On the Existence of Positive Solutions of Resonant and Nonresonant Multipoint Boundary Value Problems for Third-Order Nonlinear Differential Equations
Abstract
Positive solutions for a kind of third-order multipoint boundary value problem under the non-resonant conditions and the resonant conditions are considered. In the nonresonant case, by using Leggett-Williams fixed-point theorem, the existence of at least three positive solutions is obtained. In the resonant case, by using Leggett-Williams norm-type theorem due to O’Regan and Zima, existence result of at least one positive solution is established. The results obtained are valid and new for the problem discussed. Two examples are given to illustrate the main results.
1. Introduction
- (1)
In the nonresonant case, Green’s function is established and the results obtained are more general than those of earlier work.
- (2)
It is the first time that the positive solution is considered for third-order boundary value problem at resonance.
2. Background Definitions and Lemmas
Let X, Y be real Banach spaces. A nonempty closed convex set C ⊂ X is said to be a cone provided that ax ∈ C, if x ∈ C, a ≥ 0 and x, −x ∈ C implies x = 0.
Definition 1. The map ψ is a nonnegative continuous concave functional on C if ψ : C → +∞ is continuous and
Definition 2. Let constants 0 < a < b be given and let ψ be a nonnegative continuous concave functional on the cone C. Define the convex sets Cr and C(ψ, a, b) as follow:
Lemma 3 (Leggett-Williams fixed-point theorem [35]). Let be a completely continuous operator and let ψ be a nonnegative continuous concave functional on C such that ψ(x) ≤ ∥x∥ for all . Suppose that there exist 0 < a < b < d ≤ c such that
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(H1) {x ∈ C(ψ, b, d)∣ψ(x) > b} ≠ ⌀ and ψ(Tx) > b for x ∈ C(ψ, b, d),
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(H2) ∥Tx∥ < a for ∥x∥ ≤ a,
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(H3) ψ(Tx) > b for x ∈ C(ψ, b, c) with ∥Tx∥ ≥ d.
Lemma 4 (Leggett-Williams norm-type theorem [28]). Assume that C is a cone in X and that Ω1 and Ω2 are open bounded subsets of X with and . Suppose that L : dom L ⊂ X → Y is a Fredholm operator of index zero and that
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(C1) QN : X → Y is bounded and continuous and KP(I − Q)N : X → X is compact on every bounded subset of X,
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(C2) Lx ≠ λNx for all x ∈ C∩∂Ω2∩dom L and λ ∈ (0,1),
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(C3) γ maps subsets of into bounded subsets of C,
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(C4) dB([I − (P + JQN)γ]|ker L, Ker L∩Ω2, 0) ≠ 0, where dB stands for the Brouwer degree,
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(C5) there exists u0 ∈ C∖{0} such that ∥x∥ ≤ σ(u0)∥Ψx∥ for x ∈ C(u0)∩∂Ω1, where C(u0) = {x ∈ C : μu0 ≤ x} for some μ > 0 and σ(u0) is such that ∥x + u0∥ ≥ σ(u0)∥x∥ for every x ∈ C,
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(C6) (P + JQN)γ(∂Ω2) ⊂ C,
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(C7) .
3. Main Results for Nonresonant Case
In this section we consider the positive solution for the nonresonant case with the condition and we always suppose that f ∈ C([0,1]×[0, ∞), [0, ∞)).
Lemma 5. Suppose y(t) ∈ C[0,1]. Then problem (11) and (12) is equivalent to
Proof. Let G(t, s) be Green’s function of problem −x′′′(t) = 0 with boundary condition (12). We can suppose
Considering the properties of Green′s function together with boundary condition (12), we have
Lemma 6. Green’s function G(t, s) satisfies that G(t, s) ≥ 0, t, s ∈ [0,1].
Proof. For ξi−1 ≤ s ≤ ξi, i = 1, 2, …, m − 1, and t ≤ s,
Lemma 7. If y(t) ≥ 0, t ∈ [0,1], and x(t) is the solution of problem (11) and (12), then
Proof. From
Theorem 8. Suppose that there exist constants 0 < a < b < b/δ ≤ c such that
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(A1) f(t, x) < a/m*, (t, x)∈[0,1]×[0, a],
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(A2) f(t, x) > b/m*, (t, x)∈[0,1]×[b, b/δ],
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(A3) f(t, x) < c/m*, (t, x) ∈ [0,1] × [0, c].
Proof. The operator T : C → E is defined by
Next, the conditions of Lemma 3 are checked. If , then ∥x∥ ≤ c and condition (A3) implies that
Similar to the proof above, we obtain that . Hence, condition (H2) of Lemma 3 is satisfied.
- (1)
f(t, x) < 81/32, [t, x]∈[0, 1]×[0, 1],
- (2)
f(t, x) > 81/4, [t, x]∈[0, 1]×[4, 10],
- (3)
f(t, x) < 81, [t, x]∈[0, 1]×[0, 32].
4. Main Results for Resonant Case
In this section the condition is considered. Obviously, problem (1) is at resonance under this condition. The norm-type Leggett-Williams fixed-point theorem will be used to establish the existence results of positive solution.
We define the spaces X = Y = C[0,1] endowed with the maximum norm. It is well known that X and Y are the Banach spaces.
Theorem 9. Suppose that there exists positive constant R ∈ (0, ∞) such that f : [0,1]×[0, R]→(−∞, + ∞) is continuous and satisfies the following conditions:
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(S1) f(t, x)≥−κx, for (t, x)∈[0,1]×[0, R],
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(S2) f(t, x) < 0 for [t, x]∈[0, 1]×[(1 − (κσ/2))R, R],
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(S3) there exist r ∈ (0, R), M ∈ (0,1), t0 ∈ [0,1], a ∈ (0,1], and continuous functions
()
Proof. Firstly we prove that
Secondly we see Y = Y1 ⊕ Im L, where
Define two projections P : X → X and Q : Y → Y by
Next we will check that every condition of Lemma 4 is fulfilled. Remark that f can be extended continuously on [0,1]×(−∞, +∞) and condition (C1) of Lemma 4 is fulfilled.
Define the set of nonnegative functions C and subsets of X Ω1, Ω2 by
Let the isomorphism J = I and (γx)(t) = |x(t)| for x ∈ X. Then γ is a retraction and maps subsets of into bounded subsets of C, which ensures that condition (C3) of Lemma 4 is fulfilled.
Then we prove that (C2) of Lemma 4 is fulfilled. For this purpose, suppose that there exist x0 ∈ C∩∂Ω2∩dom L and λ0 ∈ (0,1) such that Lx0 = λ0Nx0. Then
- (1)
We show that t0 ≠ 1. Suppose, on the contrary, that x0(t) achieves maximum value R only at t0 = 1. Then the boundary condition in combination with the resonant condition yields that max 1≤i≤m−2x0(ξi) ≥ R, which is a contradiction.
- (2)
We claim that t0 ≠ 0. Suppose, on the contrary, that x0(t) achieves maximum value R at t0 = 0. From condition (S2), there exists t1 > 0 near to zero such that
()Then()which contradicts the fact that x0(t) achieves maximum value at t0 = 0. - (3)
Thus there exists t0 ∈ (0,1) such that x0(t0) = R = max 0≤t≤1x0(t). We may choose η < t0 nearest to t0 with . From the mean value theory, we claim that there exists ξ ∈ (η, t0) such that
()
Remark 10. The sign of third-order derivative of a function h(t) at point t0 cannot be confirmed when t0 is a maximal value of h(t). Thus the methods in [28] are not applicable directly to this problem.
Thus with the application of Lemma 4, we confirm that the equation Lx = Nx has a solution , which implies that the resonant problem (1) has at least one positive solution.
- (1)
f(t, x)>−(31/81)x, for all (t, x)∈[0,1]×[0, 1],
- (2)
f(t, x) < 0, for all (t, x)∈[0,1]×[1055/1458, 1],
- (3)
, and is nonincreasing on (0, 1/4] with
()
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
The project is supported by the National Natural Science Foundation of China (11201109) and the Natural Science Foundation of Anhui Educational Department (KJ2012B144 and KJ2012Z335).