Research Article Existence and Uniqueness Results for Perturbed Neumann Boundary Value Problems

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1 Hindawi Publishing Corporation Boundary Value Problems Volume 2, Article ID 4942, pages doi:.55/2/4942 Research Article Existence and Uniqueness Results for Perturbed Neumann Boundary Value Problems Jieming Zhang and Chengbo Zhai 2 Business College of Shanxi Universit, Taiyuan, Shanxi 33, China 2 School of Mathematical Sciences, Shanxi University, Taiyuan, Shanxi 36, China Correspondence should be addressed to Chengbo Zhai, cbzhai@sxu.edu.cn Received 2 April 2; Accepted 28 June 2 Academic Editor: Irena Rachůnková Copyright q 2 J. Zhang and C. Zhai. 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. Using a fixed point theorem of general α-concave operators, we present in this paper criteria which guarantee the existence and uniqueness of positive solutions for two classes of nonlinear perturbed Neumann boundary value problems for second-order differential equations. The theorems for Neumann boundary value problems obtained are very general.. Introduction and Preliminaries In this paper, we are interested in the existence and uniqueness of positive solutions for the following nonlinear perturbed Neumann boundary value problems NBVPs : ±u t m 2 u t f t, u t g t, <t<, P ± u u,. where m is a positive constant, f :,,, and g :,, are continuous. It is well known that Neumann boundary value problem for the ordinary differential equations and elliptic equations is an important kind of boundary value problems. During the last two decades, Neumann boundary value problems have deserved the attention of many researchers. By using-fixed point theorems in cone, in, 5, 7 9, the authors discussed the existence of positive solutions for ordinary differential equation Neumann boundary value problems.

2 2 Boundary Value Problems Recently, the authors 4 discussed second-order superlinear repulsive singular Neumann boundary value problems by using a nonlinear alternative of Leray-Schauder and Krasnosel skii fixed-point theorem on compression and expansion of cones, and obtained the existence of at least two positive solutions under reasonable conditions. In 6, the authors established the existence of sign-changing solutions and positive solutions for fourth-order Neumann boundary value problem by using the fixed-point index and the critical group. Besides the above methods mentioned, the method of upper and lower solutions is also used in the literature 2, 3,. However, to the best of our knowledge, few papers can be found in the literature on the existence and uniqueness of positive solutions for the NBVPs P ±. Different from the above works mentioned, in this paper, we will use a fixed-point theorem of general α-concave operators to show the existence and uniqueness of positive solutions for the NBVPs P ±. By a positive solution of P ±, we understand a function u t C 2, which is positive on < t < and satisfies the differential equation and the boundary conditions in P ±. We now present a fixed point theorem of general α-concave operators which will be used in the latter proofs. Let E be a real Banach space and P be a cone in E, θ denotes the null element. Given h>θ i.e., h θ and h / θ, we denote by P h the set P h { x P λ x,μ x > such that λ x h x μ x h }..2 See for further information. Theorem. see. Assume that cone P is normal and operator A satisfies the following conditions: B A : P h P h is increasing in P h, B 2 for for all x P h and t,, there exists α t, such that A tx t α t Ax, B 3 there is a constant l such that x θ, lh. Then operator equation x Ax x has a unique solution in P h. Remark.2. An operator A is said to be general α-concave if A satisfies condition B Positive Solutions for the Problems P ± In this section, we will apply Theorem. to study the general NBVPs P ± and then we will obtain new results on the existence and uniqueness of positive solutions for the problems P ±. The following conditions will be assumed: H f t, x is increasing in x for fixed t, H 2 for any γ, and x, there exists ϕ γ γ, such that f t, γx ϕ γ f t, x for t,, H 3 for any t,,f t, a >, where a /2 ch m. In the following, we will work in the Banach space C, and only the sup-norm is used. Set P {x C, x t,t, }, the standard cone. It is easy to see that P is

3 Boundary Value Problems 3 a normal cone of which the normality constant is. Let G t, s be the Green s function for the boundary value problem u t m 2 u t, u u. <t<, 2. Then, G t, s ψ s ψ t, s t, ρ ψ t ψ s, t s, 2.2 where ρ m sh m, ψ t ch mt. It is obvious that ψ t is increasing on,, and <G t, s G t, t, t, s. 2.3 Lemma 2. see 9. Let G t, s be the Green s function for the NBVP 2.. then, G t, s ch 2 m ch mt ch t m G t,s, t,t,s,. 2.4 Theorem 2.2. Assume H H 3 hold. Then the NBVP P has a unique positive solution u in P h, where h t ψ t ψ t /2 ch m ch m 2mt, t,. Remark 2.3. Let b /2 e m e m. Then it is easy to check that a min{h t : t, } /2 ch m,b max{h t : t, } ch m. Proof of Theorem 2.2. It is well known that u is a solution of the NBVP P if and only if u t G t, s [ f s, u s g s ] ds, 2.5 where G t, s is the Green s function for the NBVP 2.. For any u P, we define Au t G t, s f s, u s ds, x t G t, s g s ds. 2.6 It is easy to check that A : P P. From H, we know that A : P P is an increasing operator. Next we show that A satisfies the conditions B, B 2 in Theorem.. From H 2, for any γ, and u P, there exists ϕ γ γ, such that A ( γu ) t G t, s f ( s, γu s ) ds G t, s ϕ ( γ ) f s, u s ds ϕ ( γ ) Au t, t,. 2.7

4 4 Boundary Value Problems That is, A γu ϕ γ Au, for all u P, γ,. Set α ( γ ) ln ϕ( γ ) ln γ, 2.8 then α γ, and A ( γu ) γ α γ Au, for γ,, u P. 2.9 In the following, we show that A : P h Lemma 2. and Remark 2.3, that P h. On one hand, it follows from H, H 3, Ah t G t, s f s, h s ds ch 2 m ψ t ψ t G t,s f s, a ds ch 2 m h t G t,s f s, a ds, t,. 2. On the other hand, from 2.3, H,andRemark 2.3, weobtain Ah t G t, s f s, h s ds G t, t f s, b ds 2. ρ h t f s, b ds, t,. Let r min f t, a, r t, 2 maxf t, b. 2.2 t, Then <r r 2. Note that t G t,s ds ρ ψ s ψ t ds ρ t ψ t ψ s ds m Consequently, r ch 2 m m 2 h t Ah t r 2 h t, t,. 2.4 mshm

5 Boundary Value Problems 5 Hence Ah P h. For any u P h, we can choose a small number t, such that t h u t h. 2.5 By 2.9,weget ( ) A γ u Au, γ,, u P. 2.6 γ α γ Thus, from 2.9, 2.6, we have Au A t h t α t Ah, ( ) Au A h t t Ah. α t 2.7 Thus, Au P h. Therefore, A : P h P h. This together with 2.9 implies that A is general α-concave. That is, A satisfies the conditions B, B 2 in Theorem.. Next we show that the condition B 3 is satisfied. If g t, then x t ; if g t /, let l ρ max t, g t, then l>. It is easy to prove that x t l ρ G t, t ds lh t. 2.8 Hence, x lh. Finally, using Theorem., u Au x has a unique solution u in P h. That is, u is a unique positive solution of the NBVP P in P h. In the following, using the same technique, we study the general NBVP P with m,π/2. Let G t, s be the Green s function for the boundary value problem u t m 2 u t, u u. <t<, 2.9 Then, cos ms cos m t, s t, G t, s m sin m cos mt cos m s, t s. 2.2 It is obvious that cos mt is decreasing on,, and G t, s G t, t, t, s. 2.2 Lemma 2.4. Let G t, s be the Green s function for the NBVP 2.9. Then, G t, s cos 2 m cos mt cos m t G t,s, t,t,s,. 2.22

6 6 Boundary Value Problems Proof. When t, t s, G t, s cos m s cos mt cos m t cos mt G t,s cos m s cos mt cos m t cos mt cos m t cos mt C cos m t cos mt. cos 2 m 2.23 If t s t, G t, s cos m s cos mt cos m t cos mt cos m s G t,s cos m t cos ms cos m t cos ms cos m t cos m t cos mt C cos m t cos mt. cos 2 m 2.24 If t s t, G t, s cos m t cos ms cos m t cos mt cos ms G t,s cos m s cos mt cos m s cos mt cos mt cos m t cos mt C cos m t cos mt. cos 2 m 2.25 For s t, t, G t, s cos m t cos ms cos m t cos mt G t,s cos m t cos ms cos m t cos mt cos m t cos mt C cos m t cos mt. cos 2 m 2.26 Therefore, G t, s This completes the proof. cos 2 m cos m t cos mt G t,s, t,t,s, Theorem 2.5. Assume H, H 2 hold and f t, cos 2 m > for any t,. Then the NBVP P has a unique positive solution u in P h, where h t cos m t cos mt, t,. Remark 2.6. It is easy to check that cos 2 m h t fort,. Proof of Theorem 2.5. It is well known that u is a solution of the NBVP P if and only if u t G t, s [ f s, u s g s ] ds, 2.28

7 Boundary Value Problems 7 where G t, s is the Green s function for the NBVP 2.9. For any u P, we define Au t G t, s f s, u s ds, x t G t, s g s ds Similar to the proof of Theorem 2.2, we know that A : P P is an increasing operator and satisfies the condition A ( γu ) ϕ ( γ ) Au γ α γ Au, u P, γ,, 2.3 where α γ ln ϕ γ / ln γ. It follows from condition H, Lemma 2.4, andremark 2.6 that Ah t G t, s f s, h s ds cos 2 m cos mt cos m t G t,s f s, ds cos 2 m h t G t,s f s, ds, t,. 2.3 From 2.2, H,andRemark 2.6, weobtain Ah t G t, s f s, h s ds ( ) G t, t f s, cos 2 m ds m sin m h t ( ) f s, cos 2 m ds, t, Let ( ) r min f t, cos 2 m, r 2 maxf t, t, t, Then <r r 2. Consequently, Ah t r 2 cos 2 m G t,s ds h t, Ah t r h t, t, m sin m

8 8 Boundary Value Problems Note that G t,s ds m sin m t cos m t cos msds m sin m cos m s cos mt ds t m, we have r 2 /cos 2 m G t,s ds >. Hence Ah P h. The same reasoning as Theorem 2.2 shows that A is general α-concave and B 3 is satisfied. Using Theorem., u Au x has a unique solution u in P h.thatis,u is a unique positive solution of the NBVP P in P h. Remark 2.7. For the case of g t, the problems P ± reduce to the usual forms of Neumann boundary value problems for ordinary differential equations. We can establish the existence and uniqueness of positive solutions for these problems by using the same method used in this paper, which is new to the literature. So the method used in this paper is different from previous ones in literature and the results obtained in this paper are new. 3. Examples To illustrate how our main results can be used in practice we present two examples. Example 3.. Consider the following NBVP: u t ln 2 2 u t u β t q t t 2, u u, <t<, 3. where β, and q :,, is a continuous function. In this example, we let m ln 2,f t, x : x β q t,g t : t 2. After a simple calculation, we get a 9/8,b 5/4and h t ( 2 2t 2 2t ), t,. 3.2 Evidently, f t, x is increasing for x, and g t /, f t, a ( ) 9 β q t > Moreover, set ϕ γ γ β,γ,. Then, f ( t, γx ) γ β x β q t γ β( ) x β q t ϕ ( γ ) f t, x, x. 3.4 Hence, all the conditions of Theorem 2.2 are satisfied. An application of Theorem 2.2 implies that the NBVP 3. has a unique positive solution u in P h.

9 Boundary Value Problems 9 Example 3.2. Consider the following NBVP: u t ( ) π 2 u t u /3 t q t t 3, <t<, 3 u u, 3.5 where q :,, is a continuous function. In this example, we let m π/3,f t, x : x /3 q t,g t : t 3. Then, m,π/2 and h t cos π 3 t cos π t, t, Evidently, f t, x is increasing for x, and g t /, ( f t, cos 2 π ) q t 3 ( ) /3 q t > Moreover, set ϕ γ γ /3,γ,. Then, f ( t, γx ) γ /3 x /3 q t γ /3( ) x /3 q t ϕ ( γ ) f t, x, x. 3.8 Hence, all the conditions of Theorem 2.5 are satisfied. An application of Theorem 2.5 implies that the NBVP 3.5 has a unique positive solution u in P h. Acknowledgment Research was supported by the Youth Science Foundation of Shanxi Province References A. Bensedik and M. Bouchekif, Symmetry and uniqueness of positive solutions for a Neumann boundary value problem, Applied Mathematics Letters, vol. 2, no. 4, pp , A. Cabada and R. L. Pouso, Existence result for the problem ϕ u f t, u, u with periodic and Neumann boundary conditions, Nonlinear Analysis: Theory, Methods & Applications, vol. 3, no. 3, pp , A. Cabada and L. Sanchez, A positive operator approach to the Neumann problem for a second order ordinary differential equation, Journal of Mathematical Analysis and Applications, vol. 24, no. 3, pp , J. Chu, X. Lin, D. Jiang, D. O Regan, and R. P. Agarwal, Positive solutions for second-order superlinear repulsive singular Neumann boundary value problems, Positivity, vol. 2, no. 3, pp , D.-Q. Jiang and H.-Z. Liu, Existence of positive solutions to second order Neumann boundary value problems, Journal of Mathematical Research and Exposition, vol. 2, no. 3, pp , 2. 6 F. Li, Y. Zhang, and Y. Li, Sign-changing solutions on a kind of fourth-order Neumann boundary value problem, Journal of Mathematical Analysis and Applications, vol. 344, no., pp , J.-P. Sun and W.-T. Li, Multiple positive solutions to second-order Neumann boundary value problems, Applied Mathematics and Computation, vol. 46, no., pp , 23.

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