Hindawi Publishing Corporation Abstract and Applied Analysis Volume 2011, Article ID 584951, 24 pages doi:10.1155/2011/584951 Research Article Some New Volterra-Fredholm-Type Discrete Inequalities and Their Applications in the Theory of Difference Equations Bin Zheng1 and Qinghua Feng1, 2 1 2 School of Science, Shandong University of Technology, Shandong, Zibo 255049, China School of Mathematical Sciences, Qufu Normal University, Shandong, Qufu 273165, China Correspondence should be addressed to Bin Zheng, zhengbin2601@126.com Received 21 March 2011; Accepted 29 June 2011 Academic Editor: Martin D. Schechter Copyright q 2011 B. Zheng and Q. Feng. 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. Some new Volterra-Fredholm-type discrete inequalities in two independent variables are established, which provide a handy tool in the study of qualitative and quantitative properties of solutions of certain difference equations. The established results extend some known results in the literature. 1. Introduction In the research of solutions of certain differential and difference equations, if the solutions are unknown, then it is necessary to study their qualitative and quantitative properties such as boundedness, uniqueness, and continuous dependence on initial data. The GronwallBellman inequality 1, 2 and its various generalizations which provide explicit bounds play a fundamental role in the research of this domain. Many such generalized inequalities e.g., see 3–30 and the references therein have been established in the literature including the known Ou-Liang’s inequality 3. In 8, Ma generalized the discrete version of Ou-Liang’s inequality in two variables to Volterra-Fredholm form for the first time, which has proved to be very useful in the study of qualitative as well as quantitative properties of solutions of certain Volterra-Fredholm-type difference equations. But since then, few results on VolterraFredholm-type discrete inequalities have been established. Recent results in this direction include the work of Ma 9 to our knowledge. We notice, in the analysis of some certain Volterra-Fredholm-type difference equations with more complicated forms, that the bounds provided by the earlier inequalities are inadequate and it is necessary to seek some new Volterra-Fredholm-type discrete inequalities in order to obtain a diversity of desired results. 2 Abstract and Applied Analysis Our aim in this paper is to establish some new generalized Volterra-Fredholm-type discrete inequalities, which extend Ma’s work in 9, and provide new bounds for unknown functions lying in these inequalities. We will illustrate the usefulness of the established results by applying them to study the boundedness, uniqueness, and continuous dependence on initial data of solutions of certain more complicated Volterra-Fredholm-type difference equations. Throughout this paper, R denotes the set of real numbers R 0, ∞, and Z denotes the set of integers, while N0 denotes the set of nonnegative integers. Let Ω : m0 , M × n0 , N Z2 , where m0 , n0 ∈ Z and M, N ∈ Z {∞} are two constants. l1 , l2 ∈ Z are two constants, and Ki > 0, i 1, 2, 3, 4, are all constants. If U is a lattice, then we denote the set of all R-valued functions on U by ℘U and denote the set of all R -valued functions on U by 1 ℘ U. Finally, for a function f ∈ ℘ U, we have m sm0 f 0 provided m0 > m1 . 2. Main Results Lemma 2.1 see 15. Assume that a ≥ 0, p ≥ q ≥ 0, and p / 0 then for any K > 0 aq/p ≤ q q−p/p p − q q/p K K . a p p 2.1 Lemma 2.2. Suppose that um, n ∈ ℘ Ω, bs, t, m, n ∈ ℘ Ω2 , α ≥ 0 is a constant. If b is nondecreasing in the third variable, then, for m, n ∈ Ω, um, n ≤ α m−1 n−1 bs, t, m, nus, t 2.2 sm0 tn0 implies that um, n ≤ α exp m−1 n−1 bs, t, m, n . 2.3 sm0 tn0 Lemma 2.3. Suppose that um, n, am, n, bm, n ∈ ℘ Ω. If am, n is nondecreasing in the first variable, then, for m, n ∈ Ω, um, n ≤ am, n m−1 bs, nus, n 2.4 sm0 implies that um, n ≤ am, n m−1 sm0 1 bs, n. 2.5 Abstract and Applied Analysis 3 Remark 2.4. Lemma 2.3 is a direct variation of 19, Lemma 2.5β1 , and we note am, n ≥ 0 here. Theorem 2.5. Suppose that um, n, am, n ∈ ℘ Ω, bi s, t, m, n, ci s, t, m, n ∈ ℘ Ω2 , i 1, 2, . . . , l1 , di s, t, m, n, ei s, t, m, n ∈ ℘ Ω2 , i 1, 2, . . . , l2 with bi , ci , di , ei nondecreasing in 0, the last two variables. p, qi , ri are nonnegative constants with p ≥ qi , p ≥ ri , i 1, 2, . . . , l1 , p / while hi , ji are nonnegative constants with p ≥ hi , p ≥ ji , i 1, 2, . . . , l2 . If, for m, n ∈ Ω, um, n satisfies l1 m−1 n−1 up m, n ≤ am, n ⎤ s t ⎣bi s, t, m, nuqi s, t ci ξ, η, m, n uri ξ, η ⎦ ⎡ i1 sm0 tn0 ξm0 ηn0 ⎤ ⎡ l2 M−1 s t N−1 h j ⎣di s, t, m, nu i s, t ei ξ, η, m, n u i ξ, η ⎦, i1 sm0 tn0 2.6 ξm0 ηn0 then um, n ≤ am, n JM, N Cm, n 1 − μM, N 1/p 2.7 , provided that μM, N < 1, where ⎧ l1 m−1 n−1 ⎨ p − qi qi /p qi qi −p/p K1 K1 bi s, t, m, n as, t Jm, n ⎩ p p i1 sm0 tn0 ⎫ s t ri ri −p/p p − ri ri /p ⎬ K K2 ci ξ, η, m, n a ξ, η ⎭ p 2 p ξm ηn0 0 ⎧ l2 M−1 N−1 ⎨ p − hi hi /p hi hi −p/p K3 K3 as, t di s, t, m, n ⎩ p p i1 sm0 tn0 2.8 ⎫ s t ji ji −p/p p − ji ji /p ⎬ , K K4 ei ξ, η, m, n a ξ, η ⎭ p 4 p ξm ηn0 0 ⎫ ⎧ l2 M−1 s t N−1 ⎨ ji ji −p/p ⎬ hi hi −p/p μm, n d s, t, m, n K3 Cs, t ei ξ, η, m, n K4 C ξ, η , ⎭ ⎩ i p p i1 sm tn ξm ηn 0 0 0 Cm, n exp Bs, t, m, n , 2.9 2.10 sm0 tn0 ⎤ s t ri ri −p/p qi qi −p/p ⎣bi s, t, m, n K ⎦. Bs, t, m, n ci ξ, η, m, n K2 p 1 p i1 ξm ηn0 l1 ⎡ m−1 n−1 0 0 2.11 4 Abstract and Applied Analysis Proof. Denote vm, n l1 m−1 n−1 ⎤ s t ⎣bi s, t, m, nuqi s, t ci ξ, η, m, n uri ξ, η ⎦ ⎡ i1 sm0 tn0 ξm0 ηn0 ⎡ l2 M−1 N−1 s i1 sm0 tn0 ξm0 ηn0 ⎣di s, t, m, nuhi s, t t ⎤ 2.12 ei ξ, η, m, n uji ξ, η ⎦. Then, we have 2.13 um, n ≤ am, n vm, n1/p , and, furthermore, from Lemma 2.1 we have uqi m, n ≤ am, n vm, nqi /p ≤ qi qi −p/p K am, n vm, n p 1 uri m, n ≤ am, n vm, nri /p ≤ p − ri ri /p K2 , p i 1, 2, . . . , l1 , hi hi −p/p K am, n vm, n p 3 uji m, n ≤ am, n vm, nji /p ≤ i 1, 2, . . . , l1 , ri ri −p/p K am, n vm, n p 2 uhi m, n ≤ am, n vm, nhi /p ≤ p − qi qi /p K1 , p p − hi hi /p K3 , p i 1, 2, . . . , l2 , ji ji −p/p K am, n vm, n p 4 p − ji ji /p K4 , p i 1, 2, . . . , l2 . 2.14 Abstract and Applied Analysis 5 So vm, n ≤ ⎧ l1 m−1 n−1 ⎨ bi s, t, m, n i1 sm0 tn0 ⎩ qi qi −p/p p − qi qi /p K1 K1 as, tvs, t p p ⎫ s t ri ri −p/p p − ri ri /p ⎬ K K2 ci ξ, η, m, n a ξ, η v ξ, η ⎭ p 2 p ξm ηn0 0 ⎧ l2 M−1 N−1 ⎨ p − hi hi /p hi hi −p/p di s, t, m, n K3 K3 as, t vs, t ⎩ p p i1 sm0 tn0 ⎫ s t ji ji −p/p p − ji ji /p ⎬ K K4 ei ξ, η, m, n a ξ, η v ξ, η ⎭ p 4 p ξm ηn0 0 Hm, n l1 m−1 n−1 ⎡ ⎣bi s, t, m, n i1 sm0 tn0 qi qi −p/p K vs, t p 1 ⎤ s t ri ri −p/p ci ξ, η, m, n K2 v ξ, η ⎦, p ξm ηn0 0 2.15 where Hm, n Jm,n l2 M−1 N−1 i1 j −p/p ξ, η, m, nji /pK4 i vξ, η} sm0 hi −p/p vs, t sξm0 tn0 {di s, t, m, nhi /pK3 t ηn0 ei and Jm, n is defined in 2.8. Then, using that Hm, n is nondecreasing in every variable, we obtain vm, n ≤ HM, N l1 m−1 n−1 ⎡ ⎣bi s, t, m, n i1 sm0 tn0 qi qi −p/p K vs, t p 1 ⎤ s t ri ri −p/p ci ξ, η, m, n K2 v ξ, η ⎦ p ξm ηn0 0 ≤ HM, N l1 m−1 n−1 ⎡ ⎣bi s, t, m, n i1 sm0 tn0 qi qi −p/p K p 1 ⎤ s t ri ri −p/p ⎦vs, t ci ξ, η, m, n K2 p ξm ηn0 0 HM, N m−1 n−1 Bs, t, m, nvs, t, sm0 tn0 where Bs, t, m, n is defined in 2.11. 2.16 6 Abstract and Applied Analysis Since bi s, t, m, n, ci s, t, m, n are nondecreasing in the last two variables, then Bs, t, m, n is also nondecreasing in the last two variables, and by a suitable application of Lemma 2.2 we obtain vm, n ≤ HM, N exp m−1 n−1 Bs, t, m, n HM, NCm, n, 2.17 sm0 tn0 where Cm, n is defined in 2.10. Furthermore, considering the definition of Hm, n and 2.17 we have HM, N JM, N ⎧ l2 M−1 N−1 ⎨ di s, t, M, N i1 sm0 tn0 ⎩ hi hi −p/p K vs, t p 3 ⎫ s t ji ji −p/p ⎬ ei ξ, η, M, N K4 v ξ, η ⎭ p ξm ηn0 0 ≤ JM, N ⎧ l2 M−1 N−1 ⎨ di s, t, M, N i1 sm0 tn0 ⎩ hi hi −p/p K HM, NCs, t p 3 ⎫ s t ⎬ ji ji −p/p ei ξ, η, m1 , n1 K4 HM, NC ξ, η ⎭ p ξm ηn0 0 JM, N HM, N ⎧ l2 M−1 N−1 ⎨ di s, t, M, N i1 sm0 tn0 ⎩ hi hi −p/p K Cs, t p 3 ⎫ s t ji ji −p/p ⎬ ei ξ, η, M, N K4 C ξ, η ⎭ p ξm ηn0 0 JM, N HM, NμM, N, 2.18 where μm, n is defined in 2.9. Then, HM, N ≤ JM, N . 1 − μM, N 2.19 Combining 2.17 and 2.19 we deduce vm, n ≤ JM, N Cm, n. 1 − μM, N Then, combining 2.13 and 2.20, we obtain the desired result. 2.20 Abstract and Applied Analysis 7 Corollary 2.6. Suppose that g1i m, n, g2i m, n, b1i m, n, c1i m, n ∈ ℘ Ω, i 1, 2, . . . , l1 with g1i , g2i nondecreasing in every variable. d1i m, n, e1i m, n ∈ ℘ Ω, i 1, 2, . . . , l2 . um, n, am, n, p, qi , ri , hi , ji are defined as in Theorem 2.5. If, for m, n ∈ Ω, um, n satisfies u m, n ≤ am, n p l1 g1i m, n ⎡ ⎣b1i s, tu s, t i1 c1i ξ, η u ri ⎤ M−1 N−1 s t sm0 tn0 ξm0 ηn0 ⎣d1i s, tuhi s, t ji ξ, η ⎦ ξm0 ηn0 ⎡ g2i m, n s t qi sm0 tn0 i1 l2 m−1 n−1 2.21 ⎤ e1i ξ, η u ξ, η ⎦, then um, n ≤ 1/p JM, N am, n CM, N , 1 − μM, N 2.22 provided that μM, N < 1, where Jm, n l1 ⎧ g1i m, n b1i s, t sm0 tn0 i1 m−1 n−1 ⎨ ⎩ p − qi qi /p qi qi −p/p K1 K1 as, t p p s t c1i ξm0 ηn0 l2 ⎧ g2i m, n d1i s, t sm0 tn0 i1 M−1 N−1 ⎨ ⎩ ⎫ ri ri −p/p p − ri ri /p ⎬ K K2 a ξ, η ξ, η ⎭ p 2 p p − hi hi /p hi hi −p/p K3 K3 as, t p p s t e1i μm, n ⎫ ji ji −p/p p − ji ji /p ⎬ , K K4 a ξ, η ξ, η ⎭ p 4 p ⎡ g2i m, n ⎩ i1 M−1 N−1 ⎣d1i s, t hi K hi −p/p Cs, t p 3 sm0 tn0 s t ξm0 ηn0 Cm, n exp ξm0 ηn0 ⎧ l2 ⎨ m−1 n−1 e1i ⎤⎫ ji ji −p/p ⎬ C ξ, η ⎦ , ξ, η K4 ⎭ p Bs, t, m, n , sm0 tn0 ⎡ ⎤ s t ri ri −p/p qi qi −p/p ⎦. Bs, t, m, n g1i m, n⎣b1i s, t K1 c1i ξ, η K2 p p ηn i1 ξm 0 l1 0 2.23 8 Abstract and Applied Analysis The proof of Corollary 2.6 can be completed by setting bi s, t, m, n g1i m,nb1i s,t, ci s,t,m,n g1i m,nc1i s,t, di s,t,m,n g2i m,nd1i s,t, ei s,t,m,n g2i m,ne1i s,t in Theorem 2.5. Corollary 2.7. Suppose that um, n, am, n, bi s, t, m, n, ci s, t, m, n, di s, t, m, n, ei s, t, m, n are defined as in Theorem 2.5. If, for m, n ∈ Ω, um, n satisfies um, n ≤ am, n l1 m−1 n−1 ⎤ s t ⎣bi s, t, m, nus, t ci ξ, η, m, n u ξ, η ⎦ ⎡ i1 sm0 tn0 ξm0 ηn0 ⎤ l2 M−1 s t N−1 ⎣di s, t, m, nus, t ei ξ, η, m, n u ξ, η ⎦, ⎡ i1 sm0 tn0 2.24 ξm0 ηn0 then um, n ≤ am, n JM, N Cm, n, 1 − μM, N 2.25 provided that μM, N < 1, where Jm, n ⎧ l1 m−1 n−1 ⎨ i1 sm0 ⎫ s t ⎬ b s, t, m, nas, t ci ξ, η, m, n a ξ, η ⎩ i ⎭ tn ξm ηn 0 i1 sm0 tn0 μm, n ⎫ s t ⎬ ei ξ, η, m, n a ξ, η , d s, t, m, nas, t ⎩ i ⎭ ξm ηn 0 0 ⎧ l2 M−1 N−1 ⎨ i1 sm0 tn0 Cm, n exp Bs, t, m, n 0 0 ⎧ l2 M−1 N−1 ⎨ l1 i1 ⎫ s t ⎬ d s, t, m, nCs, t ei ξ, η, m, n C ξ, η , ⎩ i ⎭ ξm ηn m−1 n−1 0 2.26 0 Bs, t, m, n , sm0 tn0 ⎤ s t ⎣bi s, t, m, n ci ξ, η, m, n ⎦. ⎡ ξm0 ηn0 Theorem 2.8. Suppose that wm, n ∈ ℘ Ω, u, a, bi , ci , di , ei , p, qi , ri , hi , ji are defined as in Theorem 2.5. Furthermore, assume that am, n is nondecreasing in the first variable. If, for m, n ∈ Ω, um, n satisfies Abstract and Applied Analysis up m, n ≤ am, n m−1 9 ws, nup m, n sm0 l1 m−1 n−1 ⎤ s t ⎣bi s, t, m, nuqi s, t ci ξ, η, m, n uri ξ, η ⎦ ⎡ i1 sm0 tn0 ξm0 ηn0 2.27 ⎤ ⎡ l2 M−1 s t N−1 ⎣di s, t, m, nuhi s, t ei ξ, η, m, n uji ξ, η ⎦, i1 sm0 tn0 ξm0 ηn0 then um, n ≤ 1/p JM, N am, n Cm, n wm, n , 1 − μM, N 2.28 provided that μM, N < 1, where ⎧ l1 m−1 n−1 ⎨ p − qi qi /p qi qi −p/p K1 K1 Jm, n bi s, t, m, n as, t ⎩ p p i1 sm0 tn0 ⎫ s t ri ri −p/p p − ri ri /p ⎬ K K2 ci ξ, η, m, n a ξ, η ⎭ p 2 p ξm ηn0 0 ⎧ ⎪ l2 M−1 ⎨ N−1 ⎪ p − hi hi /p hi hi −p/p K3 K3 di s, t, m, n as, t ⎪ p p i1 sm0 tn0 ⎪ ⎩ ⎫ s t ji ji −p/p p − ji ji /p ⎬ , K K4 ei ξ, η, m, n a ξ, η ⎭ p 4 p ξm ηn0 0 2.29 bi s, t, m, n bi s, t, m, nws, tqi /p , ci s, t, m, n ci s, t, m, nws, tri /p , i 1, 2, . . . , l1 , di s, t, m, n di s, t, m, nws, thi /p , ei s, t, m, n ei s, t, m, nws, tji /p , i 1, 2, . . . , l2 , 2.30 2.31 10 Abstract and Applied Analysis wm, n m−1 1 ws, n, sm0 ⎧ l2 M−1 N−1 ⎨ hi h −p/p μm, n di s, t, m, n K3 i Cs, t ⎩ p i1 sm tn 0 2.32 0 ⎫ s t ji ji −p/p ⎬ ei ξ, η, m, n K4 C ξ, η ⎭ p ξm ηn0 0 Cm, n exp m−1 n−1 2.33 Bs, t, m, n , sm0 tn0 ⎡ ⎤ l1 s t ri ri −p/p qi qi −p/p ⎣bi s, t, m, n K ⎦. Bs, t, m, n ci ξ, η, m, n K2 p 1 p i1 ξm ηn0 2.34 0 Proof. Denote zm, n am, n l1 m−1 n−1 ⎡ ⎤ s t ri ⎣bi s, t, m, nu s, t ci ξ, η, m, n u ξ, η ⎦ qi i1 sm0 tn0 ξm0 ηn0 ⎤ l2 M−1 s t N−1 ⎣di s, t, m, nuhi s, t ei ξ, η, m, n uji ξ, η ⎦. ⎡ i1 sm0 tn0 2.35 ξm0 ηn0 Then, we have up m, n ≤ zm, n m−1 ws, nup m, n. 2.36 sm0 Obviously zm, n is nondecreasing in the first variable. So by Lemma 2.3 we obtain up m, n ≤ zm, n m−1 1 ws, n zm, nwm, n, 2.37 sm0 where wm, n vm, n m−1 sm0 1 l1 m−1 n−1 ws, n. Define ⎤ s t ri ⎣bi s, t, m, nu s, t ci ξ, η, m, n u ξ, η ⎦ ⎡ qi i1 sm0 tn0 ξm0 ηn0 ⎤ l2 M−1 s t N−1 ⎣di s, t, m, nuhi s, t ei ξ, η, m, n uji ξ, η ⎦. ⎡ i1 sm0 tn0 ξm0 ηn0 2.38 Abstract and Applied Analysis 11 Then, um, n ≤ am, n vm, nwm, n1/p , 2.39 and, furthermore, by 2.39 and Lemma 2.1 we have vm, n ≤ ⎧ l1 m−1 n−1 ⎨ bi s, t, m, nas, t vs, tws, tqi /p i1 sm0 ⎩ tn 0 ⎫ s t ri /p⎬ ci ξ, η, m, n a ξ, η v ξ, η w ξ, η ⎭ ξm ηn 0 0 ⎧ l2 M−1 N−1 ⎨ di s, t, m, nas, t vs, tws, thi /p i1 sm0 tn0 ⎩ ⎫ s t ji /p⎬ ei ξ, η, m, n a ξ, η v ξ, η w ξ, η ⎭ ξm ηn 0 ≤ 0 ⎧ l1 m−1 n−1 ⎨ bi s, t, m, nws, tqi /p i1 sm0 tn0 ⎩ qi qi −p/p p − qi qi /p K K1 as, t vs, t p 1 p s t r /p ci ξ, η, m, n w ξ, η i ξm0 ηn0 ⎫ p − ri ri /p ⎬ ri ri −p/p K2 × K2 a ξ, η v ξ, η ⎭ p p ⎧ l2 M−1 N−1 ⎨ di s, t, m, nws, thi /p i1 sm0 tn0 ⎩ p − hi hi /p hi qi −p/p K3 K3 as, t vs, t p p s t j /p ei ξ, η, m, n w ξ, η i ξm0 ηn0 ⎫ ji ji −p/p p − ji ji /p ⎬ K4 × K4 a ξ, η v ξ, η ⎭ p p ⎧ l1 m−1 n−1 ⎨ qi qi −p/p p − qi qi /p K1 K1 bi s, t, m, n as, t vs, t ⎩ p p i1 sm0 tn0 ⎫ s t ri ri −p/p p − ri ri /p ⎬ K K2 ci ξ, η, m, n a ξ, η v ξ, η ⎭ p 2 p ξm ηn0 0 12 Abstract and Applied Analysis ⎧ l2 M−1 N−1 ⎨ p − hi hi /p hi qi −p/p K3 K3 di s, t, m, n as, t vs, t ⎩ p p i1 sm0 tn0 ⎫ s t ji ji −p/p p − ji ji /p ⎬ a ξ, η v ξ, η K K4 ei ξ, η, m, n ⎭ p 4 p ξm ηn0 0 Hm, n l1 m−1 n−1 ⎡ ⎣bi s, t, m, n i1 sm0 tn0 qi qi −p/p K vs, t p 1 ⎤ s t ri ri −p/p ci ξ, η, m, n K2 v ξ, η ⎦, p ηn ξm 0 0 2.40 where Hm, n Jm, n l2 M−1 N−1 i1 j −p/p ξ, η, m, nji /pK4 i vξ, η}, sm0 hi −p/p vs, t sξm0 tn0 {di s, t, m, nhi /pK3 t ηn0 ei and Jm, n, bi , ci , di , ei are defined in 2.29–2.31 respec- tively. Similar to the process of 2.15–2.20 we deduce vm, n ≤ JM, N Cm, n, 1 − μM, N 2.41 where μm, n, Cm, n are defined in 2.32 and 2.33. Combining 2.39 and 2.41, we get the desired result. Remark 2.9. If we set bi s,t,m,n g1i m,nb1i s,t, ci s,t,m,n g1i m,nc1i s,t, di s,t,m,n g2i m,nd1i s,t, ei s,t,m,n g2i m,ne1i s,t or set p qi ri hi ji 1 in Theorem 2.8, then immediately we get two corollaries which are similar to Corollaries 2.6 and 2.7, and we omit the details for them. Theorem 2.10. Suppose that u, a, bi , ci , di , ei , p, qi , ri , hi , ji are defined as in Theorem 2.5. Li , Ti : Ω × R → R , i 1, 2, . . . , l2 , satisfies 0 ≤ Li m, n, u − Li m, n, v ≤ Ti m, n, vu − v for u ≥ v ≥ 0. If, for m, n ∈ Ω, um, n satisfies up m, n ≤ am, n l1 m−1 n−1 i1 sm0 tn0 ⎤ s t ⎣bi s, t, m, nuqi s, t ci ξ, η, m, n uri ξ, η ⎦ ⎡ ξm0 ηn0 ⎡ ⎤ l2 M−1 s t N−1 ⎣di s, t, m, nLi s, t, uhi s, t ei ξ, η, m, n uji ξ, η ⎦, i1 sm0 tn0 ξm0 ηn0 2.42 Abstract and Applied Analysis 13 then um, n ≤ ! JM, N ! am, n Cm, n 1−μ ! M, N 1/p 2.43 , provided that μ ! M, N < 1, where ! Jm, n ⎧ l1 m−1 n−1 ⎨ bi s, t, m, n i1 sm0 tn0 ⎩ p − qi qi /p qi qi −p/p K1 K1 as, t p p ⎫ s t ri ri −p/p p − ri ri /p ⎬ K K2 ci ξ, η, m, n a ξ, η ⎭ p 2 p ξm ηn0 0 ⎧ l2 M−1 N−1 ⎨ di s, t, m, nLi s, t, i1 sm0 tn0 ⎩ p − hi hi /p hi hi −p/p K K3 as, t p 3 p 2.44 ⎫ s t ji ji −p/p p − ji ji /p ⎬ , K K4 ei ξ, η, m, n a ξ, η ⎭ p 4 p ξm ηn0 0 μ ! m, n ⎧ l2 M−1 N−1 ⎨ hi h −p/p d!i s, t, m, n K3 i Cs, t ⎩ p i1 sm0 tn0 s ξm0 2.45 ⎫ ⎬ t ji j −p/p ei ξ, η, m, n K4 i C ξ, η , ⎭ p ηn0 p − hi hi /p hi h −p/p d!i s, t, m, n di s, t, m, nTi s, t, K3 i K3 , as, t p p ! Cm, n exp m−1 n−1 i 1, 2, . . . , l2 , 2.46 ! t, m, n , Bs, 2.47 sm0 tn0 ⎤ s t q ri r −p/p ⎦ ! t, m, n ⎣bi s, t, m, n i K qi −p/p . Bs, ci ξ, η, m, n K2 i 1 p p i1 ξm ηn0 l1 ⎡ 0 2.48 14 Abstract and Applied Analysis Proof. Denote vm, n l1 m−1 n−1 ⎤ s t ri ⎣bi s, t, m, nu s, t ci ξ, η, m, n u ξ, η ⎦ ⎡ qi i1 sm0 tn0 ξm0 ηn0 ⎤ 2.49 ⎡ l2 M−1 s t N−1 ⎣di s, t, m, nLi s, t, uhi s, t ei ξ, η, m, n uji ξ, η ⎦. i1 sm0 tn0 ξm0 ηn0 Then 2.50 um, n ≤ am, n vm, n1/p , and, furthermore, from Lemma 2.1 we have vm, n ≤ ⎧ l1 m−1 n−1 ⎨ bi s, t, m, n i1 sm0 tn0 ⎩ qi qi −p/p p − qi qi /p K1 K1 as, t vs, t p p ⎫ s t ri ri −p/p p − ri ri /p ⎬ K K2 ci ξ, η, m, n a ξ, η v ξ, η ⎭ p 2 p ξm ηn0 0 ⎧ l2 M−1 N−1 ⎨ p − hi hi /p hi h −p/p di s, t, m, nLi s, t, K3 i K3 as, t vs, t ⎩ p p i1 sm0 tn0 ⎫ s t ji ji −p/p p − ji ji /p ⎬ K K4 ei ξ, η, m, n a ξ, η v ξ, η ⎭ p 4 p ξm ηn0 0 ⎧ l1 m−1 n−1 ⎨ qi qi −p/p p − qi qi /p K1 K1 bi s, t, m, n ≤ as, t vs, t ⎩ p p i1 sm0 tn0 ⎫ s t ri ri −p/p p − ri ri /p ⎬ K K2 ci ξ, η, m, n a ξ, η v ξ, η ⎭ p 2 p ξm ηn0 0 ⎧ l2 M−1 N−1 ⎨ p − hi hi /p hi h −p/p K3 di s, t, m, n Li s, t, K3 i as, t vs, t ⎩ p p i1 sm0 tn0 Abstract and Applied Analysis 15 p − hi hi /p hi h −p/p K3 as, t − Li s, t, K3 i p p p − hi hi /p hi h −p/p K3 Li s, t, K3 i as, t p p ⎫ s t ji ji −p/p p − ji ji /p ⎬ K K4 ei ξ, η, m, n a ξ, η v ξ, η ⎭ p 4 p ξm ηn0 0 ⎧ l1 m−1 n−1 ⎨ qi qi −p/p p − qi qi /p bi s, t, m, n ≤ K1 K1 as, t vs, t ⎩ p p i1 sm0 tn0 ⎫ s t ri ri −p/p p − ri ri /p ⎬ K K2 ci ξ, η, m, n a ξ, η v ξ, η ⎭ p 2 p ξm ηn0 0 ⎧ l2 M−1 N−1 ⎨ p − hi hi /p hi hi −p/p hi h −p/p di s, t, m, n Ti s, t, K3 i K3 K as, t ⎩ p p p 3 i1 sm0 tn0 p − hi hi /p hi h −p/p K3 ×vs, t Li s, t, K3 i as, t p p ⎫ s t ji ji −p/p p − ji ji /p ⎬ K K4 ei ξ, η, m, n a ξ, η v ξ, η ⎭ p 4 p ξm ηn0 0 " Hm, n l1 m−1 n−1 ⎡ ⎣bi s, t, m, n i1 sm0 tn0 qi qi −p/p K vs, t p 1 ⎤ s t ri ri −p/p ci ξ, η, m, n K2 v ξ, η ⎦, p ξm ηn0 0 2.51 2 M−1 N−1 ! hi −p/p " ! where Hm, n Jm, n li1 vs, t sξm0 tηn0 ei sm0 tn0 {di s, t, m, nhi /pK3 j −p/p ! ξ, η, m, nji /pK4 i vξ, η} and Jm, n, d!i s, t, m, n are defined in 2.44 and 2.46 respectively. Similar to the process of 2.15–2.20 we deduce vm, n ≤ ! JM, N ! CM, N, 1−μ ! M, N ! where μ ! m, n, Cm, n are defined in 2.45 and 2.47 respectively. Combining 2.50 and 2.52, we get the desired result. 2.52 16 Abstract and Applied Analysis Theorem 2.11. Suppose that wm, n ∈ ℘ Ω, u, a, bi , ci , di , ei , p, qi , ri , hi , ji are defined as in Theorem 2.5. Furthermore, assume am, n is nondecreasing in the first variable. Li , Ti , i 1, 2, . . . , l2 , are defined as in Theorem 2.10. If, for m, n ∈ Ω, um, n satisfies up m, n ≤ am, n m−1 ws, nup m, n sm0 l1 m−1 n−1 ⎤ s t ⎣bi s, t, m, nuqi s, t ci ξ, η, m, n uri ξ, η ⎦ ⎡ i1 sm0 tn0 ξm0 ηn0 ⎡ ⎤ l2 M−1 s t N−1 ⎣di s, t, m, nLi s, t, uhi s, t ei ξ, η, m, n uji ξ, η ⎦, i1 sm0 tn0 ξm0 ηn0 2.53 then 1/p # JM, N # # n am, n Cm, n wm, , 1−μ # M, N um, n ≤ 2.54 provided that μ # M, N < 1, where # Jm, n ⎧ l1 m−1 n−1 ⎨ p − qi qi /p qi qi −p/p K1 K1 as, t b#i s, t, m, n ⎩ p p i1 sm0 tn0 ⎫ s t ri ri −p/p p − ri ri /p ⎬ K K2 c#i ξ, η, m, n a ξ, η ⎭ p 2 p ξm ηn0 0 ⎧ l2 M−1 N−1 ⎨ $ % p−hi hi /p hi hi−p/p di s, t, m, nLi s, t, ws, K3 K3 as, t # thi /p ⎩ p p i1 sm0 tn0 ⎫ s t ji ji −p/p p − ji ji /p ⎬ K K4 e#i ξ, η, m, n a ξ, η , ⎭ p 4 p ξm ηn0 0 # tqi /p , b#i s, t, m, n bi s, t, m, nws, c#i s, t, m, n ci s, t, m, nws, # tri /p , i 1, 2, . . . , l1 , $ % p − hi hi /p hi hi −p/p K3 K3 d#i s, t, m, n di s, t, m, nws, as, t # thi /p , # thi /p Ti s, t, ws, p p e#i s, t, m, n ei s, t, m, nws, # tji /p , i 1, 2, . . . , l2 , Abstract and Applied Analysis wm, # n m−1 17 1 ws, n, sm0 μ # m, n ⎧ l2 M−1 N−1 ⎨ hi h −p/p # d#i s, t, m, n K3 i Cs, t ⎩ p i1 sm0 tn0 ⎫ s t ji ji −p/p ⎬ # ξ, η , e#i ξ, η, m, n K4 C ⎭ p ηn ξm 0 0 # Cm, n exp m−1 n−1 # Bs, t, m, n , sm0 tn0 ⎡ ⎤ l1 s t q ri r −p/p ⎦ # t, m, n ⎣bi s, t, m, n i K qi −p/p . Bs, ci ξ, η, m, n K2 i 1 p p i1 ξm ηn0 0 2.55 The proof for Theorem 2.11 is similar to the combination of Theorems 2.8 and 2.10, and we omit the details here. Remark 2.12. If we take g1i m, n ≡ 1, c1i m, n ≡ 0, i 1, 2, . . . , l1 , and g2i m, n ≡ 1, e1i m, n ≡ 0, i 1, 2, . . . , l2 in Corollary 2.6, then Corollary 2.6 reduces to 9, Theorem 2.5. If furthermore l1 l2 1, then Corollary 2.6 reduces to 9, Theorem 2.1. If we take bi s, t, m, n b1i s, t, ci s, t, m, n ≡ 0, i 1, 2, . . . , l1 and di s, t, m, n d1i s, t, ei s, t, m, n ≡ 0, hi 1, i 1, 2, . . . , l2 in Theorem 2.10, then Theorem 2.10 reduces to 9, Theorem 2.7. If furthermore l1 l2 1, then Theorem 2.10 reduces to 9, Theorem 2.6. 3. Applications In this section, we will present some applications for the established results above and show that they are useful in the study of boundedness, uniqueness, and continuous dependence of solutions of certain difference equations. Example 3.1. Consider the following Volterra-Fredholm sum-difference equation: m−1 n−1 u m, n am, n p ⎡ ⎣F1 s, t, m, n, us, t sm0 tn0 ⎡ s t ξm0 ηn0 M−1 M−1 s t sm0 tn0 ξm0 ηn0 ⎣G1 s, t, m, n, us, t ⎤ F2 ξ, η, m, n, u ξ, η ⎦ ⎤ G2 ξ, η, m, n, u ξ, η ⎦, 3.1 18 Abstract and Applied Analysis where um, n, am, n ∈ ℘Ω, p ≥ 1 is an odd number, Fi , Gi : Ω2 × R → R, i 1, 2, M, N are two integers defined the same as in Theorem 2.5. Theorem 3.2. Suppose that um, n is a solution of 3.1, and |F1 s, t, m, n, u| ≤ f1 s, t, m, n|u|q , |F2 s, t, m, n, u| ≤ f2 s, t, m, n|u|r , |G1 s, t, m, n, u| ≤ g1 s, t, m, n|u|h , |G2 s, t, m, n, u| ≤ g2 s, t, m, n|u|j , where q, r, h, j are nonnegative constants satisfying p ≥ q, p ≥ r, p ≥ h, p ≥ j, fi , gi ∈ ℘ Ω2 , i 1, 2 and fi , gi are nondecreasing in the last two variables; then one has |um, n| ≤ |am, n| JM, N Cm, n 1 − μM, N 1/p 3.2 , provided that μM, N < 1, where ⎧ Jm, n m−1 n−1 ⎨ f1 s, t, m, n sm0 tn0 ⎩ ⎧ M−1 N−1 ⎨ g1 s, t, m, n ⎩ sm0 tn0 ⎫ r r−p/p & & p − r r/p ⎬ &a ξ, η & K2 K2 f2 ξ, η, m, n ⎭ p p ηn0 s t ξm0 q q−p/p p − q q/p K1 K1 |as, t| p p s t ξm0 ηn0 p − h h/p h h−p/p K3 K3 |as, t| p p ⎫ j j−p/p & & p − j j/p ⎬ &a ξ, η & K K4 , g2 ξ, η, m, n ⎭ p 4 p ⎧ μm, n M−1 N−1 ⎨ sm0 tn0 h h−p/p Cs, t g1 s, t, m, n K3 p ⎩ ξm0 Cm, n exp m−1 n−1 ⎫ j j−p/p ⎬ g2 ξ, η, m, n K4 C ξ, η , ⎭ p ηn0 s t Bs, t, m, n , sm0 tn0 s t r r−p/p q q−p/p Bs, t, m, n f1 s, t, m, n K1 f2 ξ, η, m, n K2 . p p ξm ηn0 0 3.3 Abstract and Applied Analysis 19 Proof. From 3.1 we have |um, n|p ≤ |am, n| m−1 n−1 sm0 tn0 ⎤ ⎡ s t & & & & &F2 ξ, η, m, n, u ξ, η &⎦ ⎣&F s, t, m, n, us, t& 1 ξm0 ηn0 ⎤ ⎡ M−1 s t & M−1 & & & &G2 ξ, η, m, n, u ξ, η &⎦ ⎣&G1 s, t, m, n, us, t& sm0 tn0 ξm0 ηn0 ≤ |am, n| m−1 n−1 ⎡ ⎤ s t &q & & &r ⎣f1 s, t, m, n &us, t& f2 ξ, η, m, n &u ξ, η & ⎦ sm0 tn0 ⎡ ξm0 ηn0 ⎤ s t &h & & &j ⎣g1 s, t, m, n &us, t& g2 ξ, η, m, n &u ξ, η & ⎦. M−1 M−1 sm0 tn0 ξm0 ηn0 3.4 Then a suitable application of Theorem 2.5 with l1 l2 1 to 3.4 yields the desired result. The following theorem deals with the uniqueness of the solutions of 3.1. Theorem 3.3. Suppose that |Fi s, t, m, n, u − Fi s, t, m, n, v| ≤ fi s, t, m, n|up − vp |, | Gi s, t, m, n, u − Gi s, t, m, n, v| ≤ gi s, t, m, n|up − vp |, i 1, 2 hold for ∀u, v ∈ R, where f , gi ∈ ℘ Ω2 , i 1, 2 with fi , gi nondecreasing in the last two variables, and μM, N s N−1 t i M−1 sm0 tn0 {g1 s, t, M, NCs, t ηn0 g2 ξ, η, M, NCξ, η} < 1, where Cm, n ξm0 m−1 n−1 exp{ sm0 tn0 Bs, t, m, n}, and Bs, t, m, n f1 s, t, m, n sξm0 tηn0 f2 ξ, η, m, n, then 3.1 has at most one solution. Proof. Suppose that u1 m, n, u2 m, n are two solutions of 3.1. Then, ⎡ n−1 & m−1 & & p & ⎣&& & p F1 s, t, m, n, u1 s, t − F1 s, t, m, n, u2 s, t& &u1 m, n − u2 m, n& ≤ sm0 tn0 ⎤ s t & & &F2 ξ, η, m, n, u1 ξ, η −F2 ξ, η, m, n, u2 ξ, η &⎦ ξm0 ηn0 ⎡ & & ⎣&G1 s, t, m, n, u1 s, t − G1 s, t, m, n, u2 s, t& M−1 M−1 sm0 tn0 ⎤ s t & & &G2 ξ, η, m, n, u1 ξ, η −G2 ξ, η, m, n, u2 ξ, η &⎦ ξm0 ηn0 20 Abstract and Applied Analysis ≤ m−1 n−1 sm0 tn0 ⎡ & & ⎣f1 s, t, m, n&&up s, t − up s, t&& 2 1 s t ξm0 ηn0 ⎤ & && p p & f2 ξ, η, m, n &u1 ξ, η − u2 ξ, η &⎦ ⎡ & & ⎣g1 s, t, m, n&&up s, t − up s, t&& 2 1 M−1 M−1 sm0 tn0 s t ξm0 ηn0 ⎤ && p && p g2 ξ, η, m, n &u1 ξ, η − u2 ξ, η &⎦. 3.5 p p Treat |u1 m, n − u2 m, n| as one variable, and a suitable application of Corollary 2.7 p p p p yields |u1 m, n − u2 m, n| ≤ 0, which implies that u1 m, n ≡ u2 m, n. Since p is an odd number, then we have u1 m, n ≡ u2 m, n, and the proof is complete. Finally we study the continuous dependence of the solutions of 3.1 on the functions a, F1 , F2 , G1 , G2 . Theorem 3.4. Suppose that um, n is a solution of 3.1, |Fi s, t, m, n, u − Fi s, t, m, n, v| ≤ fi s, t, m, n|up − vp |, |Gi s, t, m, n, u − Gi s, t, m, n, v| ≤ gi s, t, m, n|up − vp |, i 1, 2 hold for ∀u, v ∈ R, where fi , gi ∈ ℘ Ω2 , i 1, 2 with fi , gi nondecreasing in the last two variables, and, furthermore, ⎧ |am, n − am, n| m−1 n−1 ⎨& & & & &F1 s, t, us, t − F 1 s, t, us, t& sm0 tn0 ⎩ ⎫ s t & &⎬ & & &F2 ξ, η, u ξ, η − F 2 ξ, η, u ξ, η & ⎭ ηn ξm 0 0 ⎧ M−1 & N−1 ⎨&& & &G1 s, t, us, t − G1 s, t, us, t& ⎩ sm tn 0 3.6 0 ⎫ s t & &&⎬ & &G2 ξ, η, u ξ, η − G2 ξ, η, u ξ, η & ≤ ε, ⎭ ξm ηn 0 0 where ε > 0 is a constant, and um, n ∈ ℘Ω is the solution of the following difference equation: Abstract and Applied Analysis 21 m−1 n−1 p u m, n am, n ⎡ ⎣F 1 s, t, m, n, us, t sm0 tn0 ⎤ F 2 ξ, η, m, n, u ξ, η ⎦ ξm0 ηn0 ⎡ s t M−1 M−1 s t sm0 tn0 ξm0 ηn0 ⎣G1 s, t, m, n, us, t ⎤ G2 ξ, η, m, n, u ξ, η ⎦, 3.7 where F i , Gi : Ω2 × R → R, i 1, 2; then one has & & p &u m, n − up m, n& ≤ ε 1 JM, N Cm, n , 1 − μM, N 3.8 provided that μM, N < 1, where ⎧ Jm, n m−1 n−1 ⎨ f1 s, t, m, n sm0 tn0 ⎩ s t ξm0 ηn0 ⎧ M−1 N−1 ⎨ μm, n ⎩ ξm0 ⎧ M−1 N−1 ⎨ g1 s, t, m, nCs, t sm0 tn0 Cm, n exp ⎫ ⎬ g2 ξ, η, m, n , ⎭ ηn s t g1 s, t, m, n sm0 tn0 ⎫ ⎬ f2 ξ, η, m, n ⎭ ⎩ 0 ⎫ ⎬ g2 ξ, η, m, n C ξ, η , ⎭ ηn s t ξm0 m−1 n−1 3.9 0 Bs, t, m, n , sm0 tn0 Bs, t, m, n f1 s, t, m, n s t f2 ξ, η, m, n . ξm0 ηn0 Proof. From 3.1 and 3.7 we deduce & p & &u m, n−up m, n& ≤ |am, n−am, n| ⎧ m−1 n−1 ⎨& & & & &F1 s, t, m, n, us, t−F 1 s, t, m, n, us, t& ⎩ sm tn 0 0 ⎫ s t & &⎬ & & &F2 ξ, η, m, n, u ξ, η −F 2 ξ, η, m, n, u ξ, η & ⎭ ξm ηn 0 0 22 Abstract and Applied Analysis ⎧ M−1 & N−1 ⎨&& & &G1 s, t, m, n, us, t − G1 s, t, m, n, us, t& ⎩ sm tn 0 0 ⎫ s t & &⎬ & & &G2 ξ, η, m, n, u ξ, η − G2 ξ, η, m, n, u ξ, η & ⎭ ξm ηn 0 0 ≤ |am, n − am, n| ⎧ m−1 n−1 ⎨& & & & &F1 s, t, m, n, us, t−F1 s, t, m, n, us, t& ⎩ sm tn 0 0 & & & & &F1 s, t, m, n, us, t − F 1 s, t, m, n, us, t& s t & & &F2 ξ, η, m, n, u ξ, η −F2 ξ, η, m, n, u ξ, η & ξm0 ηn0 ⎫ & &&⎬ & &F2 ξ, η, m, n, u ξ, η −F 2 ξ, η, m, n, u ξ, η & ⎭ ⎧ M−1 N−1 ⎨& sm0 tn0 ⎩ & & & &G1 s, t, m, n, us, t − G1 s, t, m, n, us, t& & & & & &G1 s, t, m, n, us, t − G1 s, t, m, n, us, t& s t & & &G2 ξ, η, m, n, u ξ, η −G2 ξ, η, m, n, u ξ, η & ξm0 ηn0 ⎫ & &&⎬ & &G2 ξ, η, m, n, u ξ, η − G2 ξ, η, m, n, u ξ, η & ⎭ ≤ε m−1 n−1 & '& &F1 s, t, m, n, us, t − F1 s, t, m, n, us, t& sm0 tn0 & &( &F2 ξ, η, m, n, u ξ, η − F2 ξ, η, m, n, u ξ, η & ⎧ M−1 N−1 ⎨& & &G1 s, t, m, n, us, t − G1 s, t, m, n, us, t& ⎩ sm tn 0 0 ⎫ s t & &⎬ &G2 ξ, η, m, n, u ξ, η −G2 ξ, η, m, n, u ξ, η & ⎭ ξm ηn 0 ≤ε m−1 n−1 sm0 tn0 ⎡ 0 & & ⎣f1 s, t, m, n&up s, t − up s, t& Abstract and Applied Analysis 23 s t ⎤ & p p & f2 ξ, η, m, n &u ξ, η −u ξ, η &⎦ ξm0 ηn0 ⎡ M−1 M−1 & & ⎣g1 s, t, m, n&up s, t − up s, t& sm0 tn0 s t ⎤ & & p g2 ξ, η, m, n &u ξ, η − up ξ, η &⎦. ξm0 ηn0 3.10 Then a suitable application of Corollary 2.7 yields the desired result. 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