Hindawi Publishing Corporation Journal of Applied Mathematics Volume 2013, Article ID 212036, 4 pages http://dx.doi.org/10.1155/2013/212036 Research Article Chaos for Discrete Dynamical System Lidong Wang,1,2 Heng Liu,1,2 and Yuelin Gao1 1 2 Information and Computational Science department, Beifang University of Nationality, Yinchuan, Ningxia 750021, China School of Science, Dalian Nationalities University, Dalian, Liaoning 116600, China Correspondence should be addressed to Lidong Wang; wld0117@yahoo.cn Received 8 January 2013; Revised 1 March 2013; Accepted 2 March 2013 Academic Editor: Ezio Venturino Copyright © 2013 Lidong Wang 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. We prove that a dynamical system is chaotic in the sense of Martelli and Wiggins, when it is a transitive distributively chaotic in a sequence. Then, we give a sufficient condition for the dynamical system to be chaotic in the strong sense of Li-Yorke. We also prove that a dynamical system is distributively chaotic in a sequence, when it is chaotic in the strong sense of Li-Yorke. 1. Introduction Since Li and Yorke first gave the definition of chaos by using strict mathematical language in 1975 [1], the research on chaos has greatly influenced modern science, not just natural sciences but also several social sciences, such as economics, sociology, and philosophy. The theory of chaos convinced scientists that a simple definite system can produce complicated features and a complex system instead possibly follows a simple law. However, scientists in different fields, finding different chaotic connotations, gave different definitions of chaos such as Li-Yorke chaos, distributional chaos, and Devaney chaos. In order to establish a satisfactory definitional and terminological framework for complex dynamical systems that are based on strict mathematical definitions, these concepts with less ambiguous are necessary, and their interdependence has to be clarified. There is no doubt that the mathematical definition of Li-Yorke chaos has a large influence than any other one, whereas distributional chaos possesses some statistical connotations besides the uncertainty of long-term behaviors. So, comparing distributional chaos with Li-Yorke chaos is a meaningful and significant problem. In order to reveal the inner relations between Li-Yorke chaos and distributional chaos, the author brought up the definition of distributional chaos in a sequence in [2]. In this paper, we mainly prove the relations between some different chaoses in discrete dynamical systems. The main theorems are stated as follows. Theorem 1. If a dynamical system (饾憢, 饾憮) exhibits transitive distributional chaos in a sequence, then, (1) it is chaotic in the sense of Martelli; (2) it is chaotic in the sense of Wiggins. Theorem 2. Let 饾憥, 饾憦 ∈ 饾憢 with 饾憥 =谈 饾憦 and 饾憹饾憳 → ∞ be a sequence of positive integers. If for any sequence 饾惗 = 饾惗1 ⋅ ⋅ ⋅ 饾惗饾憳 ⋅ ⋅ ⋅ where 饾惗饾惥 = 饾惖(饾憥, 1/饾憳) or 饾惖(饾憦, 1/饾憳)(饾惖(饾憥, 1/饾憳) = {饾懃 | 饾憫(饾憥, 饾懃) < (1/饾憳)}) there exists 饾懃(饾惗) ∈ 饾憢, such that for each 饾憳 ≥ 1, 饾憮饾憹饾憳 (饾懃(饾惗)) ∈ 饾惗饾憳 then system (饾憢, 饾憮) is chaotic in the strong sense of Li-Yorke. Theorem 3. If a dynamical system (饾憢, 饾憮) exhibits chaotic in the strong sense of Li-Yorke, then it is distributively chaotic in a sequence. 2. Problem Statement and Preliminaries Throughout this paper 饾憢 will denote a compact metric space with metric 饾憫. 2.1. Several Definitions and Lemmas. 饾惙 ⊂ 饾憢 is said to be a chaotic set of 饾憮 if for any pair (饾懃, 饾懄) ∈ 饾惙 × 饾惙, 饾懃 =谈 饾懄, we have lim inf 饾憫 (饾憮饾憶 (饾懃) , 饾憮饾憶 (饾懄)) = 0, 饾憶→∞ lim sup 饾憫 (饾憮饾憶 (饾懃) , 饾憮饾憶 (饾懄)) > 0. 饾憶→∞ (1) 2 Journal of Applied Mathematics Definition 4. 饾憮 is said to be chaotic in the sense of Li and Yorke (for short: Li-Yorke chaotic), if it has a chaotic set 饾惙 which is uncountable. Let {饾憹饾憱 } be an increasing sequence of positive integers, 饾懃, 饾懄 ∈ 饾憢, 饾憽 > 0. Let 1 饾憶 饾惞饾懃饾懄 (饾憽, {饾憹饾憱 }) = lim inf ∑ 饾湌[0,饾憽) (饾憫 (饾憮饾憹饾憳 (饾懃) , 饾憮饾憹饾憳 (饾懄))) , 饾憶→∞ 饾憶 饾憳=1 1 饾憶 ∗ (饾憽, {饾憹饾憱 }) = lim sup ∑ 饾湌[0,饾憽) (饾憫 (饾憮饾憹饾憳 (饾懃) , 饾憮饾憹饾憳 (饾懄))) , 饾惞饾懃饾懄 饾憶→∞ 饾憶 饾憳=1 (2) where 饾湌饾惔 (饾懄) is 1 if 饾懄 ∈ 饾惔 and 0 otherwise. Obviously, 饾惞饾懃饾懄 ∗ and 饾惞饾懃饾懄 are both nondecreasing functions. If for 饾憽 ≤ 0 we ∗ ∗ define 饾惞饾懃饾懄 (饾憽) = 饾惞饾懃饾懄 (饾憽) = 0, then 饾惞饾懃饾懄 and 饾惞饾懃饾懄 are probability distributional functions. Definition 5. Let 饾惙 ⊂ 饾憢, If ∀饾懃, 饾懄 ∈ 饾惙, 饾懃 =谈 饾懄, we have (1) ∃饾浛 > 0, 饾惞饾懃饾懄 = (饾浛, {饾憹饾憱 }) = 0, (2) ∀饾憽 > 0, ∗ (饾憽, {饾憹饾憱 }) = 1, 饾惞饾懃饾懄 (3) then 饾惙 is said to be a distributively chaotic set in a sequence. The two points are said to be distributively chaotic point pair in a sequence. 饾憮 is said to be distributively chaotic in a sequence, if 饾憮 has a distributively chaotic set in a sequence which is uncountable. Definition 6. Let 饾憜 ⊂ 饾憢. If there exist two strictly increasing sequences of positive integers {饾憹饾憱 } and {饾憺饾憱 } such that for any 饾懃, 饾懄 ∈ 饾憜, 饾懃 =谈 饾懄, lim 饾憫 (饾憮饾憹饾憱 (饾懃) , 饾憮饾憹饾憱 (饾懄)) = 0, 饾憱→∞ lim 饾憫 (饾憮饾憺饾憱 (饾懃) , 饾憮饾憺饾憱 (饾懄)) > 0, (4) 饾憱→∞ then 饾憜 is said to be a strong scrambled set. 饾憮 is said to be chaotic in the strong sense of Li-Yorke, if 饾憮 has an uncountable strong scrambled set. Definition 7. Let {饾憹饾憱 } be an increasing sequence of positive integers, then, PR (饾憮, {饾憹饾憱 }) = {(饾懃, 饾懄) ∈ 饾憢 × 饾憢 | ∀饾渶 > 0, (5) ∃饾憱 ∈ 饾憗 such that 饾憫 (饾憮饾憹饾憱 (饾懃) , 饾憮饾憹饾憱 (饾懄)) < 饾渶} is called proximal relation with respect to {饾憹饾憱 }. Thus 饾憱→∞ is called asymptotic relation with respect to {饾憹饾憱 }. DR (饾憮, {饾憹饾憱 }) = 饾憢 × 饾憢 − PR (饾憮, {饾憹饾憱 }) (7) is called distal relation with respect to {饾憹饾憱 }. So DCR (饾憮, {饾憹饾憱 }) = {(饾懃, 饾懄) ∈ 饾憢 × 饾憢 | (饾懃, 饾懄) is a distributively chaotic point pair of 饾憮 in 饾憹饾憱 } (8) is called distributively chaotic respect to {饾憹饾憱 }. Definition 8 (see [3]). 饾憮 is (topologically) transitive if for any two nonempty open sets 饾憟, 饾憠 ⊂ 饾憢 there exists 饾憶 > 0 such that 饾憮饾憶 (饾憟) ∩ 饾憠 =谈 0. 饾憮 is (topologically) weakly mixing if for any three nonempty open sets 饾憟, 饾憠, 饾憡 ⊂ 饾憢 there exists 饾憶 > 0 such that 饾憮饾憶 (饾憡) ∩ 饾憠 =谈 0 and 饾憮饾憶 (饾憡) ∩ 饾憟 =谈 0. Definition 9 (see [4–6]). Let 饾憮 be a continuous map from a compact metric space (饾憢, 饾憫) into itself. The orbit of a point 饾懃 ∈ 饾憢 is said to be unstable if there exists 饾憻 > 0 such that for every 饾湒 > 0 there are 饾懄 ∈ 饾憢 and 饾憶 ≥ 1 satisfying inequalities 饾憫(饾懃, 饾懄) < 饾湒 and 饾憫(饾憮饾憶 (饾懃), 饾憮饾憶 (饾懄)) > 饾憻. The map 饾憮 is said to be chaotic in the sense of Martelli if there exists 饾懃0 ∈ 饾憢 such that 饾懃0 has dense orbit which is unstable. Definition 10 (see [7]). Let 饾憮 be a continuous map from a compact metric space (饾憢, 饾憫) into itself. We say 饾憮 has sensitive dependence on initial conditions if there exists 饾憻 > 0 such that for any 饾懃 ∈ 饾憢 and 饾湒 > 0, there is some 饾懄 ∈ 饾憢 and a nonnegative integer 饾憶 satisfying 饾憫(饾懃, 饾懄) < 饾湒 and 饾憫(饾憮饾憶 (饾懃), 饾憮饾憶 (饾懄)) > 饾憻. 饾憮 is said to be chaotic in the sense of Wiggins, if 饾憮 is transitive and has sensitive dependence on initial conditions. Definition 11. Let (饾憢, 饾憫) be a compact metric space, 饾憮 : 饾憢 → 饾憢 be a continuous map, and 饾惙 be an uncountable distributively scrambled set in a sequence. We say that 饾憮 exhibits dense distributional chaos in a sequence if the set 饾惙 may be chosen to be dense. If 饾惙 is not only dense but additionally consists of points with dense orbits, then we say that 饾憮 exhibits transitive distributional chaos in a sequence. Lemma 12. Let Σ be an infinite sequence set of {0, 1}. Then, there exists an uncountable subset 饾惛 ⊂ Σ such that for any different points 饾憼 = 饾憼1 饾憼2 , . . . , 饾憽 = 饾憽1 饾憽2 , . . . , 饾憼饾憵 =谈 饾憽饾憵 for infinitely many 饾憵 and 饾憼饾憶 = 饾憽饾憶 for infinitely many 饾憶. Proof. For a proof, see [8]. AR (饾憮, {饾憹饾憱 }) = {(饾懃, 饾懄) ∈ 饾憢 × 饾憢 | lim 饾憫 (饾憮饾憹饾憱 (饾懃) , 饾憮饾憹饾憱 (饾懄)) = 0} Thus (6) Lemma 13. If {饾憹饾憱 } and {饾憺饾憱 } are both infinite increasing subsequences of {饾憵饾憱 } which is a sequence of positive integers, then there exists an infinite increasing subsequence {饾憽饾憱 } ⊂ {饾憵饾憱 } such that AR (饾憮, {饾憹饾憱 }) ∩ DR (饾憮, {饾憺饾憱 }) ⊂ DCR (饾憮, {饾憽饾憱 }) . (9) Journal of Applied Mathematics 3 Proof. For a proof, see [9]. Lemma 14. 饾憮 is weakly mixing if and only if for any 饾憵 ≥ 2, 饾憮饾憵 is transitive. Proof of Theorem 3. Because 饾憮 is chaotic in the strong sense of Li-Yorke, there exists an infinite increasing sequence {饾憹饾憱 } ⊂ N and uncountable set 饾憜 ⊂ 饾憢, such that for any 饾懃, 饾懄 ∈ 饾憜 with 饾懃 =谈 饾懄, we have lim 饾憫 (饾憮饾憹饾憱 (饾懃) , 饾憮饾憹饾憱 (饾懄)) = 0, Proof. For a proof, see [10]. 饾憱→∞ 3. Proof of Main Theorem Proof of Theorem 1. (1) There is no isolated points in 饾憢 as otherwise the set of points with dense orbit is at most countable. But in the case of compact set without isolated points, the existence of dense orbit implies transitivity. Let 饾惙 be a dense scrambled set in the sequence {饾憹饾憳 } consisting of transitive points, and let 饾憻 > 0 be such that 饾惞饾懃饾懄 (饾憻, {饾憹饾憳 }) = 0 for all distinct 饾懃, 饾懄 ∈ 饾惙. Let us fix any 饾懃0 ∈ 饾惙. Because orbit of 饾懃0 is dense, for any 饾湒 > 0, there exists 饾懄 ∈ 饾惙 and 饾憳 ≥ 1 satisfying the inequalities 饾憫(饾懃0 , 饾懄) < 饾湒 and 饾惞饾懃0 饾懄 (饾憻, {饾憹饾憳 }) = 0. This implies that 饾憫(饾憮饾憹饾憳 (饾懃0 ), 饾憮饾憹饾憳 (饾懄)) > 饾憻 for some 饾憳 ≥ 1. This shows that the orbit of 饾懃0 is unstable. So, (饾憢, 饾憮) is chaotic in the sense of Martelli. (2) Fix any 饾湒 > 0. In 饾湒-neighborhood of any point 饾懃, we can find points 饾懄, 饾懅 ∈ 饾惙 such that 饾憫(饾憮饾憹饾憳 (饾懄), 饾憮饾憹饾憳 (饾懅)) > 饾憻. Then, 饾憫(饾憮饾憹饾憳 (饾懃), 饾憮饾憹饾憳 (饾懄)) > 饾憻/2 or 饾憫(饾憮饾憹饾憳 (饾懃), 饾憮饾憹饾憳 (饾懅)) > 饾憻/2. Proof of Theorem 2. Let 饾惛 be an uncountable subset of Σ, as in Lemma 12. For each 饾憼 = 饾憼0 饾憼1 ⋅ ⋅ ⋅ 饾憼饾憶 ⋅ ⋅ ⋅ ∈ 饾惛, by the hypotheses, we can choose a point 饾懃(饾憼) ∈ 饾憢 such that for any 饾憳, if 饾憶! < 饾憳 ≤ (饾憶 + 1)! then, 饾懃饾憹饾憼 饾憳 1 { { {饾惖 (饾憥, 饾憳 ) if 饾憼饾憶 = 0, ∈{ { 1 { 饾惖 (饾憦, ) if 饾憼饾憶 = 1. { 饾憳 (10) Put 饾惙 = {饾懃饾憼 | 饾憼 ∈ 饾惛}. Clearly, if 饾憼 =谈 饾憽 then 饾懃饾憼 =谈 饾懃饾憽 . It follows that 饾惛 being uncountable implies so is 饾惙. Let 饾懃饾憼 , 饾懃饾憽 ∈ 饾惙 be any different points, where 饾憼 = 饾憼0 饾憼1 ⋅ ⋅ ⋅ 饾憼饾憱 ⋅ ⋅ ⋅ , 饾憽 = 饾憽0 饾憽1 ⋅ ⋅ ⋅ 饾憽饾憱 ⋅ ⋅ ⋅ ∈ 饾惛. By the property of 饾惛, we know that there exist sequences of positive integers 饾憵饾憱 → ∞, 饾憶饾憱 → ∞ such that 饾憼饾憵饾憱 =谈 饾憽饾憵饾憱 , 饾憼饾憶饾憱 = 饾憽饾憶饾憱 for all 饾憱, and for 饾憵饾憱 饾憼 饾憽 large enough 1/饾憵 < 饾憫(饾憥, 饾憦)/4 = 饾浛, we have 饾憫(饾懃饾憵 , 饾懃饾憵 ) > 饾浛. 饾憱 饾憱 Thus, 饾憼 饾憽 lim 饾憫 (饾懃饾憵 , 饾懃饾憵 ) ≥ 饾浛, 饾憱 饾憱 (11) 饾憼 饾憽 lim sup 饾憫 (饾懃饾憵 , 饾懃饾憵 ) ≥ 饾浛. 饾憱 饾憱 (12) 饾憱→∞ this shows 饾憱→∞ Meanwhile, for 饾憶饾憱 large enough, 饾懃饾憶饾憼 饾憱 of diameter less than 1/饾憶饾憱 . Thus, and 饾懃饾憶饾憽 饾憱 lie in the same ball 饾憫(饾懃饾憶饾憼 饾憱 , 饾懃饾憶饾憽 饾憱 ) < 1/饾憶, so lim 饾憫 (饾懃饾憶饾憼 饾憱 , 饾懃饾憶饾憽 饾憱 ) = 0. (13) lim inf 饾憫 (饾懃饾憶饾憼 饾憱 , 饾懃饾憶饾憽 饾憱 ) = 0. (14) 饾憱→∞ This shows 饾憱→∞ Above all, (饾憢, 饾憮) is chaotic in the strong sense of Li-Yorke. (15) so that (饾懃, 饾懄) ∈ AR(饾憮, {饾憹饾憱 }). Again, by the definition of chaos in the strong sense of Li-Yorke, there exists {饾憺饾憱 } ⊂ N, such that lim 饾憫 (饾憮饾憺饾憱 (饾懃) , 饾憮饾憺饾憱 (饾懄)) > 0, 饾憱→∞ (16) so that (饾懃, 饾懄) ∈ DR(饾憮, {饾憺饾憱 }). Hence, 饾憜 × 饾憜 − Δ ⊂ AR(饾憮, {饾憹饾憱 }) ∩ DR(饾憮, {饾憺饾憱 }), where Δ = {(饾懃, 饾懃); 饾懃 ∈ 饾憢}. Then by Lemma 13, there exists a subsequence {饾憽饾憱 } ⊂ N such that 饾憜 × 饾憜 ⊂ DCR(饾憮, {饾憽饾憱 }). This shows that 饾憜 is a distributively chaotic set in the sequence {饾憽饾憱 } of 饾憮. Corollary 15. If system (饾憢, 饾憮) satisfies conditions of Theorem 2. then it is distributively chaotic in the sequence. Proof. By Theorems 2 and 3, we can easily prove it. Corollary 16. Let (饾憢, 饾憫) be a locally compact metric space containing at least two points. If system (饾憢, 饾憮) is weakly mixing, then it must be chaotic in the strong sense of Li-Yorke. Proof. Let 饾憮 be weakly mixing, 饾憥, 饾憦 ∈ 饾憢 with 饾憥 =谈 饾憦. Take arbitrarily a nonempty open set 饾憠0 ⊂ 饾憢 such that 饾憠0 is compact. Since 饾憮 is weakly mixing, there exists 饾憹1 > 0 such that 饾憮饾憹1 (饾憠0 ) ∩ 饾惖(饾憥, 1/饾憳) =谈 0 and 饾憮饾憹1 (饾憠0 ) ∩ 饾惖(饾憦, 1/饾憳) =谈 0. Thus, we find points 饾懃1 , 饾懃2 such that 饾憮饾憹1 (饾懃1 ) ∈ 饾惖(饾憥, 1/饾憳), 饾憮饾憹1 (饾懃2 ) ∈ 饾惖(饾憦, 1/饾憳). Assume that there exist positive integers 饾憹1 < 饾憹2 < ⋅ ⋅ ⋅ < 饾憹饾憳 such that for each finite sequence 饾惔 1 饾惔 2 ⋅ ⋅ ⋅ 饾惔 饾憳 , where 饾惔 饾憱 ∈ {饾惖(饾憥, 1/饾憱), 饾惖(饾憦, 1/饾憱)}, there is a point 饾懃 ∈ 饾憠0 satisfying 饾憮饾憹饾憱 (饾懃) ∈ 饾惔 饾憱 for 饾憱 = 1, 2, . . . , 饾憳, the set of all such points will denoted by 饾憜饾憳 . By continuity of 饾憮, each 饾懃 ∈ 饾憜饾憳 has an open nonempty neighborhood 饾憡饾懃 ⊂ 饾憠0 such that 饾憮饾憹饾憱 (饾憡饾懃 ) ⊂ 饾惔 饾憱 , if 饾憮饾憹饾憱 (饾懃) ∈ 饾惔 饾憱 , it follows from Lemma 14 that there exists 饾憹饾憳+1 > 饾憹饾憳 such that for each 饾懃 ∈ 饾憜饾憳 , 饾憮饾憹饾憳+1 (饾憡饾懃 )∩饾惖(饾憥, 1/(饾憳+1)) =谈 0 and 饾憮饾憹饾憳+1 (饾憡饾懃 ) ∩ 饾惖(饾憦, 1/(饾憳 + 1)) =谈 0. Thus by induction, we know that there exists a sequence 饾憹饾憳 → ∞ of positive integers such that for any finite sequence 饾惔 1 ⋅ ⋅ ⋅ 饾惔 饾憳 , there is a point 饾懃 ∈ 饾憠0 satisfying 饾憮饾憹饾憱 (饾懃) ∈ 饾惔 饾憱 , 1 ≤ 饾憱 ≤ 饾憳. Let 饾惗 = 饾惗1 饾惗2 ⋅ ⋅ ⋅ be an infinite sequence, where 1 1 饾惗饾憳 ∈ {饾惖 (饾憥, ), 饾惖 (饾憦, )} . 饾憳 饾憳 (17) For each 饾憳, we can take a point 饾懃饾憳 ∈ 饾惗饾憳 such that 饾憮饾憹饾憱 (饾懃饾憳 ) ∈ 饾惗饾憱 , 1 ≤ 饾憱 ≤ 饾憳. (18) Since 饾憠0 is compact, the infinite sequence {饾懃饾憱 } has a limit point in 饾憠0 , say 饾懃饾惗, it is not difficult to show 饾憮饾憹饾憳 (饾懃饾惗) ⊂ 饾惗饾憳 . Thus by Theorem 2, 饾憮 is a strong chaos in the sense of LiYorke. 4 Acknowledgment This work is supported by the NSFC no. 11271061, the NSFC no. 11001038, the NSFC no. 61153001, and the Independent Research Foundation of the Central Universities no. DC 12010111. References [1] T. Y. Li and J. A. Yorke, “Period three implies chaos,” The American Mathematical Monthly, vol. 82, no. 10, pp. 985–992, 1975. [2] L. D. Wang, G. F. Huang, and S. M. Huan, “Distributional chaos in a sequence,” Nonlinear Analysis, vol. 67, no. 7, pp. 2131–2136, 2007. [3] P. 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