Electronic Journal of Differential Equations, Vol. 2010(2010), No. 166, pp. 1–10. ISSN: 1072-6691. URL: http://ejde.math.txstate.edu or http://ejde.math.unt.edu ftp ejde.math.txstate.edu IMPULSIVE BOUNDARY-VALUE PROBLEMS FOR FIRST-ORDER INTEGRO-DIFFERENTIAL EQUATIONS XIAOJING WANG, CHUANZHI BAI Abstract. This article concerns boundary-value problems of first-order nonlinear impulsive integro-differential equations: y 0 (t) + a(t)y(t) = f (t, y(t), (T y)(t), (Sy)(t)), t ∈ J0 , ∆y(tk ) = Ik (y(tk )), k = 1, 2, . . . , p, Z c y(0) + λ y(s)ds = −y(c), λ ≤ 0, 0 where J0 = [0, c] \ {t1 , t2 , . . . , tp }, f ∈ C(J × R × R × R, R), Ik ∈ C(R, R), a ∈ C(R, R) and a(t) ≤ 0 for t ∈ [0, c]. Sufficient conditions for the existence of coupled extreme quasi-solutions are established by using the method of lower and upper solutions and monotone iterative technique. Wang and Zhang [18] studied the existence of extremal solutions for a particular case of this problem, but their solution is incorrect. 1. Introduction In recent years, many authors have paid attention to the research of differential equations with impulsive boundary conditions, because of their potential applications; see for example [4, 6, 9, 12, 13, 15, 17]. First-order and second-order impulsive differential equations with anti-periodic boundary conditions have also drawn much attention; see [1, 2, 3, 5, 7, 8, 14, 16, 19]. Recently, Wang and Zhang [18] studied the existence of extremal solutions of the following nonlinear anti-periodic boundary value problem of first-order integrodifferential equation with impulse at fixed points y 0 (t) = f (t, y(t), (T y)(t), (Sy)(t)), ∆y(tk ) = Ik (y(tk )), t ∈ J0 , k = 1, 2, . . . , p, (1.1) y(0) = −y(T ), where J = [0, T ], J0 = J \ {t1 , t2 , . . . , tp }, 0 < t1 < t2 < · · · < tp < T , f ∈ − C(J × R × R × R, R), Ik ∈ C(R, R), ∆y(tk ) = y(t+ k ) − y(tk ) denotes the jump of 2000 Mathematics Subject Classification. 34A37, 34B15. Key words and phrases. Impulsive integro-differential equation; coupled lower-upper quasi-solutions; monotone iterative technique. c 2010 Texas State University - San Marcos. Submitted November 1, 2010. Published November 17, 2010. Supported by grant 10771212 from the National Natural Science Foundation of China. 1 2 X. WANG, C. BAI EJDE-2010/166 − y(t) at t = tk ; y(t+ k ) and y(tk ) represent the right and left limits of y(t) at t = tk , respectively. Z t Z T (T y)(t) = k(t, s)y(s)ds, (Sy)(t) = h(t, s)y(s)ds, 0 0 + k ∈ C(D, R ), D = {(t, s) ∈ J × J : t ≥ s}, h ∈ C(J × J, R+ ). Unfortunately, their extremal solutions y∗ (t), y ∗ (t) are wrong. In fact, by [18, Theorem 3.1] we obtain y∗0 (t) = f (t, y∗ (t), (T y∗ )(t), (Sy∗ )(t)), ∆y∗ (tk ) = Ik (y∗ (tk )), t ∈ J0 , k = 1, 2, . . . , p, ∗ y∗ (0) = −y (T ), and y ∗0 (t) = f (t, y ∗ (t), (T y ∗ )(t), (Sy ∗ )(t)), ∗ ∗ ∆y (tk ) = Ik (y (tk )), t ∈ J0 , k = 1, 2, . . . , p, ∗ y (0) = −y∗ (T ), which implies that y∗ (t), y ∗ (t) are not solutions of (1.1). So the conclusions of [18] are reconsidered here, for a more general equation. In this paper, we investigate the following integral boundary value problem for first-order integro-differential equation with impulses at fixed points y 0 (t) + a(t)y(t) = f (t, y(t), (T y)(t), (Sy)(t)), t ∈ J0 , ∆y(tk ) = Ik (y(tk )), k = 1, 2, . . . , p, Z c y(0) + λ y(s)ds = −y(c), λ ≤ 0, (1.2) 0 where J = [0, c], J0 = J \ {t1 , t2 , . . . , tp }, 0 < t1 < t2 < · · · < tp < c, f ∈ C(J × R × R × R, R), Ik ∈ C(R, R), a ∈ C(R, R) and a(t) ≤ 0 for t ∈ J. Z t Z c (T y)(t) = k(t, s)y(s)ds, (Sy)(t) = h(t, s)y(s)ds, 0 0 k ∈ C(D, R+ ), D = {(t, s) ∈ J × J : t ≥ s}, h ∈ C(J × J, R+ ). Remark 1.1. If a(t) ≡ 0 and λ ≡ 0, then (1.2) reduces to (1.1). We will give the concept of coupled quasi-solutions of BVP (1.2) in next section. It is well known that the monotone iterative technique offers an approach for obtaining approximate solutions of nonlinear differential equations, for details, see [10, 11] and the references therein. The aim of this paper is to investigate the existence of coupled quasi-solutions of (1.2) by using the method of upper and lower solutions combined with a monotone iterative technique. Our result correct and generalize the main result of [18]. 2. Preliminaries In this section, we present some definitions needed for introducing the concept of quasi-solutions for (1.2). Let P C(J) = {y : J → R : y is continuous at t ∈ J0 ; − − y(0+ ), y(T − ), y(t+ k ), y(tk ) exist and y(tk ) = y(tk ), k = 1, . . . , p}, EJDE-2010/166 IMPULSIVE BOUNDARY-VALUE PROBLEMS 3 P C 1 (J) = {y ∈ P C(J) : y is continuously differentiable for t ∈ J0 ; 0 − y 0 (0+ ), y 0 (T − ), y 0 (t+ k ), y (tk ) exist, k = 1, . . . , p}, The sets P C(J) and P C 1 (J) are Banach spaces with the norms kykP C(J) = sup{|y(t)| : t ∈ J}, kykP C 1 (J) = kykP C(J) + ky 0 kP C(J) . Definition 2.1. Functions α0 , β0 ∈ P C 1 (J) are said to be coupled lower-upper quasi-solutions to the problem (1.2) if α00 (t) + a(t)α0 (t) ≤ f (t, α0 (t), (T α0 )(t), (Sα0 )(t)), t ∈ J0 , ∆α0 (tk ) ≤ Ik (α0 (tk )), k = 1, 2, . . . , p, Z c α0 (0) + λ α0 (s)ds ≤ −β0 (c), λ ≤ 0, 0 β00 (t) + a(t)β0 (t) ≥ f (t, β0 (t), (T β0 )(t), (Sβ0 )(t)), t ∈ J0 , (2.1) ∆β0 (tk ) ≥ Ik (β0 (tk )), k = 1, 2, . . . , p, Z c β0 (0) + λ β0 (s)ds ≥ −α0 (c), λ ≤ 0. 0 Note that if α0 (c) = β0 (c), then the above definition reduces to the notion of lower and upper solutions of (1.2). Definition 2.2. Functions v, w ∈ P C 1 (J) are said to be coupled quasi-solutions to (1.2) if v 0 (t) + a(t)v(t) = f (t, v(t), (T v)(t), (Sv)(t)), t ∈ J0 , ∆v(tk ) = Ik (v(tk )), k = 1, 2, . . . , p, Z c v(0) + λ v(s)ds = −w(c), λ ≤ 0, 0 w0 (t) + a(t)w(t) = f (t, w(t), (T w)(t), (Sw)(t)), t ∈ J0 , (2.2) ∆w(tk ) = Ik (w(tk )), k = 1, 2, . . . , p, Z c w(0) + λ w(s)ds = −v(c), λ ≤ 0. 0 1 Let α0 , β0 ∈ P C (J) and α0 (t) ≤ β0 (t) for t ∈ J0 . In what follows we define the segment [α0 , β0 ] = {u ∈ P C 1 (J) : α0 (t) ≤ u(t) ≤ β0 (t), t ∈ J}. Definition 2.3. Let u, v be coupled quasi-solutions of (1.2) such as u(t) ≤ v(t) for t ∈ J0 . Assume that α0 , β0 ∈ P C 1 (J) and α0 (t) ≤ β0 (t) for t ∈ J0 . Coupled quasi-solutions u, v of (1.2) are called coupled minimal-maximal quasi-solutions in segment [α0 , β0 ] if α0 (t) ≤ u(t), v(t) ≤ β0 (t) for t ∈ J0 and for any U, V coupled quasi-solutions of (1.2), such as α0 (t) ≤ U (t), V (t) ≤ β0 (t) for t ∈ J0 we have u(t) ≤ U (t) and V (t) ≤ v(t), t ∈ J0 . For convenience, we assume the following conditions are satisfied (H1) Functions α0 (t), β0 (t) are coupled lower-upper quasi-solutions of (1.2) such that α0 (t) ≤ β0 (t) for t ∈ J0 . (H2) There exist M > 0, N, N1 ≥ 0 such that f (t, x1 , y1 , z1 ) − f (t, x2 , y2 , z2 ) ≥ −M (x1 − x2 ) − N (y1 − y2 ) − N1 (z1 − z2 ), 4 X. WANG, C. BAI EJDE-2010/166 for α0 ≤ x2 ≤ x1 ≤ β0 , T α0 ≤ y2 ≤ y1 ≤ T β0 , Sα0 ≤ z2 ≤ z1 ≤ Sβ0 , t ∈ J. (H3) There exist 0 ≤ Lk < 1, k = 1, 2, . . . , p, satisfy Ik (x) − Ik (y) ≥ −Lk (x − y), for α0 ≤ y ≤ x ≤ β0 , t ∈ J. Now we consider the problem y 0 (t) + M y(t) + N (T y)(t) + N1 (Sy)(t) = σ(t), ∆y(tk ) = −Lk y(tk ) + bk , t ∈ J0 , k = 1, 2, . . . , p, (2.3) y(0) = b, where M > 0, N, N1 ≥ 0, Lk < 1, k = 1, 2, . . . , p. Lemma 2.4. If y ∈ P C 1 (J), M > 0, N, N1 ≥ 0, Lk < 1, k = 1, 2, . . . , p, and k̄ + h̄ + p X Li < 1, (2.4) i=1 where −1 −M c ), if M > 1, k0 cM (1 − e −1 k̄ = k0 cM (1 − M e−M c ), if 0 < M ≤ 1, 1 2 if M = 0. 2 k0 c , ( h0 cM −1 (1 − e−M c ), if M > 0, h̄ = h0 c2 , if M = 0, where k0 = max0≤s≤t≤c k(t, s) and h0 = max0≤t,s≤c h(t, s). Then (2.3) has a unique solution. Proof. If y ∈ P C 1 (J) is a solution of (2.3), then, by integrating, we obtain Z t −M t y(t) = be + e−M (t−s) [σ(s) − N (T y)(s) − N1 (Sy)(s)]ds 0 X + e−M (t−ti ) (−Li y(ti ) + bi ). (2.5) 0<ti <t Conversely, if y(t) ∈ P C(J) is solution of the above-mentioned integral equation (2.5), then it is easy to check that y 0 (t) = −M y(t) − N (T y)(t) − N1 (Sy)(t) + σ(t), t 6= tk , ∆y(tk ) = −Lk y(tk ) + bk , k = 1, 2, . . . , p, and y(0) = b. So (2.3) is equivalent to the integral equation (2.5). Now, we define operator B : P C(J) → P C(J) as Z t (By)(t) = be−M t + e−M (t−s) [σ(s) − N (T y)(s) − N1 (Sy)(s)]ds 0 (2.6) X + e−M (t−ti ) (−Li y(ti ) + bi ). 0<ti <t For each u, v ∈ P C(J), we have EJDE-2010/166 IMPULSIVE BOUNDARY-VALUE PROBLEMS 5 Z t |(Bu)(t) − (Bv)(t)| ≤N e−M (t−s) (T u − T v)(s)ds 0 Z t + N1 e−M (t−s) (Su − Sv)(s)ds 0 X + Li |e−M (t−ti ) (u(ti ) − v(ti ))|. (2.7) 0<ti <t We easily check that Z t −M (t−s) e (T u − T v)(s)ds 0 −1 −M t )ku − vkP C , if M > 1, k0 tM (1 − e −1 ≤ k0 tM (1 − M e−M t )ku − vkP C , if 0 < M ≤ 1, 1 2 if M = 0, k0 2 t ku − vkP C , (2.8) and ( h0 cM −1 (1 − e−M t )ku − vkP C , if M > 0, e h0 ctku − vkP C , if M = 0. 0 (2.9) Substituting (2.8) and (2.9) into (2.7), we obtain Z t −M (t−s) (Su − Sv)(s)ds ≤ kBu − BvkP C ≤ (k̄ + h̄ + p X Li )ku − vkP C . i=1 This indicates that B is a contraction mapping (by (2.4)). Then there is one unique y ∈ P C(J) such that By = y, that is, (2.3) has a unique solution. Lemma 2.5 ([18]). Assume that y ∈ P C 1 (J) satisfies y 0 (t) + M y(t) + N (T y)(t) + N1 (Sy)(t) ≤ 0, ∆y(tk ) ≤ −Lk y(tk ), t ∈ J0 , k = 1, 2, . . . , p, (2.10) y(0) ≤ 0, where M > 0, N, N1 ≥ 0, Lk < 1, k = 1, 2, . . . , p, and Z c p Y q(s)ds ≤ (1 − L̄j ) 0 with L̄k = max{Lk , 0}, k = 1, 2, . . . , p, Z t Z Y q(t) = N k(t, s)eM (t−s) (1−Lk )ds+N1 0 (2.11) j=1 s<tk <c 0 c h(t, s)eM (t−s) Y (1−Lk )ds, s<tk <c then y ≤ 0. 3. Main result Theorem 3.1. If (H1),(H2),(H3) are satisfied, and, in addition, if there exist M > 0, N, N1 ≥ 0, Lk < 1, k = 1, 2, . . . , p, such that (2.4) and (2.11) hold, then (1.2) has, in segment [α0 , β0 ] the coupled minimal-maximal quasi-solutions. 6 X. WANG, C. BAI EJDE-2010/166 Proof. For convenience, let (Kφ)(t) = N (T φ)(t) + N1 (Sφ)(t). We now construct two sequences {αn (t)} and {βn (t)} that satisfy the following problems αi0 (t) + a(t)αi−1 (t) + M αi (t) + (Kαi )(t) = f (t, αi−1 (t), (T αi−1 )(t), (Sαi−1 )(t)) + M αi−1 (t) + (Kαi−1 )(t), t ∈ J0 , ∆αi (tk ) = Ik (αi−1 (tk )) − Lk (αi (tk ) − αi−1 (tk )), k = 1, 2, . . . , p, Z c αi (0) + λ αi−1 (s)ds = −βi−1 (c), (3.1) 0 and βi0 (t) + a(t)βi−1 (t) + M βi (t) + (Kβi )(t) t ∈ J0 , = f (t, βi−1 (t), (T βi−1 )(t), (Sβi−1 )(t)) + M βi−1 (t) + (Kβi−1 )(t), ∆βi (tk ) = Ik (βi−1 (tk )) − Lk (βi (tk ) − βi−1 (tk )), k = 1, 2, . . . , p, Z c βi (0) + λ βi−1 (s)ds = −αi−1 (c). (3.2) 0 For each φ, ψ ∈ [α0 , β0 ], we consider the equation y 0 (t) + M y(t) + (Ky)(t) = f (t, φ(t), (T φ)(t), (Sφ)(t)) − a(t)φ(t) + M φ(t) + (Kφ)(t), t ∈ J0 , ∆y(tk ) = Ik (φ(tk )) − Lk (y(tk ) − φ(tk )), k = 1, 2, . . . , p, Z c y(0) + λ φ(s)ds = −ψ(c). (3.3) 0 By condition (2.4) and Lemma 2.4, we know that (3.3) has a unique solution y(t) ∈ P C 1 (J). Define the operator A : P C 1 (J) × P C 1 (J) → P C 1 (J) as A(φ, ψ) = y. Let αn (t) = A(αn−1 , βn−1 )(t) and βn (t) = A(βn−1 , αn−1 )(t), n = 1, 2, . . . , we will prove that {αn }, {βn } have the following properties. (i) αi−1 ≤ αi , βi ≤ βi−1 ; (ii) αi ≤ βi , i = 1, 2, .... Firstly, we prove that α0 ≤ α1 . Set p(t) = α0 (t) − α1 (t), it follows that p0 (t) + M p(t) + N (T p)(t) + N1 (Sp)(t) = p0 (t) + M p(t) + (Kp)(t) ≤ 0, ∆p(tk ) ≤ −Lk p(tk ), k = 1, 2, . . . , p, (3.4) p(0) ≤ 0. Then by condition (2.11) and Lemma 2.5, we get p(t) ≤ 0, which implies that α0 (t) ≤ α1 (t), for all t ∈ J0 . In a similar way, it can be proved that β1 (t) ≤ β0 (t), for all t ∈ J0 . Now we prove that α1 (t) ≤ β1 (t), for all t ∈ J0 . In fact, setting p(t) = α1 (t) − β1 (t) and using assumption, we obtain p0 (t) + M p(t) + N (T p)(t) + N1 (Sp)(t) = α10 (t) − β10 (t) + M (α1 (t) − β1 (t)) + N (T α1 (t) − T β1 (t)) + N1 (Sα1 (t) − Sβ1 (t)) = f (t, α0 (t), (T α0 )(t), (Sα0 )(t)) − a(t)α0 (t) + M α0 (t) + N (T α0 )(t) + N1 (Sα0 )(t) − f (t, β0 (t), (T β0 )(t), (Sβ0 )(t)) + a(t)β0 (t) − M β0 (t) − N (T β0 )(t) − N1 (Sβ0 )(t) ≤ a(t)(β0 (t) − α0 (t)) ≤ 0, t ∈ J0 , and ∆p(tk ) = −Lk p(tk ) + Ik (α0 (tk )) − Ik (β0 (tk )) + Lk α0 (tk ) − Lk β0 (tk ) ≤ −Lk p(tk ), EJDE-2010/166 IMPULSIVE BOUNDARY-VALUE PROBLEMS 7 c Z p(0) = α1 (0) − β1 (0) = λ (β0 (s) − α0 (s))ds + α0 (c) − β0 (c) ≤ 0. 0 Again by Lemma 2.5, we obtain p(t) ≤ 0, that is, α1 (t) ≤ β1 (t) for all t ∈ J0 . Thus we have α0 (t) ≤ α1 (t) ≤ β1 (t) ≤ β0 (t) for all t ∈ J0 . Continuing this process, by induction, one can obtain monotone sequence {αn (t)} and {βn (t)} such that α0 (t) ≤ α1 (t) ≤ · · · ≤ αn (t) ≤ · · · ≤ βn (t) ≤ . . . β1 (t) ≤ β0 (t), t ∈ J0 , where each αi (t), βi (t) ∈ P C 1 (J) satisfies (3.1) and (3.2). As the sequences {αn }, {βn } are uniformly bounded and equi-continuous, by employing the standard arguments Ascoli-Arzela criterion [12], we conclude that the sequences {αn } and {βn } converge uniformly on J0 with lim αn (t) = y∗ (t), n→∞ lim βn (t) = y ∗ (t). n→∞ Obviously, y∗ (t), y ∗ (t) are coupled lower-upper quasi-solutions of (1.2). Now we have to prove that (y∗ , y ∗ ) are coupled minimal-maximal quasi-solutions of problem (1.2) in segment [α0 , β0 ]. Let x, z be coupled quasi-solutions of (1.2) such that αn (t) ≤ x(t), z(t) ≤ βn (t), t ∈ J0 for some n ∈ N. Put q(t) = αn+1 (t) − x(t), for t ∈ J0 . Form definition of αn+1 and properties of quasi-solution x(t), we obtain q 0 (t) + M q(t) + N (T q)(t) + N1 (Sq)(t) = f t, αn (t), (T αn )(t), (Sαn )(t) − a(t)αn (t) + M αn (t) + N (T αn )(t) + N1 (Sαn )(t) − f t, x(t), (T x)(t), (Sx)(t) + a(t)x(t) − M x(t) − N (T x)(t) − N1 (Sx)(t) ≤ a(t)(x(t) − αn (t)) ≤ 0, t ∈ J0 , and ∆q(tk ) = −Lk q(tk ) + Ik (αn (tk )) − Ik (x(tk )) + Lk αn (tk ) − Lk x(tk ) ≤ −Lk q(tk ), Z c q(0) = αn+1 (0) − x(0) = λ (x(s) − αn (s))ds + z(c) − βn (c) ≤ 0. 0 By Lemma 2.5, we have q(t) ≤ 0 for all t ∈ J0 , that is αn+1 (t) ≤ x(t). Similarly, we can prove that z(t) ≤ βn+1 (t) for all t ∈ J0 . By induction, we obtain αm (t) ≤ x(t), z(t) ≤ βm (t), t ∈ J0 , for m ∈ N. If m → ∞, it yields y∗ (t) ≤ x(t), z(t) ≤ y ∗ (t), t ∈ J0 . It shows that (y∗ , y ∗ ) are coupled minimal-maximal quasi-solutions of problem (1.2) in segment [α0 , β0 ]. 8 X. WANG, C. BAI EJDE-2010/166 Example 3.2. Consider the problem Z Z t 1 t −(t−s) 5 1 y 0 (t) − (1 − e−t )y(t) = −y(t) − te y(s)ds − y(s)ds, 4 8 0 6 0 t ∈ [0, t1 ) ∪ (t1 , 1], 1 1 ∆y(t1 ) = − y(t1 ), t1 = 9 3 Z 1 1 y(0) − y(s)ds = −y(1). 6 0 (3.5) where a(t) = − 4t (1 − e−t ) ≤ 0, I1 (x) = − 19 x, L1 = 91 and λ = − 61 < 0. Let f (t, x, y, z) = −M x − N y − N1 z, M = 1, N = 83 , N1 = 65 , J = [0, 1], c = 1, k(t, s) = 3t e−(t−s) , h(t, s) = 1, then for t ∈ J, xi , yi , zi ∈ R, i = 1, 2, x1 ≥ x2 , y 1 ≥ y 2 , z1 ≥ z2 , 3 5 f (t, x1 , y1 , z1 ) − f (t, x2 , y2 , z2 ) = −(x1 − x2 ) − (y1 − y2 ) − (z1 − z2 ). 8 6 Thus the condition (H2) holds. It is easy to see that k0 = 1 −1 ) and 3 (1 − e 1 3, h0 = 1, k̄ = 1 3 h̄ = h̄ + k̄ + L1 = 0.9359 < 1. Hence the condition (2.4) holds. Moreover, we have Z 1 Z 5 1 (t−s) t −(t−s) (t−s) e e (1 − L1 )ds + e (1 − L1 )ds dt 8 0 3 6 0 0 Z 1 2 20 t + (1 − e−1 )et dt = 18 27 0 1 20 = + (e + e−1 − 2) = 0.8231 < 0.8889 = 1 − L1 , 54 27 Z q(s)ds ≤ 0 1 3 Z t which implies that the condition (2.11) holds. Let 5 α0 (t) = − , 4 β0 (t) = 2 − t, t ∈ [0, 1]. Then α0 (t) and β0 (t) are coupled lower-upper quasi-solutions of problem (??). In fact, 5 5 t(1 − e−t ) ≤ 2 + t(1 − e−t ) 16 32 Z t Z 5 25 1 5 −(t−s) < + te ds + ds 4 32 0 24 0 = f (t, α0 (t), (T α0 )(t), (Sα0 )(t)), 5 = −L1 α0 (1/3) ∆α0 (1/3) = 0 < 36 Z 1 1 25 α0 (0) − α0 (s)ds = − < −1 = −β0 (1), 6 0 24 α00 (t) + a(t)α0 (t) = EJDE-2010/166 IMPULSIVE BOUNDARY-VALUE PROBLEMS 9 and 1 β00 (t) + a(t)β0 (t) = −1 − t(1 − e−t )(2 − t) 4 1 ≥ −1 − (1 − e−1 ) 4 27 3 −1 >− + e 12 8 1 3 15 ≥ t − 2 − t(3 − t) + te−t − 8 8 12 Z Z 1 t −(t−s) 5 1 =t−2− te (2 − s)ds − (2 − s)ds 8 0 6 0 = f (t, β0 (t), (T β0 )(t), (Sβ0 )(t)), 5 = −L1 β0 (1/3) ∆β0 (1/3) = 0 > − 27 Z 1 1 7 5 β0 (0) − β0 (s)ds = > = −α0 (1). 6 0 4 4 Obviously, α0 (t) ≤ β0 (t). Thus, all the conditions of Theorem 3.1 are satisfied, so problem (3.5) has the coupled minimal-maximal quasi-solutions in the segment [α0 (t), β0 (t)]. Acknowledgments. 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