Boundary Value Problems

, 2011:51 | Cite as

Multiple unbounded solutions for a boundary value problem on infinite intervals

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Abstract

This paper is concerned with the existence of multiple unbounded solutions for a Sturm-Liouville boundary value problem on the half-line. By assuming the existence of two pairs of unbounded upper and lower solutions, the existence of at least three solutions is obtained using the degree theories. Nagumo condition plays an important role in the nonlinear term involved in the first-order derivative. It is an interesting point that the method of unbounded upper and lower solutions is extended to obtain conditions for the existence of multiple solutions.

Mathematics Subject Classification (2000 34B10, 34B40

Keywords

infinite interval problem multiplicity unbounded upper solutions unbounded lower solutions degree theory 

1 Introduction

In this paper, we will employ the method of unbounded upper and lower solutions to study the existence of Sturm-Liouville boundary value problem on the half-line
u ( t ) + ϕ ( t ) f ( t , u ( t ) , u ( t ) ) = 0 , t ( 0 , + ) , u ( 0 ) - a u ( 0 ) = B , u ( + ) = C , Open image in new window
(1)

where ϕ : (0, +) → (0, +), f : [0, +∞) × ℝ2 → ℝ are continuous, a > 0, B, C ∈ ℝ.

The method of upper and lower solutions is a powerful tool to prove the existence of differential equation subject to certain boundary conditions. It is well known that nonlinear problems always have at least one solution in the ordered interval defined by one pair of well-ordered upper and lower solutions. To show this kind of result, we can employ the topological degree theory or monotone iterative technique, etc, see [1, 2, 3, 4, 5] and the reference therein.

Boundary value problems to differential equations on the half-line arise naturally in the study of radially symmetric solutions of nonlinear elliptic equations, and many works have been done in this area, see [6]. When applying the method of upper and lower solution method to discuss the infinite interval problem, most of the results are concerned with the existence of the bounded or positive solutions. Chen and Zhang [7], with a = 0, discussed the lower and upper solution technique and presented the existence of positive solutions with sublinearity conditions. In [8], Agarwal and O'Regan studied the equation (1/p)(py')' = q(t)f(t, y) with the boundary condition y(t) bounded on [0, +∞) or limt→+∞y(t) = 0 using the upper and lower solution technique. The methods used therein were based on diagonalization arguments and existence results of appropriate boundary value problems on finite intervals using upper and lower solutions. In [9], Palamides and Galanis proved the existence of a global, monotone, positive and unbounded solution. Recently, Yan, Agarwal and O'Regan [10], with C > 0 in BVP (1), presented sufficient conditions for the existence of unbounded positive solutions. In [11], the authors established further the unbounded upper and lower theory for such boundary value problem. Using such technique coupled with the Schäuder fixed point theorem, the existence of the unbounded solution is obtained.

How many solutions exist when infinite interval nonlinear problem has two pairs of well-ordered lower and upper solutions? Eloe, Kaufmann and Tisdell [12] have presented some sufficient conditions for the existence of three solutions for the equation x″ - a(t)x + f(t, x) = 0 with the boundary conditions x(0) = x0 and x(t) bounded on [0, +∞) by employing the degree theory on compact domains and the sequential arguments on unbounded domain. But, to the best knowledge of the authors, when considering the upper and lower solution method, there is no paper to find the existence of multiple unbounded solutions for infinite interval problem.

Inspirited by the papers mentioned above, in this paper, we aim to use the upper and lower solution method to discuss the existence of multiple unbounded solutions for infinite interval problem. For BVP (1), by assuming two unbounded lower solutions α1, α2 and two unbounded upper solutions β1, β2 satisfying α1α2, β1β2, α2β1, the Nagumo condition and additional suitable ones on f to yield prior bounds of u and u', we show that the infinite interval problem (1) has at least three solutions. The bases of the degree theory and the truncations analysis are exposed in this paper, which are somewhat different from those in [12]. The solutions obtained in this paper are admitted to be unbounded. And the extension of the unbounded upper and lower solution method to obtain conditions for the existence of multiple solutions is interesting.

2 Definitions

In this section, we present the definition of unbounded upper solutions, unbounded lower solutions, Nagumo condition and a special Banach space.

Definition 2.1. A function αC1[0, +∞) ∩ C2(0, +∞) is called a lower solution of BVP (1) if
α ( t ) + ϕ ( t ) f ( t , α ( t ) , α ( t ) ) 0 , t ( 0 , + ) , α ( 0 ) - a α ( 0 ) B , α ( + ) < C . Open image in new window
(2)
A function βC1[0, +∞) ∩ C2(0, +∞) is called an upper solution of BVP (1) if
β ( t ) + ϕ ( t ) f ( t , β ( t ) , β ( t ) ) 0 , t ( 0 , + ) , β ( 0 ) - a β ( 0 ) B , β ( + ) > C . Open image in new window
(3)

A function αC1[0, +∞) ∩ C2(0, +∞) is called a strict lower solution of BVP (1) if the inequality in (2) is strict for t ∈ (0, +∞). A strict upper solution is defined similarly.

Remark 2.1. Since the continuous functions α, β are defined on [0, +∞), they are admitted to be unbounded. So, we call them unbounded lower and upper solutions.

Definition 2.2. Given a pair of functions α, βC1[0, +∞) satisfying α(t) ≤ β(t), t ∈ [0, +∞). A function f : [0, +∞) × R2R is said to satisfy Nagumo condition with respect to the pair of functions α, β, if there exists a nonnegative function ψC[0, +∞) and a positive one hC[0, +∞) such that
f ( t , x , y ) ψ ( t ) h ( y ) Open image in new window
for all 0 ≤ t < +∞, α(t) ≤ xβ(t) and
0 + ψ ( s ) ϕ ( s ) d s < + , + s h ( s ) d s = + . Open image in new window
Consider the space X defined by
X = x C 1 [ 0 , + ) , lim t + x ( t ) exists , Open image in new window
(4)

with the norm ||x|| = max{||x||1, ||x'||}, where x 1 = sup t [ 0 , + ) x ( t ) 1 + t Open image in new window, ||x'|| = supt∈[0,+∞)|x'(t)|. By the standard arguments, we can prove that (X, ||·||) is a Banach space.

Remark 2.2. Banach space X is introduced in order to estimate the prior bound of the solutions to BVP (1), which is necessary to apply the degree theory. The solution of BVP (1) is bounded with the norm ||·|| in X as well as the upper and lower solutions, but they are not as a continuous function.

3 Existence of solutions

For the sake of convenience, we list the assumptions to be used in this section.

(H1) BVP (1) has two pairs of upper-lower solutions β i , α i X, i = 1, 2 satisfying
α 1 ( t ) α 2 ( t ) β 2 ( t ) , α 1 ( t ) β 1 ( t ) β 2 ( t ) , α 2 ( t ) β 1 ( t ) , t [ 0 , + ) , Open image in new window

where α2, β1 are strict lower and upper solutions, respectively.

(H2) f satisfies Nagumo condition with respect to α1, β2. ϕL1[0, +∞) and there exists γ > 1 such that sup 0 t < + ( 1 + t ) γ ϕ ( t ) ψ ( t ) < + Open image in new window.

(H3) For any r > 0, there exists φ r such that for 0 ≤ t < +∞, α1(t) ≤ xβ2(t), 0 ≤ yr, we have
f ( t , x , y ) φ r ( t ) and 0 + ϕ ( s ) φ r ( s ) d s < + . Open image in new window
Theorem 3.1. Suppose conditions (H1) and (H2) hold. Then, BVP (1) has at least three solutions u1, u2, u3satisfying
α 1 u 1 β 1 , α 2 u 2 β 2 , u 3 β 1 , u 3 α 2 . Open image in new window
(5)
Proof. Choose R > C,
η max sup t [ δ , + ) β 2 ( t ) - α 1 ( 0 ) t , sup t [ δ , + ) β 2 ( 0 ) - α 1 ( t ) t Open image in new window
(6)
with δ > 0 a certain constant such that
η R s h ( s ) d s M sup t [ 0 , + ) β 2 ( t ) ( 1 + t ) γ - inf t [ 0 , + ) α 1 ( t ) ( 1 + t ) γ + γ γ - 1 sup t [ 0 , + ) β 2 ( t ) 1 + t , Open image in new window
where M = sup 0 t < + ( 1 + t ) γ ϕ ( t ) ψ ( t ) Open image in new window. And consider the truncated boundary value problem
u ( t ) + ϕ ( t ) f 1 * ( t , u ( t ) , u ( t ) ) = 0 , t ( 0 , + ) , u ( 0 ) - a u ( 0 ) = B , u ( + ) = C , Open image in new window
(7)
where
f 1 * ( t , x , y ) = f R ( t , α 1 ( t ) , y ) + α 1 ( t ) - x 1 + x - α 1 ( t ) , x < α 1 ( t ) , f R ( t , x , y ) , α 1 ( t ) x β 1 ( t ) , f R ( t , β 2 ( t ) , y ) + β 2 ( t ) - x 1 + x - β 2 ( t ) , x > β 2 ( t ) , Open image in new window
and
f R ( t , x , y ) = f ( t , x , - R ) , y < - R , f ( t , x , y ) , - R y R , f ( t , x , R ) , y > R . Open image in new window

Obviously, if (7) has triple solutions u i , i = 1, 2, 3 satisfying α1u i β2 and u i < R Open image in new window, then we can complete the proof. Next, we will prove sequentially that if u is a solution of (7), it holds α1uβ2, ||u'|| < R and (7) has at least three solutions.

Step 1: If u is a solution of (7), it holds α1(t) ≤ u(t) ≤ β2(t), t ∈ [0, +∞).

We just show u(t) ≤ β2(t), t ∈ [0, +∞). If it does not hold, we have
sup 0 t < + ( u ( t ) - β 2 ( t ) ) > 0 . Open image in new window

Because u ( + ) - β 2 ( + ) < 0 Open image in new window, so there are two cases.

Case 1. lim t 0 + ( u ( t ) - β 2 ( t ) ) = sup 0 t < + ( u ( t ) - β 2 ( t ) ) > 0 Open image in new window.

Easily, it holds u ( 0 + ) - β 2 ( 0 + ) 0 Open image in new window. While from the boundary condition, we have
u ( 0 ) - β 2 ( 0 ) 1 a ( u ( 0 ) - β 2 ( 0 ) ) > 0 , Open image in new window

which is a contraction.

Case 2. There exists t* ∈ (0, +∞) such that
u ( t * ) - β 2 ( t * ) = sup 0 t < + ( u ( t ) - β 2 ( t ) ) > 0 . Open image in new window
So we have u ( t * ) - β 2 ( t * ) = 0 , u ( t * ) - β 2 ( t * ) 0 Open image in new window. Unfortunately,
u ( t * ) - β 2 ( t * ) ϕ ( t * ) ( f ( t * , β 2 ( t * ) , β 2 ( t * ) ) - f 1 * ( t * , u ( t * ) , u ( t * ) ) ) = ϕ ( t * ) u ( t * ) - β 2 ( t * ) 1 + u ( t * ) - β 2 ( t * ) > 0 . Open image in new window

Which is also a contraction. Here, we note that ||β2|| < R holds with similar discussions to those in step 2.

Consequently, u(t) ≤ β2(t) holds for all t ∈ [0, +∞). Similarly, we can show that α1(t) ≤ u(t) for all t ∈ [0, +∞).

Step 2: If u is a solution of (7), then ||u'||R.

From step 1, we know that α1uβ2 if u is a solution of (7). If ||u'||R is untrue, we have the following three cases.

Case 1. |u'(t)| > η, ∀t ∈ [0, +∞).

Without loss of generality, we suppose u'(t) > η, t ∈ [0, +∞). While for any Tδ,
β 2 ( T ) - α 1 ( 0 ) T u ( T ) - u ( 0 ) T = 1 T 0 T u ( s ) d s > η β 2 ( T ) - α 1 ( 0 ) T , Open image in new window

which is a contraction. So, there must exist t0 ∈ [0, +∞) such that |u'(t0)| ≤ η.

Case 2. |u'(t)| ≤ η, ∀t ∈ [0, +∞).

Just take R such as the definition of η in (6) and we can complete the proof.

Case 3. There exists [t1, t2] ⊂ [0, +∞) such that |u'(t1)| = η, |u'(t)| > η, t ∈ (t1, t2] or |u'(t2)| = η, |u'(t)| > η, t ∈ [t1, t2).

Without loss of generality, we suppose that u'(t1) = η, u'(t) > η, t ∈ (t1, t2]. Obviously,
u ( t 1 ) u ( t 2 ) s h ( s ) d s = t 1 t 2 u ( s ) h ( u ( s ) ) u ( s ) d s = t 1 t 2 - ϕ ( s ) f 1 * ( s , u ( s ) , u ( s ) ) u ( s ) h ( u ( s ) ) d s t 1 t 2 u ( s ) ϕ ( s ) ψ ( s ) d s M t 1 t 2 u ( s ) ( 1 + s ) γ d s = M t 1 t 2 u ( s ) ( 1 + s ) γ d s + t 1 t 2 γ u ( s ) ( 1 + s ) 1 + γ d s M sup t [ 0 , + ) β 2 ( t ) ( 1 + t ) γ - inf t [ 0 , + ) α 1 ( t ) ( 1 + t ) γ + sup t [ 0 , + ) β 2 ( t ) 1 + t 0 + γ ( 1 + s ) γ d s η R s h ( s ) d s , Open image in new window

which concludes that u'(t2) ≤ R. For t1 and t2 are arbitrary, we obtain that if u'(t) ≥ η, then u'(t) ≤ R, t ∈ [0, +∞).

Similarly, we can also obtain that if u'(t1) = -η, u'(t) < -η, t ∈ (t1, t2], then u'(t) ≥ -R, t ∈ [0, +∞).

Step 3: (7) has at least three solutions.

Define T1 : XX as
( T 1 u ) ( t ) = a C + B + C t + 0 + G ( t , s ) ϕ ( s ) f 1 * ( s , u ( s ) , u ( s ) ) d s , Open image in new window
where
G ( t , s ) = a + s , 0 s t < + , a + t , 0 t s < + . Open image in new window
(8)

Easily, the fixed point of T1 coincides with the solution of BVP (7). It is enough to prove that T1 has at least three fixed points.

With the similar discussions to those in [11], we can show that T1 is completely continuous. Let N > max { max { C , a C + B } + max { a , 1 } H R 0 + ϕ ( s ) ψ ( s ) d s , α 1 , β 2 } Open image in new window, where H R = max0 ≤ sRh(s). Set Ω = {uX1, ||u|| < N}. Then for any u Ω ̄ Open image in new window, it holds
T 1 u = max { T 1 u 1 , ( T 1 u ) } max { C , a C + B } + max { a , 1 } 0 + ϕ ( s ) f 1 * ( s , u ( s ) , u ( s ) ) d s max { C , a C + B } + max { a , 1 } H R 0 + ϕ ( s ) ψ ( s ) d s < N , Open image in new window
so we obtain deg(I - T1, Ω, 0) = deg(I, Ω, 0) = 1. Let
Ω α 2 = u Ω , u ( t ) > α 2 ( t ) , t [ 0 , + ) , Ω β 1 = u Ω , u ( t ) < β 1 ( t ) , t [ 0 , + ) . Open image in new window
Because α2β1, α1α2β2 and α1β1β2, we have Ω α 2 Open image in new window, Ω \ Ω α 2 Ω β 1 ¯ Open image in new window and Ω α 2 Ω β 2 = Open image in new window. Noticing that α2, β1 are strict lower and upper solutions, T1 has no fixed point in the set Ω α 2 Ω β 2 Open image in new window. Therefore,
deg ( I - T 1 , Ω , 0 ) = deg ( I - T 1 , Ω \ Ω α 2 Ω β 2 ¯ , 0 ) + deg ( I - T 1 , Ω α 2 , 0 ) + deg ( I - T 1 , Ω β 1 , 0 ) . Open image in new window
In order to show deg ( I - T 1 , Ω α 2 , 0 ) = deg ( I - T 1 , Ω β 1 , 0 ) = 1 Open image in new window, we define the operator T 2 : Ω ̄ Ω ̄ Open image in new window as
( T 2 u ) ( t ) = a C + B + C t + 0 + G ( t , s ) f 2 * ( s , u ( s ) , u ( s ) ) d s , Open image in new window
where f 2 * Open image in new window has the same expression as f 1 * Open image in new window except changing α1 to α2. Similarly, we have α2(t) ≤ u(t) ≤ β2(t) for t ∈ [0, +∞) when u is a fixed point of T2. In another word, deg ( I - T 2 , Ω \ Ω α 2 ¯ , 0 ) = 0 Open image in new window. Meanwhile, T 2 Ω ̄ Ω ̄ Open image in new window, so from Schäuder fixed point theorem, it holds
deg ( I - T 2 , Ω , 0 ) = 1 . Open image in new window
And we have
deg ( I - T 1 , Ω α 2 , 0 ) = deg ( I - T 2 , Ω α 2 , 0 ) = deg ( I - T 2 , Ω , 0 ) - deg ( I - T 2 , Ω \ Ω α 2 ¯ , 0 ) = 1 . Open image in new window
Similarly, we can deduce that deg ( I - T 1 , Ω β 1 , 0 ) = 1 Open image in new window. Therefore,
deg ( I - T 1 , Ω \ Ω α 2 Ω β 2 ¯ , 0 ) = - 1 . Open image in new window

Using the properties of the degree, we can obtain that T1 has at least three fixed point u 1 Ω α 2 , u 2 Ω β 1 , u 3 Ω \ Ω α 2 Ω β 1 ¯ Open image in new window.

Remark 3.1. The strictness of the lower solution α2 and the upper one β1 can be weakened, see [3].

If f : [0, +∞)3 → [0, +∞), we can establish a criteria for the existence of positive solutions.

Theorem 3.2. Let f : [0, +∞)3 → [0, +∞) be continuous and ϕL1[0, +∞). Suppose conditions (H1) and (H3) hold with α1(t) > 0, t ∈ (0, +∞). Then, BVP (1) with B, C ≥ 0 has at least three positive solutions satisfying the inequality (5).

Proof. Choose R = 1 a ( B + β ( 0 ) ) Open image in new window and consider the boundary value problem (7) except f R substituting by
f R ( t , x , y ) = f ( t , x , 0 ) , y < 0 , f ( t , x , y ) , 0 y R f ( t , x , R ) , y > R . Open image in new window
Similarly, we can obtain that the truncated problem has at least three solutions u i , i = 1, 2, 3 satisfying α1(t) ≤ u i (t) ≤ β2(t), t ∈ [0, +∞). Because
u i ( t ) = - ϕ ( t ) f * ( t , u i ( t ) , u i ( t ) ) 0 Open image in new window
and u i ( + ) = C 0 Open image in new window, we have
0 u i ( t ) u i ( 0 ) = 1 a ( B + u i ( 0 ) ) R . Open image in new window

Consequently, u i , i = 1, 2, 3 are positive solutions of (1).

Notes

Acknowledgements

The authors would like to thank the referee for helpful comments and suggestions. This work was supported by the Fundamental Research Funds for the Central Universities (No. 2011YXL044) and the National Natural Science Foundation of China (No. 111010385).

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© Lian and Geng; licensee Springer. 2011

This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Authors and Affiliations

  1. 1.School of Information EngineeringChina University of GeoscienceBeijingPeople's Republic of China

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