Hindawi Publishing Corporation Discrete Dynamics in Nature and Society Volume 2011, Article ID 506373, 12 pages doi:10.1155/2011/506373 Research Article Global Attractivity of a Family of Max-Type Difference Equations Xiaofan Yang,1 Wanping Liu,1 and Jiming Liu2 1 2 College of Computer Science, Chongqing University, Chongqing 400044, China Department of Computer Science, Hong Kong Baptist University, Kowloon Tong, Hong Kong Correspondence should be addressed to Wanping Liu, wanping liu@yahoo.cn Received 20 October 2010; Accepted 13 January 2011 Academic Editor: Ibrahim Yalcinkaya Copyright q 2011 Xiaofan Yang 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 propose to study a generalized family of max-type difference equations and then prove the global attractivity of a particular case of it under some parameter conditions. Through some numerical results of other cases, we finally pose a generic conjecture. 1. Introduction The study of max-type difference equations is a hotspot in the area of discrete dynamics because such equations are often closely related to automatic control theory and competitive dynamics. For recent advances in this direction see 1–8 and the references therein. Motivated by 9, Liu et al. 10 studied the following nonautonomous max-type difference equation: yn p ryn−s , q φn yn−1 , . . . , yn−m yn−s n ∈ N0 , 1.1 where p ≥ 0, r, q > 0, s, m ∈ N, and φn : R m → R , n ∈ N0 are mappings satisfying the condition β min{x1 , . . . , xm } ≤ φn x1 , x2 , . . . , xm ≤ β max{x1 , . . . , xm }, for some fixed β ∈ 0, ∞. When p 0, β ∈ 0, 1, they proved that every positive solution to 1.1 converges to zero if r ≤ q, while r − q/1 β if r > q. If p > 0 and rq ≥ p, then each positive solution to 1.1 converges to q − r2 4p1 β − q − r/21 β, for some β ∈ 0, ∞, except for the case q < r, β ∈ β0 , ∞, where β0 4p/q − r2 1. Note that the behavior of positive solutions to 1.1 for the case q < r, β ∈ β0 , ∞, is still an unsolved open problem as was mentioned in 10. 2 Discrete Dynamics in Nature and Society Here, we propose to investigate the asymptotic behavior of positive solutions to the generalized family of max-type difference equations pi ri xn−s , xn max 1≤i≤k qi xn−s fi xn−1 , . . . , xn−m n ∈ N0 , 1.2 where pi ≥ 0, ri , qi > 0, s, m, k ∈ N, k ≥ 2 and the functions fi : 0, ∞m → 0, ∞, i 1, 2, . . . , k satisfy the condition β min{u1 , . . . , um } ≤ fi u1 , u2 , . . . , um ≤ β max{u1 , . . . , um }, 1.3 for some fixed β ∈ 0, 1. In this paper, we mainly consider the particular case that all pi are zero, and then obviously 1.2 reduces to the following form: xn xn−s × max 1≤i≤k ri , qi xn−s fi xn−1 , . . . , xn−m n ∈ N0 . 1.4 Let x∗ be a nonnegative equilibrium point of 1.4, then we have ri . x x × max 1≤i≤k qi 1 β x∗ ∗ ∗ 1.5 It follows directly from 1.5 that if 0 < ri ≤ qi for all i 1, 2, . . . , k, then 1.4 has the unique nonnegative equilibrium x∗ 0, while if there exists at least one j ∈ {1, 2, . . . , k} such that rj > qj , then 1.4 has a zero equilibrium x∗ 0 and a unique positive equilibrium x∗ max1≤i≤k {ri − qi }/1 β. Finally, the following two beautiful theorems are derived. Theorem 1.1. Consider 1.4 with condition 1.3. If 0 < ri ≤ qi for all i 1, 2, . . . , k, then every positive solution to 1.4 converges to the unique nonnegative equilibrium zero. Theorem 1.2. Consider 1.4 with positive initial values and positive ri and qi . Let fi : 0, ∞m → 0, ∞ be functions such that for some fixed β ∈ 0, 1, there hold β min{u1 , . . . , um } ≤ fi u1 , . . . , um ≤ β max{u1 , . . . , um }, i 1, 2, . . . , k. 1.6 If there exists at least one j ∈ {1, 2, . . . , k} such that rj > qj , then the unique positive equilibrium of 1.4 is a global attractor. 2. Preliminary Lemmas For the purpose of establishing the main results, some auxiliary lemmas are essential. Discrete Dynamics in Nature and Society 3 Lemma 2.1. Consider the first-order difference equation xn xn−1 × max 1≤i≤k ri , qi xn−1 n ∈ N0 , 2.1 with positive initial value x−1 and positive ri and qi . If there exists at least one j ∈ {1, 2, . . . , k} such that rj > qj , then lim xn max ri − qi : i 1, 2, . . . , k . n→∞ 2.2 Proof. Suppose that max{ri − qi : i 1, 2, . . . , k} rτ − qτ , which is positive, for some τ ∈ {1, 2, . . . , k}. By making the variable change xn rτ − qτ yn into 2.1 and then canceling the positive term rτ − qτ from the resulting equation, we can derive ri yn yn−1 × max , 1≤i≤k qi rτ − qτ yn−1 n ∈ N0 . 2.3 Note that min{a1 /b1 , a2 /b2 } ≤ a1 a2 /b1 b2 ≤ max{a1 /b1 , a2 /b2 } for ai , bi > 0, i 1, 2. Then it follows from 2.3 that yn1 qi yn ri − qi yn qi yn rτ − qτ yn ≤ max ≤ max yn , 1 . max 1≤i≤k 1≤i≤k qi rτ − qτ yn qi rτ − qτ yn 2.4 In addition, the following two inequalities hold: qτ yn − 1 ri yn rτ yn yn1 − 1 max 2.5 −1 ≥ −1 , 1≤i≤k qi rτ − qτ yn qτ rτ − qτ yn qτ rτ − qτ yn rτ − qτ yn 1 − yn ri yn rτ yn . yn1 − yn max − yn ≥ − yn 1≤i≤k qi rτ − qτ yn qτ rτ − qτ yn qτ rτ − qτ yn 2.6 In the following, we are confronted with three possibilities. Case 1. If there exists n0 ≥ −1 such that yn0 1, then it follows from 2.4 and 2.5 that yn 1 holds for all n ≥ n0 . Case 2. If there exists n0 ≥ −1 such that yn0 > 1, then it follows from 2.5 and 2.6 that yn0 ≥ yn0 1 ≥ yn0 2 ≥ · · · > 1. 2.7 4 Discrete Dynamics in Nature and Society Thus there is a finite limit γ limn → ∞ yn ≥ 1. By taking the limits on both sides of 2.3 and canceling the positive factor γ from the resulting equation, we obtain 1 max ri 2.8 , qi rτ − qτ γ 1≤i≤k which implies γ 1. Because if γ > 1, then 1 max ri qi rτ − qτ γ 1≤i≤k ri < max 1≤i≤k qi rτ − qτ 1, 2.9 leading to a contradiction. Case 3. If yn < 1 for all n ≥ −1, then it follows from 2.5 and 2.6 that y−1 < y0 < y1 < · · · < yn < · · · < 1. 2.10 Therefore, the limit of yn exists, denoted by 0 < γ limn → ∞ yn ≤ 1. By taking the limits on both sides of 2.3 and canceling the nonzero factor γ from the resulting equation, there hold 1 max 1≤i≤k ri 2.11 , qi rτ − qτ γ which implies γ 1. Because if 0 < γ < 1, then 1 max 1≤i≤k ri qi rτ − qτ γ ri > max 1≤i≤k qi rτ − qτ 1, 2.12 which is a contradiction. In either of the above three cases, we get limn → ∞ yn 1, implying limn → ∞ xn rτ − qτ . From Lemma 2.1, we have the following result. Lemma 2.2. Consider the s-order difference equation ri xn xn−s × max , 1≤i≤k qi xn−s n ∈ N0 , 2.13 with positive initial values and ri , qi > 0. If there exists at least one j ∈ {1, 2, . . . , k} such that rj > qj , then lim xn max ri − qi : i 1, 2, . . . , k . n→∞ 2.14 Discrete Dynamics in Nature and Society 5 Proof. Let {xn }n≥−s be an arbitrary positive solution to 2.13. Apparently we know that the sequence {xn }n≥−s can be divided into s subsequences {xjsk }k≥0 , j −s, −s 1, . . . , −1, which are, respectively, positive solutions to the first-order equation 2.1 with positive initial values x−s , x−s1 , . . . , x−1 . According to Lemma 2.1, we derive limk → ∞ xjsk max{ri − qi : i 1, 2, . . . , k} for all j −s, −s 1, . . . , −1, which directly lead to limn → ∞ xn max{ri − qi : i 1, 2, . . . , k}. Lemma 2.3. Let a > b > 0, 0 < β < 1, and 0 < < 1 − β/1 βa − b. Define two sequences {mk } and {Mk } in the following way: M1 a − b, , k 1, 2, . . . , m k M1 − β M k k Mk M1 − β mk−1 − , k 2, 3, . . . . k − 1 2.15 Then limk → ∞ mk limk → ∞ Mk . Proof. Observe that M2 − M1 −β 1 − β a − b − β 1 < 0, , k 1, 2, . . . , mk1 − mk β Mk − Mk1 kk 1 , k 2, 3, . . . . Mk1 − Mk −β mk − mk−1 kk − 1 2.16 It follows by induction that {mk } is increasing and {Mk } is decreasing. Again by induction we derive mk < a − b and Mk > 0, k 1, 2, . . . . Hence there are two finite limits ξ limk → ∞ mk and η limk → ∞ Mk . By taking limits on both sides of 2.15, we derive ξ a − b − βη, η a − b − βξ, 2.17 which imply 1 − βξ − η 0. Therefore ξ η a − b/1 β. 3. Proofs of Main Theorems In this section, we are in a position to prove the main theorems presented in Section 1. Proof of Theorem 1.1. Note that for the case ri < qi , i 1, 2, . . . , k, the behavior of positive solutions to 1.4 is quite simple. In this case, we have that ri xn ≤ xn−s × max 1≤i≤k qi μxn−s , 3.1 6 Discrete Dynamics in Nature and Society where μ max1≤i≤k {ri /qi } < 1. Easily the subsequences {xlsj }l∈N0 , j ∈ {0, 1, . . . , s − 1} converge to zero, hence the sequence {xn } also converges to zero. For the case ri ≤ qi , i 1, 2, . . . , k with at least one j ∈ {1, 2, . . . , k} such that rj qj , we can obtain that ri xn ≤ xn−s × max 1≤i≤k qi xn−s . 3.2 In this case, the subsequences {xlsj }l∈N0 , j 0, 1, . . . , s − 1 are all positive and nonincreasing, thus they converge, respectively, to some nonnegative limits ψj : liml → ∞ xlsj , j 0, 1, . . . , s−1. If we replace n in 1.4 by sl j, l ∈ N0 for an arbitrary fixed j ∈ {0, 1, . . . , s − 1} and let l → ∞, we can get ri ψj ψj × max , 1≤i≤k qi ψj fi ψv1 , . . . , ψvm 3.3 where vi ∈ {0, 1, . . . , s − 1}, i 1, . . . , m. Without loss of generality, assume that ψj / 0, then we obtain that 1 rτ , qτ ψj fτ ψv1 , . . . , ψvm 3.4 with some fixed number τ ∈ {1, 2, . . . , k}. Because rτ ≤ qτ , then it follows from 3.4 that qτ ψj fτ ψv1 , . . . , ψvm rτ ≤ qτ . 3.5 ψj fτ ψv1 , . . . , ψvm 0, 3.6 Therefore we have leading to ψj 0, which is a contradiction. Hence we have that ψj 0, j 0, 1, . . . , s − 1, and every positive solution to 1.4 converges to zero, if ri ≤ qi for all i 1, 2, . . . , k. Proof of Theorem 1.2. Suppose that max{ri − qi : i 1, 2, . . . , k} rτ − qτ > 0 for some τ ∈ {1, 2, . . . , k}. Let be an arbitrary fixed real number with 0 < < 1 − β/1 βrτ − qτ . Define two sequences {Mk } and {mk } in the way shown in 2.15 with a rτ , b qτ . Let {xn } be an arbitrary positive solution to 1.4. Next, we proceed by proving two claims. Claim 1. There exists N1 ∈ N such that m1 − ≤ xn ≤ M1 for all n ≥ N1 . Proof of Claim 1. Note that ri xn ≤ xn−s × max , 1≤i≤k qi xn−s n 0, 1, 2, . . . . 3.7 Discrete Dynamics in Nature and Society 7 Consider the following difference equation: 1 zn 1 zn−s × max 1≤i≤k ri 1 qi zn−s , n 0, 1, 2, . . . . 1 1 3.8 1 1 Let {zn } be a positive solution to 3.7 with the initial values z−1 x−1 , z−2 x−2 , . . . , z−s x−s . Note that the mapping hx rx/q x is strictly increasing on the interval 0, ∞. 1 1 It follows by induction that xn ≤ zn for all n ≥ −s. By Lemma 2.2, we have limn → ∞ zn rτ − qτ M1 . Hence there is an integer N1 ∈ N such that xn ≤ M1 for n ≥ N1 . Let t max{s, m}. Note that ri , xn ≥ xn−s × max 1≤i≤k qi xn−s βM1 n ≥ N1 t. 3.9 Consider the difference equation 1 yn 1 yn−s 1 1 1 1 × max 1≤i≤k ri 1 qi yn−s βM1 n ≥ N1 t, , 3.10 1 with yN t−1 xN1 t−1 , yN t−2 xN1 t−2 , . . . , yN xN1 . Note the monotonicity of hx, it 1 1 1 follows by induction that xn ≥ yn for all n ≥ N1 . By Lemma 2.2, we get that limn → ∞ yn m1 . Thus there exists an integer N1 ≥ N1 such that xn ≥ m1 − for all n ≥ N1 . Working inductively, we will reach the following claim. Claim 2. For every k ∈ N, there exists Nk ∈ N such that mk − ≤ xn ≤ Mk , k k 3.11 for all n ≥ Nk . Proof of Claim 2. Obviously, the case k 1 follows directly from Claim 1. In the following, we proceed by induction. Assume that the assertion is true for k ωω ≥ 1. Then it suffices to prove the assertion is also true for k ω 1. Note that xn ≤ xn−s × max 1≤i≤k ri , qi xn−s βmω − /ω n ≥ Nω t. 3.12 Consider the difference equation ω1 zn ω1 zn−s × max 1≤i≤k ri ω1 qi zn−s βmω − /ω , n ≥ Nω t, 3.13 8 Discrete Dynamics in Nature and Society ω1 ω1 ω1 with zNω t−1 xNω t−1 , zNω t−2 xNω t−2 , . . . , zNω ω1 zn xNω . Note the monotonicity of hx, it follows by induction that xn ≤ for all n ≥ Nω . By Lemma 2.2, we have that ω1 limn → ∞ zn Mω1 . So there is an integer Nω1 ∈ N such that xn ≤ Mω1 /ω 1 for all n ≥ Nω1 . Then note that ri , xn ≥ xn−s × max 1≤i≤k qi xn−s βMω1 /ω 1 n ≥ Nω1 t. 3.14 Consider the following difference equation ω1 yn ω1 ω1 t−1 with yN ω1 yn−s × max 1≤i≤k ri ω1 qi yn−s ω1 ω1 t−2 xNω1 t−1 , zN βMω1 /ω 1 ω1 xNω1 t−2 , . . . , zN ω1 ω1 yn , n ≥ Nω1 t, 3.15 xNω1 . By the monotonicity of hx, it follows by induction that xn ≥ for all n ≥ Nω1 . By Lemma 2.2, we have that ω1 limn → ∞ yn mω1 . So there is an integer Nω1 ≥ Nω1 such that xn ≥ mω1 − /ω 1 for all n ≥ Nω1 . From Claim 2, we derive ≤ lim xn ≤ lim xn ≤ lim Mk lim Mk . lim mk lim mk − n→∞ k→∞ k→∞ k→∞ k→∞ k k n→∞ 3.16 This plus Lemma 2.3 leads to that lim xn lim mk lim Mk n→∞ k→∞ k→∞ rτ − qτ . 1β 3.17 4. Simulations and Future Work In the previous section, we proved the global attractivity of 1.2 when all pi are zero. In this section, we investigate the dynamic behavior of 1.2 provided that all pi are not zero. First, it is trivial to confirm that when all pi are not zero, 1.2 has the following unique positive equilibrium point x∗ max1≤i≤k { qi − ri 2 4pi 1 β ri − qi }/21 β. In the following, some numerical results are presented. Experiment 1. Consider the first-order difference equation xn max 0.2 0.6xn−1 rxn−1 , , 0.6 xn−1 0.3xn−1 q xn−1 0.3xn−1 where r, q > 0 and the initial value x0 > 0. See Figures 1 and 2. n ∈ N, 4.1 Discrete Dynamics in Nature and Society 9 0.7 0.4 0.38 0.65 0.36 0.6 0.34 0.32 0.55 0.3 0.28 0.5 0.26 0.45 0.24 0.4 0.22 0.2 0 10 20 30 40 50 0.35 0 a Initial value x0 0.2 0.9 0.8 0.88 0.75 0.86 0.7 0.84 0.65 0.82 0.6 0.8 0.55 0.78 10 20 30 30 40 50 40 50 √ 26/13 ≈ 0.3922. 0.85 0 20 b Initial value x0 0.7 Figure 1: r 1, q 2; x∗ 0.5 10 40 50 0.76 0 a Initial value x0 0.5 10 20 30 b Initial value x0 0.9 Figure 2: r 2, q 1; x∗ 10/13 ≈ 0.7692. Experiment 2. Consider the second-order difference equation xn max 0.5 xn−2 0.8 rxn−2 , , 1 xn−2 0.5xn−1 q xn−2 0.5xn−1 n ≥ 2, 4.2 where r, q > 0 and the initial values x0 , x1 > 0. See Figures 3 and 4. Experiment 3. Consider the third-order difference equation xn max ⎧ ⎪ ⎨ ⎫ ⎪ ⎬ 0.5 xn−3 3xn−3 , , ⎪ ⎩ 1 xn−3 0.9 x2 x2 /2 2 xn−3 0.9 x2 x2 /2 ⎪ ⎭ n−2 n−2 n−1 n−1 n ≥ 3, 4.3 where the initial values x0 , x1 , x2 > 0. See Figure 5. 10 Discrete Dynamics in Nature and Society 0.65 0.8 0.6 0.75 0.55 0.7 0.5 0.65 0.6 0.45 0.55 0.4 0.5 0.35 0.45 0.3 0.4 0.25 0.35 0.2 0.3 0 10 20 30 40 50 0 a Initial values x0 0.2, x1 0.4 10 20 30 40 50 b Initial values x0 0.8, x1 0.3 Figure 3: r 1, q 2; x∗ √ 3/3 ≈ 0.5774. 1.4 1.3 1.35 1.2 1.3 1.1 1.25 1 1.2 0.9 1.15 0.8 1.1 0.7 1.05 0.6 1 0.5 0 10 20 30 40 50 0 a Initial values x0 1.3, x1 1.0 10 20 30 40 50 b Initial values x0 0.5, x1 0.9 √ Figure 4: r 2, q 1; x∗ 5.8 1/3 ≈ 1.1361. Inspired by this work and the results of 10, here we pose the following conjecture. Conjecture 4.1. Consider 1.2 with nonnegative pi and positive ri and qi . Let fi : 0, ∞m → 0, ∞, i 1, 2, . . . , k be k functions such that for some fixed β ∈ 0, 1, there hold β min{u1 , . . . , uk } ≤ fi u1 , . . . , uk ≤ β max{u1 , . . . , uk }. 4.4 If ri qi ≥ pi for all i 1, 2, . . . , k, then every positive solution to 1.2 converges to the equilibrium point 1 x∗ max 2 1 β 1≤i≤k qi − ri 2 4pi 1 β ri − qi . 4.5 Discrete Dynamics in Nature and Society 11 0.6 1 0.55 0.9 0.5 0.8 0.45 0.4 0.7 0.35 0.6 0.3 0.5 0.25 0.4 0.2 0.3 0.15 0.1 0.2 0 50 100 0 150 a Initial values x0 0.1, x1 0.3, x2 0.5 0.8 1.1 0.7 1 0.6 0.9 0.5 0.8 0.4 0.7 0.3 0.6 0.2 0.5 0.1 0 50 100 150 c Initial values x0 0.8, x1 0.3, x2 0.1 50 100 150 b Initial values x0 0.3, x1 0.7, x2 1.0 0.4 0 50 100 150 d Initial values x0 1.1, x1 0.8, x2 0.6 Figure 5: x∗ 10/19 ≈ 0.5263. Acknowledgments This work was financially supported by the Fundamental Research Funds for the Central Universities no. CDJXS10181130, the National Natural Science Foundation of China no. 10771227, and the New Century Excellent Talent Project of China no. NCET-05-0759. References 1 K. S. Berenhaut, J. D. Foley, and S. Stević, “Boundedness character of positive solutions of a max difference equation,” Journal of Difference Equations and Applications, vol. 12, no. 12, pp. 1193–1199, 2006. 2 G. Ladas, “On the recursive sequence xn max{A1 /xn−1 , A2 /xn−2 , . . . , Ap /xn−p },” Journal of Difference Equations and Applications, vol. 2, no. 3, pp. 339–341, 1996. 3 W. Liu and X. Yang, “Quantitative bounds for positive solutions of a Stević difference equation,” Discrete Dynamics in Nature and Society, vol. 2010, Article ID 235808, 14 pages, 2010. p p 4 S. Stević, “On the recursive sequence xn1 max{c, xn /xn−1 },” Applied Mathematics Letters, vol. 21, no. 8, pp. 791–796, 2008. 5 S. 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