Hindawi Publishing Corporation Abstract and Applied Analysis Volume 2013, Article ID 871512, 6 pages http://dx.doi.org/10.1155/2013/871512 Research Article Umbral Calculus and the Frobenius-Euler Polynomials Dae San Kim,1 Taekyun Kim,2 and Sang-Hun Lee3 1 Department of Mathematics, Sogang University, Seoul 121-742, Republic of Korea Department of Mathematics, Kwangwoon University, Seoul 139-701, Republic of Korea 3 Division of General Education, Kwangwoon University, Seoul 139-701, Republic of Korea 2 Correspondence should be addressed to Taekyun Kim; tkkim@kw.ac.kr Received 27 November 2012; Accepted 19 December 2012 Academic Editor: Juan J. Trujillo Copyright © 2013 Dae San Kim 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 study some properties of umbral calculus related to the Appell sequence. From those properties, we derive new and interesting identities of the Frobenius-Euler polynomials. 1. Introduction Let C be the complex number field. For π ∈ C with π =ΜΈ 1, the Frobenius-Euler polynomials are defined by the generating function to be ∞ π‘π 1 − π π₯π‘ π»(π₯|π)π‘ = π = π» π , | π) ∑ (π₯ π ππ‘ − π π! π=0 For π ∈ Z+ , the Frobenius-Euler polynomials of order π are defined by the generating function to be ( 1 − π π π₯π‘ 1−π 1 − π π₯π‘ )π =( π‘ ) × ⋅⋅⋅ × ( π‘ )π ππ‘ − π π −π π −π βββββββββββββββββββββββββββββββββββββββββββββββββββ π-times ∞ (1) = ∑ π»π(π) (π₯ | π) π=0 (see [1–5]) with the usual convention about replacing π»π (π₯ | π) by π»π (π₯ | π). In the special case, π₯ = 0, π»π (0 | π) = π»π (π) are called the πth Frobenius-Euler numbers. By (1), we get π π (π) π»π(π) (π₯ | π) = ∑ ( ) π»π−π (π) π₯π , π (2) π=0 π=0 (see [6–9]) with the usual convention about replacing π»π (π) by π»π (π). Thus, from (1) and (2), we note that (π» (π) + 1)π − ππ»π (π) = (1 − π) πΏ0,π , where πΏπ,π is the kronecker symbol (see [1, 10, 11]). π‘ . π! In the special case, π₯ = 0, π»π(π) (0 | π) = π»π(π) (π) are called the πth Frobenius-Euler numbers of order π (see [1, 10]). From (4), we can derive the following equation: π π π»π (π₯ | π) = ∑ ( ) π»π−π (π) π₯π = (π» (π) + π₯)π , π (4) π (3) π»π(π) (π) = ∑ π1 +⋅⋅⋅+ππ =π π ( ) π»π1 (π) ⋅ ⋅ ⋅ π»ππ (π) . π1 , . . . , ππ (5) By (5), we see that π»π(π) (π₯ | π) is a monic polynomial of degree π with coefficients in Q(π). Let P be the algebra of polynomials in the single variable π₯ over C and let P∗ be the vector space of all linear functionals on P. As is known, β¨πΏ | π(π₯)β© denotes the action of the linear functional πΏ on a polynomial π(π₯) and we remind that 2 Abstract and Applied Analysis the addition and scalar multiplication on P∗ are, respectively, defined by β¨πΏ + π | π (π₯)β© = β¨πΏ | π (π₯)β© + β¨π | π (π₯)β©, β¨ππΏ | π (π₯)β© = πβ¨πΏ | π (π₯)β©, (6) where π is a complex constant (see [3, 12]). Let F denote the algebra of formal power series: 2. The Frobenius-Euler Polynomials and Umbral Calculus ∞ ππ π π‘ | ππ ∈ C} π! π=0 F = {π (π‘) = ∑ (7) (see [3, 12]). The formal power series define a linear functional on P by setting β¨π (π‘) | π₯π β© = ππ , ∀π ≥ 0. where π(π‘) is the compositional inverse of π(π‘) (see [3]). In this paper, we study some properties of umbral calculus related to the Appell sequence. For those properties, we derive new and interesting identities of the Frobenius-Euler polynomials. (8) By (4) and (12), we see that π»π(π) (π₯ | π) ∼ (( π Indeed, by (7) and (8), we get (π, π ≥ 0) (9) (see [3, 12]). This kind of algebra is called an umbral algebra. The order π(π(π‘)) of a nonzero power series π(π‘) is the smallest integer π for which the coefficient of π‘π does not vanish. A series π(π‘) for which π(π(π‘)) = 1 is said to be an invertible series (see [2, 12]). For π(π‘), π(π‘) ∈ F, and π(π₯) ∈ P, we have β¨π (π‘) π (π‘) | π (π₯)β© = β¨π (π‘) | π (π‘) π (π₯)β© = β¨π (π‘) | π (π‘) π (π₯)β© ∞ β¨β (π‘) | π π (π₯)β© π (π‘) π(π‘)π , π! π=0 β (π‘) = ∑ ∞ π (π₯) = ∑ β¨π (π‘) π(π‘)π | π (π₯)β© π! π=0 π π (π₯) , β¨π (π‘) | π (πΌπ₯)β© = β¨π (πΌπ‘ | π (π₯)β© , π (π (π‘)) ∞ π π (π¦) π π‘ , π! π=0 ππ¦π(π‘) = ∑ ∀π¦ ∈ C, (12) (14) (15) Then it is an (π + 1)-dimensional vector space over Q(π). So we see that {π»0(π) (π₯ | π), π»1(π) (π₯ | π), . . . , π»π(π) (π₯ | π)} is a basis for Pπ (π). For π(π₯) ∈ Pπ (π), let π π (π₯) = ∑ πΆπ π»π(π) (π₯ | π) , (π ≥ 0) . (16) π=0 Then, by (13), (14), and (16), we get π ππ‘ − π π β¨( ) π‘ | π (π₯)β© 1−π π π = ∑πΆπ β¨( π=0 ππ‘ − π π ) π‘ | π»π(π) (π₯ | π)β© 1−π (17) π = ∑πΆπ π!πΏπ,π = π!πΆπ . π=0 From (17), we have π πΆπ = 1 ππ‘ − π π β¨( ) π‘ | π (π₯)β© π! 1−π π = 1 ππ‘ − π β¨( ) | π·π π (π₯)β© π! 1−π = π 1 π π−π β¨πππ‘ | π·π π (π₯)β© π ∑ (π) (−π) π!(1 − π) π=0 = π 1 π π−π β¨π‘0 | πππ‘ π·π π (π₯)β© π ∑ (π) (−π) π!(1 − π) π=0 = π 1 π ( ) (−π)π−π β¨π‘0 | π·π π (π₯ + π)β© . ∑ π!(1 − π)π π=0 π (11) π (π‘) π π (π₯) = ππ π−1 (π₯) , 1 Pπ (π) = {π (π₯) ∈ Q (π) [π₯] | deg π (π₯) ≤ π} . β (π‘) ∈ F, π (π₯) ∈ P, ππ‘ − π π ) π‘ | π»π(π) (π₯ | π)β© = π!πΏπ,π . 1−π Let (10) (see [12]). One should keep in mind that each π(π‘) ∈ F plays three roles in the umbral calculus: a formal power series, a linear functional, and a linear operator. To illustrate this, let π(π₯) ∈ P and π(π‘) = ππ¦π‘ ∈ F. As a linear functional, ππ¦π‘ satisfies β¨ππ¦π‘ | π(π₯)β© = π(π¦). As a linear operator, ππ¦π‘ satisfies ππ¦π‘ π(π₯) = π(π₯ + π¦) (see [12]). Let π π (π₯) denote a polynomial in π₯ with degree π. Let us assume that π(π‘) is a delta series and π(π‘) is an invertible series. Then there exists a unique sequence π π (π₯) of polynomials such that β¨π(π‘)π(π‘)π | π π (π₯)β© = π!πΏπ,π for all π, π ≥ 0 (see [3, 12]). This sequence π π (π₯) is called the Sheffer sequence for (π(π‘), π(π‘)) which is denoted by π π (π₯) ∼ (π(π‘), π(π‘)). If π π (π₯) ∼ (1, π(π‘)), then π π (π₯) is called the associated sequence for π(π‘). If π π (π₯) ∼ (π(π‘), π‘), then π π (π₯) is called the Appell sequence. Let π π (π₯) ∼ (π(π‘), π(π‘)). Then we see that (13) Thus, by (13), we get β¨( β¨π‘π | π₯π β© = π!πΏπ,π π ππ‘ − π ) , π‘) . 1−π (18) Therefore, by (16) and (18), we obtain the following theorem. Abstract and Applied Analysis 3 Theorem 1. For π(π₯) ∈ Pπ (π), let For π ∈ Z+ , from (25), we have π½ππ (π»π(π) (π₯ | π)) = π»π(π−π ) (π₯ | π) . π π (π₯) = ∑ πΆπ π»π(π) (π₯) . (19) On the other hand, by (12), (13), and (25), π=0 Then one has πΆπ = π 1 π π−π π π· π (π) , π ∑ (π) (−π) π!(1 − π) π=0 π½ππ (π»π(π) (π₯ | π)) = ( (20) π 1 (1 − π)π Thus, by (28), we get π { π } 1 π ⋅ ∑ { ∑ ( ) (−π)π−π π·π π (π)} π»π(π) (π₯ | π) . π π! π=0 π=0 { } (21) π½ππ (π»π(π) (π₯ | π)) Μ π π(π₯) = π(π₯ + 1) − Μ π with Δ Let us consider the operator Δ Μ π . Then we have ππ(π₯) and let π½π = (1/(1 − π))Δ 1 {π (π₯ + 1) − ππ (π₯)} . 1−π π ( ππ ) ∞ ∑ π∑( π=0 (1 − π) π=π π +⋅⋅⋅+π = ∑ 1 (22) ( ππ ) ∞ 1 ∑ π∑ ( π=0 (1 − π) π=π π! π +⋅⋅⋅+π π 1 1 {π»π(π) (π₯ + 1 | π) − ππ»π(π) (π₯ | π)} . 1−π (23) ∞ ∑ {π»π(π) (π₯ + 1 | π) − ππ»π(π) (π₯ | π)} π=0 1 − π π π₯π‘ 1 − π π (π₯+1)π‘ π − π( ) )π ππ‘ − π ππ‘ − π =( 1 − π π π₯π‘ π‘ 1 − π π−1 π₯π‘ ) π (π − π) = (1 − π) ( π‘ ) π π‘ π −π π −π π=0 min{π ,π} ∑ π=0 π‘π π! =( = (1 − π) ∑ π»π(π−1) (π₯ | π) π ( ) π·π ) π1 , . . . , ππ = ( ππ ) π π ∑( ) ∑ (1 − π)π π=π π π +⋅⋅⋅+π 1 π =π π (π) ( ) π»π−π (π₯ | π) π1 , . . . , ππ ππ ≥1 min{π ,π} { { { π ( ππ ) π ∑ {( ) ∑ { π π=0 (1 − π)π π=0 { { } } } (π) π ⋅ ∑ ( )} π»π−π (π₯ | π) π1 , . . . , ππ } } π1 +⋅⋅⋅+ππ =π ππ ≥1 } π π‘ . π! (24) By (23) and (24), we get π =π ππ ≥1 ⋅ π»π(π) (π₯ | π) = From (4), we can derive ∞ π ππ ≥1 1 ) π‘π (π»π(π) (π₯ | π)) π ! ⋅ ⋅ ⋅ π ! 1 π =π = ∑ Thus, by (22), we get π½π (π»π(π) (π₯ | π)) = (28) ⋅ (π»π(π) (π₯ | π)) . From Theorem 1, we note that π½π (π) (π₯) = π ππ‘ − π ) (π»π(π) (π₯ | π)) 1−π ∞ π π‘ 1 ((1 − π) + ) = ∑ π π! (1 − π) π=1 where π·π(π₯) = ππ(π₯)/ππ₯. π (π₯) = (27) { { { π min{π ,π} ( ππ ) + ( ) ∑ ∑ { { π π=0 (1 − π)π π=min{π ,π}+1 { { π π½π (π»π(π) (π₯ | π)) = π»π(π−1) (π₯ | π) . (25) From (25), we have π½ππ (π»π(π) (π₯ | π)) = π½ππ−1 (π»π(π−1) (π₯ | π)) = ⋅ ⋅ ⋅ = π»π(0) (π₯ | π) = π₯π , π½ππ (π₯π ) = π½ππ π»π(0) (π₯ | π) = π»π(−π) (π₯ | π) = π½π2π π»π(π) (π₯ | π) . (26) } } } (π) π ⋅ )} π»π−π ∑ ( (π₯ | π) . π1 , . . . , ππ } } π1 +⋅⋅⋅+ππ =π ππ ≥1 } (29) Therefore, by (27) and (29), we obtain the following theorem. 4 Abstract and Applied Analysis Theorem 2. For any π, π ≥ 0, one has { { { π min{π,π} ( ππ ) ( ) ∑ + ∑ { { π π=0 (1 − π)π π=min{π,π}+1 { { π π»π(π−π ) (π₯ | π) = min{π ,π} { { { } π } } ( ππ ) π π )} ∑ {( ) ∑ ∑ ( π , . . . , π π π { 1 π } } π=0 (1 − π) π1 +⋅⋅⋅+ππ =π π=0 { π ≥1 π { } } } } (π) π ⋅ ∑ ( )} π»π−π (π₯ | π) . π1 , . . . , ππ } } π1 +⋅⋅⋅+ππ =π ππ ≥1 } (32) (π) ⋅ π»π−π (π₯ | π) { { { π min{π ,π} ( ππ ) + ( ) ∑ ∑ { { π π=0 (1 − π)π π=min{π ,π}+1 { { π } } } (π) π ⋅ ∑ ( )} π»π−π (π₯ | π) . π1 , . . . , ππ } } π1 +⋅⋅⋅+ππ =π ππ ≥1 } (30) Now, we define the analogue of Stirling numbers of the second kind as follows: ππ (π, π) = = ∑ π=0 By (33) and (34), we get { { { π min{π−1,π} ( π−1 π ) + ( ) ∑ ∑ { π { (1 − π)π π=0 π=min{π−1,π}+1 { { π } } } (π) π ⋅ ∑ ( )} π»π−π (π₯ | π) . π1 , . . . , ππ } } π1 +⋅⋅⋅+ππ =π ππ ≥1 } (31) (35) π½ππ π₯π = min{2π,π} { { { } π } } ( 2π π π π) ( ) ( ) ∑ ∑ π { } , . . . , π π π { 1 π } { } π=0 (1 − π) π1 +⋅⋅⋅+ππ =π ππ ≥1 { } ∑ π=0 (π) ⋅ π»π−π (π₯ | π) { { { π min{2π,π} ( 2π π) + ( ) ∑ ∑ π { π { π=0 (1 − π) π=min{2π,π}+1 { { π } } } (π) π ( )} π»π−π (π₯ | π) , π1 , . . . , ππ } } π1 +⋅⋅⋅+ππ =π ππ ≥1 } ⋅ Corollary 4. For π ≥ 0, π ≥ 1, one has } } } π π ( ) ∑ {( ) ∑ ∑ π { π π=0 (1 − π) π +⋅⋅⋅+π =π π1 , . . . , ππ } } } π=0 { 1 π ππ ≥1 { } (π) ⋅ π»π−π (π₯ | π) (π, π ≥ 0) . Let us take π = 2π. Then we have Let us take π = π (π ≥ 1) in Theorem 2. Then we obtain the following corollary. π₯π = 1 Μπ π Δ 0 , π! π = π»π(−π) (π₯ | π) (π) ⋅ π»π−π (π₯ | π) min{π,π} { { { (34) π=0 ππ (π, π) = } π } } ( π−1 π π π ) ( ) ( ) ∑ ∑ π { } , . . . , π π π { (1 − π) 1 π } { } π=0 π1 +⋅⋅⋅+ππ =π π ≥1 π { } (33) π Μ π π (0) = ∑ (π) (−π)π−π π (π) . Δ π π π»π (π₯ | π) min{π−1,π} { { { (π, π ≥ 0) . Note that π1 (π, π) = π(π, π) is the Stirling number of the second kind. Μ π , we have From the definition of Δ Let us take π = π−1 (π ≥ 1) in Theorem 2. Then we obtain the following corollary. Corollary 3. For π ≥ 0, π ≥ 1, one has 1 π π ∑ ( ) (−π)π−π ππ , π! π=0 π π ( ππ ) π½ππ π₯π = ( = ∑ 1 Μ π π Δ )π₯ 1−π π π 1 π π ( ) (−π)π−π (π₯ + π) . ∑ (1 − π)π π=0 π (36) Abstract and Applied Analysis 5 Let us consider π = 2π − 1 in the identity of Theorem 2. Then we have By (36), we get π 1 π π π−π (π₯ + π) π ∑ (π) (−π) − π) (1 π=0 = = π½ππ−1 π₯π = π»π−(π−1) (π₯ | π) 1 Μ π π₯π Δ (1 − π)π π = min{2π,π} { { { } π } } ( 2π π π π) ( ) ( ) ∑ ∑ π { } , . . . , π π π { 1 π } { } π=0 (1 − π) π1 +⋅⋅⋅+ππ =π π ≥1 π { } ∑ π=0 (π) ⋅ π»π−π (π₯ | π) { { { π min{2π−1,π} ( 2π−1 π ) + ( ) ∑ ∑ { π { (1 − π)π π=0 π=min{2π−1,π}+1 { { π { { { π min{2π,π} ( 2π π) ( ) ∑ π { { π π=0 (1 − π) π=min{2π,π}+1 { { π ∑ } } } } (π) π ⋅ ∑ ( )} π»π−π (π₯ | π) . π1 , . . . , ππ } } π1 +⋅⋅⋅+ππ =π } ππ ≥1 } (37) Let us take π₯ = 0 in (37). Then we obtain the following theorem. Theorem 5. We have π! π (π, π) (1 − π)π π = } } } π } } ( 2π π π π) )} ( )∑ ∑ ( π { , . . . , π π π { − π) (1 1 π } } { π=0 π1 +⋅⋅⋅+ππ =π } { } { ≥1 π π } { min{2π,π} { { { π=0 ⋅ (π) π»π−π + (π) { { { π min{2π,π} ( 2π π) ( ) ∑ { π { (1 − π)π π=0 π=min{2π,π}+1 { { π ∑ π=0 = 1 Μ π−1 π₯π . Δ π (1 − π)π−1 (39) (π − 1)! ππ (π, π − 1) (1 − π)π−1 = Μ π−1 0π (π − 1)! Δ π (1 − π)π−1 (π − 1)! min{2π−1,π} { { { } π } } ( 2π−1 π π π ) ( ) ( ) ∑ ∑ π { } , . . . , π π π { 1 π } { } π=0 (1 − π) π1 +⋅⋅⋅+ππ =π ππ ≥1 { } ∑ π=0 (π) ⋅ π»π−π (π) ∑ min{π,π} π−1 1 π−1 π ) (−π)π−1−π (π₯ + π) ∑( π−1 π (1 − π) π=0 = { { { π min{2π−1,π} ( 2π−1 π ) + ( ) ∑ ∑ { π { − (1 π)π π=0 π=min{2π−1,π}+1 { { π } } } (π) π ⋅ ∑ ( ) } π»π−π (π) π1 , . . . , ππ } } π1 +⋅⋅⋅+ππ =π ππ ≥1 } = = Theorem 6. For π ≥ 0 and π ≥ 1, one has Μ π 0π π! Δ π (1 − π)π π! ∑ } } } (π) π ⋅ ∑ ( )} π»π−π (π₯ | π) π1 , . . . , ππ } } π1 +⋅⋅⋅+ππ =π ππ ≥1 } Let us take π₯ = 0 in (39). Then we obtain the following theorem. { { = } π } } ( 2π−1 π π π ) ( ) ( ) ∑ ∑ π { } , . . . , π π π { 1 π } { } π=0 (1 − π) π1 +⋅⋅⋅+ππ =π ππ ≥1 { } ∑ π=0 (π) ⋅ π»π−π (π₯ | π) + min{2π−1,π} { { { ( ππ ) ∑ (1 − π)π π +⋅⋅⋅+π 1 π =π ππ ≥1 π ( ). π1 , . . . , ππ (38) } } } (π) π ⋅ ∑ ( )} π»π−π (π) . π1 , . . . , ππ } } π1 +⋅⋅⋅+ππ =π ππ ≥1 } (40) 6 Abstract and Applied Analysis Remark 7. Note that (π − 1)! ππ (π, π − 1) (1 − π)π−1 = min{π,π} { { { } π } } ( ππ ) π π ( ) ∑ {( ) ∑ ∑ π { π π=0 (1 − π) π +⋅⋅⋅+π =π π1 , . . . , ππ } } } π=0 { 1 π π ≥1 π { } ⋅ π»π−π (π) + { { { π min{π,π} ( ππ ) ( ) ∑ { { π π=0 (1 − π)π π=min{π,π}+1 { { π ∑ } } } π ⋅ ∑ ( ) } π»π−π (π) . π1 , . . . , ππ } } π1 +⋅⋅⋅+ππ =π ππ ≥1 } (41) Acknowledgment The authors would like to express their gratitude to the referees for their valuable suggestions. References [1] T. 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