Hindawi Publishing Corporation Discrete Dynamics in Nature and Society Volume 2009, Article ID 618413, 13 pages doi:10.1155/2009/618413 Research Article Successive Iteration and Positive Solutions for Nonlinear m-Point Boundary Value Problems on Time Scales Yanbin Sang Department of Mathematics, North University of China, Taiyuan, Shanxi 030051, China Correspondence should be addressed to Yanbin Sang, sangyanbin@126.com Received 3 June 2009; Accepted 16 September 2009 Recommended by Yong Zhou We study the existence of positive solutions for a class of m-point boundary value problems on time scales. Our approach is based on the monotone iterative technique and the cone expansion and compression fixed point theorem of norm type. Without the assumption of the existence of lower and upper solutions, we do not only obtain the existence of positive solutions of the problem, but also establish the iterative schemes for approximating the solutions. Copyright q 2009 Yanbin Sang. 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. 1. Introduction The purpose of this paper is to consider the existence of positive solutions and establish the corresponding iterative schemes for the following m-point boundary value problems BVP on time scales: uΔ∇ t ft, ut 0, βu0 − γuΔ 0 0, u1 t ∈ 0, 1T , m−2 αi uξi , m ≥ 3. 1.1 i1 The study of dynamic equations on time scales goes back to its founder Hilger 1, and is a new area of still fairly theoretical exploration in mathematics. Motivating the subject is the notion that dynamic equations on time scales can build bridges between continuous and discrete mathematics. Some preliminary definitions and theorems on time scales can be found in 2–5 which are good references for the calculus of time scales. 2 Discrete Dynamics in Nature and Society In recent years, by applying fixed point theorems, the method of lower and upper solutions and critical point theory, many authors have studied the existence of positive solutions for two-point and multipoint boundary value problems on time scales, for details, see 2, 3, 6–18 and references therein. However, to the best of our knowledge, there are few papers which are concerned with the computational methods of the multipoint boundary value problems on time scales. We would like to mention some results of Sun and Li 16, Aykut Hamel and Yoruk 12, Anderson and Wong 10, Wang et al. 18, and Jankowski 13, which motivated us to consider the BVP 1.1. In 16, Sun and Li considered the existence of positive solutions of the following dynamic equations on time scales: uΔ∇ t atft, ut 0, βu0 − γuΔ 0 0, t ∈ 0, T , 1.2 αu η uT , where β, γ ≥ 0, βγ > 0, η ∈ 0, ρT , 0 < α < T/η. They obtained the existence of single and multiple positive solutions of 1.2 by using a fixed point theorem and the Leggett-Williams fixed point theorem, respectively. Very recently, in 12, Aykut Hamel and Yoruk discussed the following dynamic equation on time scales: uΔ∇ t ft, ut 0, βu0 − γuΔ 0 0, u1 t ∈ 0, 1 ⊂ T, m−2 αi uξi , m ≥ 3. 1.3 i1 They obtained some results for the existence of at least two and three positive solutions to the BVP 1.3 by using fixed point theorems in a cone and the associated Green’s function. In related paper, in 10, Anderson and Wong studied the second-order time scale semipositone boundary value problem with Sturm-Liouville boundary conditions or multipoint conditions as in puΔ ∇ t λft, ut 0, αua − β puΔ a 0, n φi puΔ ti , αua − β puΔ a i1 t ∈ a, bT , γuσ b δ puΔ b 0, or n ψi puΔ ti . γuσ b δ puΔ b i1 1.4 Discrete Dynamics in Nature and Society 3 On the other hand, the method of lower and upper solutions has been effectively used for proving the existence results for dynamic equations on time scales. In 18, Wang et al. considered a method of generalized quasilinearization, with even-order k k ≥ 2 convergence, for the BVP ∇ − ptxΔ qtxσ ft, xσ gt, xσ , τ1 x ρa − τ2 xΔ ρa 0, t ∈ a, bT , xσb − τ3 x η 0. 1.5 The main contribution in 18 relaxed the monotone conditions on f i t, x, g i t, x 1 < i < k including a more general concept of upper and lower solution in mathematical biology, so that the high-order convergence of the iterations was ensured for a larger class of nonlinear functions on time scales. In 13, Jankowski investigated second-order differential equations with deviating arguments on time scales of the form −xΔΔ t ft, xt, xαt ≡ Fxt, x0 k1 ∈ R, t ∈ J, xT k2 ∈ R. 1.6 They formulated sufficient conditions, under which such problems had a minimal and a maximal solution in a corresponding region bounded by upper-lower solutions. We would also like to mention the result of Yao 19. In 19, Yao considered the positive solutions to the following two classes of nonlinear second-order three-point boundary value problems: u t f t, ut, u t 0, u t ft, ut 0, u0 0, 0 ≤ t ≤ 1, 0 ≤ t ≤ 1, 1.7 αu η u1, where both η and α are given constants satisfying 0 < η < 1, 0 < α < 1/η. By improving the classical monotone iterative technique of Amann 20, two successive iterative schemes were established for the BVP 1.7. It was worth stating that the first terms of the iterative schemes were constant functions or simple functions. We note that Ma et al. 21 and Sun et al. 22, 23 have also applied the similar methods to p-laplacian boundary value problems with T R. In this paper, we will investigate the iterative and existence of positive solutions for the BVP 1.1, by considering the “heights” of the nonlinear term f on some bounded sets and applying monotone iterative techniques on a Banach space, we do not only 4 Discrete Dynamics in Nature and Society obtain the existence of positive solutions for the BVP 1.1, but also give the iterative schemes for approximating the solutions. We should point out that the monotone condition imposed on the nonlinear term f will play crucial role in obtaining the iterative schemes for approximating the solutions. In essence, we combine the method of lower and upper solutions with the cone expansion and compression fixed point theorem of norm type. The idea of this paper comes from Yao 19, 24, 25. Let T be a time scale which has the subspace topology inherited from the standard topology on R. For each interval I of R, we define IT I ∩ T. For the remainder of this article, we denote the set of continuous functions from 0, 1T to R by C0, 1T , R. Let C0, 1T , R be endowed with the ordering x ≤ y if xt ≤ yt for all t ∈ 0, 1T , and u maxt∈0,1T |ut| is defined as usual by maximum norm. The C0, 1T , R is a Banach space. Throughout this paper, we will assume that the following assumptions are satisfied: H1 β, γ ≥ 0, 0 < β γ ≤ 1, ξi ∈ 0, ρ1T for i 1, 2, . . . , m − 2 with 0 < ξ1 < ξ2 < · · · < ξm−2 < ρ1; H2 m−2 α ∈ 0, 1 with αi ∈ 0, ∞ for i 1, 2, . . . , m − 2 and d β1 − i γ1 − m−2 i1 αi > 0; i1 m−2 i1 αi ξi H3 f : 0, 1T × 0, ∞ → 0, ∞ is continuous. 2. Preliminaries and Several Lemmas To prove the main results in this paper, we will employ several lemmas. These lemmas are based on the linear boundary value problem uΔ∇ t ht 0, βu0 − γuΔ 0 0, t ∈ 0, 1T , u1 m−2 αi uξi , m ≥ 3. 2.1 i1 Lemma 2.1 see 12. It holds d β1 − for the BVP m−2 i1 αi ξi γ1 − −uΔ∇ t 0, βu0 − γuΔ 0 0, m−2 i1 αi / 0; then the Green’s function t ∈ 0, 1T , u1 m−2 αi uξi , i1 m ≥ 3, 2.2 Discrete Dynamics in Nature and Society 5 is given by Gt, s ⎤ ⎡ ⎧ m−2 ⎪ ⎪ ⎪ ⎪ βs γ ⎣1 − t − aj ξj − t ⎦, ⎪ ⎪ ⎪ j1 ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ if 0 ≤ t ≤ 1, 0 ≤ s ≤ ξ1 , s ≤ t; ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ m−2 i−1 ⎪ ⎪ ⎪ ⎪ βs γ ξ α − t βs γ αj βξj γ t − s, − t − 1 j j ⎪ ⎪ ⎪ ⎪ ji j1 ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ 1 ⎨ if ξr−1 ≤ t ≤ ξr , 2 ≤ r ≤ m − 1, ξi−1 ≤ s ≤ ξi , 2 ≤ i ≤ r, s ≤ t; d⎪ ⎤ ⎡ ⎪ ⎪ ⎪ m−2 ⎪ ⎪ ⎪ ⎪ αj ξj − s ⎦, βt γ ⎣1 − s − ⎪ ⎪ ⎪ ⎪ ji ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ if ξr−1 ≤ t ≤ ξr , 2 ≤ r ≤ m − 2, ξi−1 ≤ s ≤ ξi , r ≤ i ≤ m − 2, t ≤ s; ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ βt γ 1 − s, ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩ if 0 ≤ t ≤ 1, ξm−2 ≤ s ≤ 1, t ≤ s. Here for the sake of convenience, one writes m2 im1 2.3 hi 0 for m2 < m1 . Lemma 2.2 see 12. Assume that conditions H1 –H3 are satisfied. Then i Gt, s ≥ 0 for t, s ∈ 0, 1T ; ii there exist a number Ψ ∈ 0, 1 and a continuous function θ : 0, 1T → R such that Gt, s ≤ θs for t, s ∈ 0, 1T , Gt, s ≥ Ψθs for t ∈ ξ1 , 1T , s ∈ 0, 1T , 2.4 where θs max 1, Ψ m−2 i1 ξ1 αi βs γ 1 − s , d 1 max 1, m−2 i1 αi /ξ1 ⎧ ⎨ 2.5 ⎫ s−1 m−2 ⎬ αj 1 − ξj , αj βξj γ αj 1 − ξj βξ1 γ 1 − ξm−2 , × min . ⎭ 2≤s≤m−2⎩ js j1 j1 m−2 6 Discrete Dynamics in Nature and Society Let B C0, 1T , R. It is easy to see that the BVP 1.1 has a solution u ut if and only if u is a fixed point of the operator equation: T ut 1 2.6 Gt, sfs, us∇s. 0 Denote K u ∈ B : u is nonnegative, concave, and min ut ≥ Ψ u , t∈ξ1 ,1T 2.7 where Ψ is the same as in Lemma 2.2. By 12, Lemma 3.1, we can obtain that T K ⊂ K and T : K → K is completely continuous. 3. Successive Iteration and One Positive Solution for 1.1 For notational convenience, we denote A −1 1 max t∈0,1T Gt, s∇s , B 0 −1 1 max t∈0,1T Gt, s∇s . 3.1 ξ1 Constants A, B are not easy to compute explicitly. For convenience, we can replace A by A , B by B , where A 1 −1 θs∇s , 1 −1 B Ψ θs∇s . 0 3.2 ξ1 Obviously, 0 < A < A < B < B . Theorem 3.1. Assume H1 –H3 hold, and there exist two positive numbers a, b with b < a such that C1 max{ft, a : t ∈ 0, 1T } ≤ aA, min{ft, Ψb : t ∈ ξ1 , 1T } ≥ bB; C2 ft, u1 ≤ ft, u2 for any t ∈ 0, 1T , 0 ≤ u1 ≤ u2 ≤ a. Then the BVP 1.1 has at least one positive solution u∗ such that b ≤ u∗ ≤ a and limn → ∞ T n u u∗ , n ∗ converges uniformly to u in 0, 1T , where u t ≡ a, t ∈ 0, 1T . that is, T u Remark 3.2. The iterative scheme in Theorem 3.1 is u1 T u , un1 T un , n 1, 2, . . . . It starts off with constant function u t ≡ a, t ∈ 0, 1T . Proof of Theorem 3.1. Denote Kb, a {u ∈ K : b ≤ u ≤ a}. If u ∈ Kb, a, then max ut ≤ a, t∈0,1T min ut ≥ Ψ u ≥ bΨ. t∈ξ1 ,1T 3.3 Discrete Dynamics in Nature and Society 7 By Assumptions C1 and C2 , we have ft, ut ≤ ft, a ≤ aA, t ∈ 0, 1T ; ft, ut ≥ ft, bΨ ≥ bB, t ∈ ξ1 , 1T . 3.4 It follows that 1 T u max Gt, sfs, us∇s t∈0,1T 0 ≤ aA max 1 t∈0,1T T u ≥ max t∈0,1T Gt, s∇s a; 0 3.5 1 Gt, sfs, us∇s ξi ≥ bB max 1 t∈0,1T Gt, s∇s b. ξi Thus, we assert that T : Kb, a → Kb, a. Let u t ≡ a, t ∈ 0, 1T , then u ∈ Kb, a. Let u1 T u , then u1 ∈ Kb, a. Denote un1 T un , n 1, 2, . . . . Since T Kb, a ⊂ Kb, a, we have un ∈ T Kb, a ⊂ Kb, a, n 1, 2, . . . . Since T is completely continuous, we assert that {un }∞ n1 has a convergent subsequence ∗ ∗ and there exists u ∈ Kb, a, such that u → u . {unk }∞ nk k1 Now, since u1 ∈ Kb, a, we have u1 t ≤ u1 ≤ a u t, t ∈ 0, 1T . 3.6 By Assumption C2 , u2 t T u1 t 1 Gt, sfs, u1 s∇s 0 ≤ 1 3.7 Gt, sfs, u s∇s 0 Tu t u1 t. By the induction, then un1 t ≤ un t, t ∈ 0, 1T , n 1, 2, . . . . 3.8 Hence, T n u un → u∗ . Applying the continuity of T and un1 T un , we get T u∗ u∗ . Since ∗ u ≥ b > 0 and u∗ is a nonnegative concave function, we conclude that u∗ is a positive solution of the BVP 1.1. 8 Discrete Dynamics in Nature and Society Corollary 3.3. Assume that H1 –H3 hold, and the following conditions are satisfied: C1 liml → 0 mint∈ξ1 ,1T ft, l/l > Ψ−1 B, liml → ∞ maxt∈0,1T ft, l/l < A (particularly, liml → 0 mint∈ξ1 ,1T ft, l/l ∞, liml → ∞ maxt∈0,1T ft, l/l 0); C2 ft, u1 ≤ ft, u2 for any t ∈ 0, 1T , u1 ≤ u2 , u1 , u2 ∈ 0, ∞. Then the BVP 1.1 has at least one positive solution u∗ ∈ K and there exists a positive number a such u∗ , that is, that limn → ∞ T n u t − u∗ t| 0, lim sup |T n u n→∞ t∈0,1T 3.9 where u t ≡ a, t ∈ 0, 1T . Theorem 3.4. Assume H1 –H3 hold, and the following conditions are satisfied: D1 there exists a > 0 such that ft, · : 0, a → 0, ∞ is nondecreasing for any t ∈ 0, 1T and max{ft, a : t ∈ 0, 1T } ≤ aA; D2 ft, 0 > 0, for any t ∈ 0, 1T . Then the BVP 1.1 has one positive solution u∗ such that 0 < u∗ ≤ a and limn → ∞ T n 0 u∗ , that is, T n 0 converges uniformly to u∗ in 0, 1T . Furthermore, if there exists 0 < ω < 1 such that ft, l2 − ft, l1 ≤ ωA|l2 − l1 |, t ∈ 0, 1T , 0 ≤ l1 , l2 ≤ a. 3.10 Then T n1 0 − u∗ ≤ ωn /1 − ω T 0 . Proof. Denote K0, a {u ∈ K : u ≤ a}. Similarly to the proof of Theorem 3.1, we can know 1 ∈ K0, a. Denote u n1 t T u n , n 1, 2, . . . . that T : K0, a → K0, a. Let u 1 T 0, then u Copying the corresponding proof of Theorem 3.1, we can prove that u n1 t ≥ u n t, t ∈ 0, 1T , n 1, 2, . . . . 3.11 Since T is completely continuous, we can get that there exists u∗ ∈ K0, a such that n1 t T u n , we can obtain that T u∗ u∗ . We u n → u∗ . Applying the continuity of T and u note that ft, 0 > 0, for all t ∈ 0, 1T , it implies that the zero function is not the solution of the problem 1.1. Therefore, u∗ is a positive solution of 1.1. Now, since ft, l2 − ft, l1 ≤ ωA|l2 − l1 |, t ∈ 0, 1T , 0 ≤ l1 , l2 ≤ a. 3.12 Discrete Dynamics in Nature and Society 9 If u1 , u2 ∈ K0, a and u2 t ≥ u1 t, t ∈ 0, 1T , then 1 T u2 − T u1 max Gt, s fs, u2 s − fs, u1 s ∇s t∈0,1T 0 ≤ ωA max 1 t∈0,1T Gt, s|u2 s − u1 s|∇s 0 3.13 ≤ ωA u2 − u1 A−1 ω u2 − u1 . Hence, we can deduce that n1 T u n ≤ ωn T 0 − 0 ωn T 0 , un2 − u n1 − T u n1 ≤ ω unk2 − u nk ω nk−1 3.14 ωn T 0 . 1−ω 3.15 ··· ω n ωn T 0 . T 0 < 1−ω It implies that T n1 0 − u∗ ≤ The proof is complete. 4. Existence of n positive solutions Theorem 4.1. Assume H1 –H3 hold, and there exist 2n positive numbers a1 , . . . , an , b1 , . . . , bn with b1 < a1 < b2 < a2 < · · · < bn < an such that E1 max{ft, ai : t ∈ 0, 1T } ≤ ai A, min{ft, Ψbi : t ∈ ξ1 , 1T } ≥ bi B, i 1, 2, . . . , n; E2 ft, u1 ≤ ft, u2 for any t ∈ 0, 1T , 0 ≤ u1 ≤ u2 ≤ an . Then the BVP 1.1 has n positive solutions u∗i , i 1, 2, . . . , n such that bi ≤ u∗i ≤ ai and i u∗i , that is, limn → ∞ T n u lim sup T n u i t − u∗i t 0, n→∞ t∈0,1T 4.1 where u i t ≡ ai , t ∈ 0, 1T , i 1, 2, . . . , n. Corollary 4.2. Assume that H1 –H3 and C1 –C2 hold, and the following condition is satisfied E there exist 2n − 1 positive numbers a1 < b2 < a2 < · · · < bn−1 < an−1 < bn such that ! " max ft, ai : t ∈ 0, 1T < ai A, i 1, . . . , n − 1, ! " min ft, Ψbi : t ∈ ξi , 1T > bi B, i 2, . . . , n. 4.2 10 Discrete Dynamics in Nature and Society Then the BVP 1.1 has n positive solutions u∗i , i 1, 2, . . . , n, and there exists a positive number i u∗i , where u i t ≡ ai , t ∈ 0, 1T , i 1, 2, . . . , n. an with an > bn such that limn → ∞ T n u 5. Examples # Example 5.1. Let T 0, 1/3 1/2, 1. Considering the following BVP: uΔ∇ t ft, u 0, u0 0, t ∈ 0, 1T , $ % $ % 1 1 1 1 u1 u u , 8 3 6 2 5.1 where ft, u 200/109u2 1, it is easy to check that ft, 0 1 > 0, for any t ∈ 0, 1T . Further calculations give us d A 7 , 8 1 −1 θs∇s 0 1 −1 8 s1 − s∇s 7 0 5.2 1/3 −1 1/2 1 8 s1 − sds s1 − s∇s s1 − sds 7 0 ρ1/2 1/2 567 . 109 Choose a 1, it is easy to check that ft, · : 0, 1T → 0, ∞ is nondecreasing for any t ∈ 0, 1T and max ft, 1 t∈0,1T 567 200 1≤1· . 109 109 5.3 Let u 0 t ≡ 0, for n 0, 1, 2, . . . , we have % $ % 200 8 1 200 u n s 1 ∇s t 1 − s u n s 1 ∇s u n1 t − t − s 109 7 0 109 0 1/3 $ 1/2 $ %$ %$ % % 200 200 8 1 1 1 1 −s u n s 1 ∇s −s u n s 1 ∇s . − t 7 8 0 3 109 6 0 2 109 5.4 t $ Discrete Dynamics in Nature and Society 11 By Theorem 3.4, the BVP 5.1 has one positive solution u∗ such that 0 < u∗ ≤ 1 and T n 0 → u∗ . On the other hand, for any 0 ≤ u1 , u2 ≤ 1, we have fu1 − fu2 200 u2 − u2 2 109 1 ≤ 400 567 400 · |u1 − u2 | |u1 − u2 | 109 109 567 400 A |u1 − u2 |. 567 5.5 Then, T n1 0 − u∗ ≤ $ % 567 400 n 400/567n 0 T 0 . T 1 − 400/567 167 567 5.6 The first and second terms of this scheme are as follows: u 0 t 0, ⎧ 2 199t t ⎪ ⎪ ⎪ ⎨− 2 378 , u 1 t ⎪ ⎪ 1 t2 199t ⎪ ⎩− , 2 378 72 & ' 1 t ∈ 0, , 3 & ' 1 t∈ ,1 . 2 5.7 Now, we compute the third term of this scheme. For t ∈ 0, 1/3, u 2 t 175 4 9950 3 t2 t − t − 327 61803 2 $ % 6720175 199 t. 70084602 378 5.8 $ % 6720175 100097 t. 70084602 185409 5.9 For t ∈ 1/2, 1, u 2 t 175 4 9950 3 503t2 t − t − 327 61803 981 # Example 5.2. Let T {0} {1/3n : n ∈ N0 }. Considering the BVP on T, uΔ∇ ( uΔ 0 0, ut 0, t ∈ 0, 1T , $ % $ % 1 1 1 1 u . u1 u 3 9 9 3 5.10 By direct computation, we can get d 5 , 9 A 9 , 16 B 81 , 8 Ψ 5 . 54 5.11 12 Discrete Dynamics in Nature and Society Choose a 100, b 1/1875, it is easy to see that the nonlinear term ft, u fu possesses the following properties ) ut a f : 0, 1T × 0, ∞ → 0, ∞ is continuous; b ft, u1 ≤ ft, u2 for any t ∈ 0, 1T and 0 ≤ u1 ≤ u2 ≤ 100; √ c ) max{ft, 100 : t ∈ 0, 1T } 100 ≤ aA 100 × 9/16, min{ft, Ψb : t ∈ ξ1 , 1T } 5/54 × 1/1875 > bB 1/1875 × 81/8. By Theorem 3.1, the BVP 5.10 has one positive solution u∗ such that 1/1875 ≤ u∗ ≤ 100 u∗ , where u t ≡ 100, t ∈ 0, 1T . Let u0 t ≡ 100, t ∈ 0, 1T . For n and limn → ∞ T n u 0, 1, 2, . . . , we have un1 1 Gt, sfs, un s∇s 0 ( ( 9 1 − t − s un s∇s 1 − s un s∇s 5 0 0 $ $ %( %( 1 1/3 1 3 1/9 1 −s −s un s∇s − un s∇s. − 5 0 9 5 0 3 t 5.12 Remark 5.3. 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