper is to study conditions under which the generalized (in this sense) equations preserve the known oscillation properties of Emden-Fowler equations and conditions under which these properties are not preserved. Oscillatory properties of almost linear and essentially nonlinear differential equation with advanced argument have already been studied in [21–28]. In this paper we study oscillation properties of th order delay Emden-Fowler equations.
2. Some Auxiliary Lemmas
In the sequel, will denote the set of all functions absolutely continuous on any finite subinternal of along with their derivatives of order up to including .
Lemma 3 (see ). Let , , for , and in any neighborhood of . Then there exist and such that is odd and
Remark 4. If is odd and , then it means that in only the second inequalities are fulfilled.
Lemma 5 (see ). Let and let be fulfilled for some with odd. Then
If, moreover, then there exists such that
The following notation will be used throughout the work:
Clearly , and is nondecreasing and coincides with the inverse of when the latter exists.
Definition 6. Let . By one denotes the set of all proper solutions of (1) satisfying the condition with some .
Lemma 7. Let the conditions (2),(3) be fulfilled, letwith odd, and let be a positive proper solution of (1). If, moreover, , , then for any there exists such that for any where is given by the first equality of (26) and
Proof. Let , with odd and (see Definition 6) is solution of (1). Since , according to (1), , and , it is clear that condition (22) holds. Thus, by Lemma 5 there exists such that the conditions –(25) with are fulfilled and
Observe that there exists such that for . Thus, by (24), for any we get
According to and , choose such that
By (34) and (35) we have
Let . Since as , without loss of generally we can assume that for . Then by from (36) we get
It is obvious that where
Thus, according to (37), and (39) from (38) we get
Hence, according to (37) and (39) where
Thus, according to (36) and (42) where
Now assume that and . Since for , without loss of generality we can assume that for . Therefore, from (36) we have
Taking into account (46), as above we can find that if , then where
According to (43)–(45) and (47)–(49), it is obvious that for any , , and there exists such that (29)–(31) hold, where is defined by (32). This proves the validity of the lemma.
Analogously we can prove.
Lemma 8. Let conditions (2), (3), be fulfilled, let with odd, , and let be a positive proper solution of (1). Then for any there exists such that for any where is defined by the second equality of (26) and
Remark 9. In Lemma 7, the condition cannot be replaced by the condition . Indeed, let . Consider (1), where is even and
It is obvious that the function is the solution of (1) and it satisfies the condition for . On the other hand, the condition holds, but the condition (22) is not fulfilled.
Theorem 10. Let with be odd, let and the conditions (2), (3), , and let be fulfilled, and for some , . Then for any there exists such that if , and if , then for any and , where is defined by first equality of (26) and is given by (30)–(32).
Proof. Let and such that . By definition, (1) has a proper solution satisfying the condition with some . Due to (1), , and , it is obvious that condition (22) holds. Thus by Lemma 5 there exists such that conditions -(24) with are fulfilled. On the other hand, according to Lemma 7 (and its proof), we see that
and there exists such that relation (30) is fulfilled. Without loss of generality we can assume that for . Therefore, by (24), from (58) we have
Assume that . Then by (44) and (59), we have
On the other hand, according to and it is obvious that
Therefore, from (60) we get
Now assume that and . Then according to (47), , and (61), from (59) we have
Thus, we obtain where .
It is obvious that there exist such that
Therefore, from (64) where
From the last inequality we get
Passing to the limit in the latter inequality, we get that is, according to (62) and (69), (56) and (57) hold, which proves the validity of the theorem.
Using Lemma 8 in a similar manner one can prove the following.
Theorem 11. Let with be odd, let (2), (3), , and be fulfilled, and for some , . Then there exists such that if , for any and if , then for any and where is defined by the second equality of (26) and is given by (51)–(53).
Theorem 12. Let with odd, let the conditions (2), (3), , and be fulfilled, and if , for any large and for some
or if , for same and
Then for any one has , where and are defined by (26) and is given by (30)–(32).
Proof. Assume the contrary. Let there exist such that (see Definition 6). Then (1) has a proper solution satisfying the condition . Since the condition of Theorem 10 is fulfilled, there exists such that if (if ), the condition (56) (the condition (57)) holds, which contradicts and . The obtained contradiction proves the validity of the theorem.
Using Theorem 11 analogously we can prove the following.
Theorem 13. Let with odd, let the conditions (2), (3), , and be fulfilled, and if , for any large and for some or if for same and Then for any we have , where is defined by the second equality of (26) and is given by (51)–(53).
Proof. By and (77), for , the condition holds, which proves the validity of the corollary.
Quite similarly one can prove the following.
Corollary 19. Let with be odd, let the conditions (2), (3), and be fulfilled, , and . Moreover, let there exist and such that and let at least one of the conditions or and for some and be fulfilled. Then for any one has , where is defined by (26).
Proof. It suffices to show that the condition is satisfied for some and . Indeed, according to and (88), there exist , , , and such that
By (77), (31), and (91), from (31) we get
Let . Choose such that
Therefore, by (91) from (31) we can find such that
Hence for any there exists such that
Assume that (89) is fulfilled. Choose such that . Then according to (92), (96), and , the condition holds for and . In this case, the validity of the corollary has already been proven.
Assume now that is fulfilled. Let and choose and such that
Then according to (96), (92), and , it is obvious that holds for . The proof of the corollary is complete.
Using Theorem 13, in a manner similar to above we can prove the following.
5. Differential Equation with Property A
Theorem 23. Let the conditions (2), (3) be fulfilled and for any with odd, the conditions and hold. Moreover for any large , if and for some let be fulfilled or if and , for some , , and , let be fulfilled. Then, if for odd then (1) has Property , where and are defined by (26) and is given by (30)–(32).
Proof. Let (1) have a proper nonoscillatory solution (the case is similar). Then by (2), (3), and Lemma 3 there exists such that is odd and conditions hold. Since, for any with odd, the conditions of Theorem 12 are fulfilled we have. Now assume that, is odd, and there exists such that for sufficiently large . According to since , from (1) we have where is a sufficiently large number. The last inequality contradicts the condition (102). The obtained contradiction proves that (1) has Property A.
Theorem 24. Let the conditions (2), (3) be fulfilled and for any with odd the conditions and hold. Let moreover, if , for some hold or if and for some , , and , hold. Then, if for odd (102) is fulfilled, then (1) has Property , where is defined by the second equality of (26) and is given by (51)–(53).
Theorem 25. Let , , let the conditions (2), (3), , and be fulfilled, and If, moreover, for some , , and , holds and for odd n (102) is fulfilled, then (1) has Property , where and are defined by (26) and is given by (30)–(32).
Proof. It suffices to note that by (104) and , for any there exist , , and such that condition is fulfilled.