On the solvability of a boundary value problem on the real line

  • Giovanni Cupini1,

    Affiliated with

    • Cristina Marcelli2 and

      Affiliated with

      • Francesca Papalini2Email author

        Affiliated with

        Boundary Value Problems20112011:26

        DOI: 10.1186/1687-2770-2011-26

        Received: 22 January 2011

        Accepted: 23 September 2011

        Published: 23 September 2011

        Abstract

        We investigate the existence of heteroclinic solutions to a class of nonlinear differential equations

        ( a ( x ) Φ ( x ( t ) ) ) = f ( t , x ( t ) , x ( t ) ) , a . e . t http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equa_HTML.gif

        governed by a nonlinear differential operator Φ extending the classical p- Laplacian, with right-hand side f having the critical rate of decay -1 as |t| → +∞, that is f ( t , , ) 1 t http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq1_HTML.gif. We prove general existence and non-existence results, as well as some simple criteria useful for right-hand side having the product structure f(t, x, x') = b(t, x)c(x, x').

        Mathematical subject classification: Primary: 34B40; 34C37; Secondary: 34B15; 34L30.

        Keywords

        boundary value problems unbounded domains heteroclinic solutions nonlinear differential operators p-Laplacian operator Φ-Laplacian operator

        1. Introduction

        Differential equations governed by nonlinear differential operators have been extensively studied in the last decade, due to their several applications in various sciences. The most famous differential operator is the well-known p-Laplacian and its generalization to the generic Φ-Laplacian operator (an increasing homeomorphism of ℝ with Φ(0) = 0). Many articles have been devoted to the study of differential equations of the type
        ( ( Φ ( x ) ) ( t ) = f ( t , x ( t ), x ( t ) ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equb_HTML.gif

        for Φ-Laplacian operators, and recently also the study of singular or non-surjective differential operators has become object of an increasing interest (see, i.e., [110]).

        On the other hand, in many applications the dynamic is described by a differential operator also depending on the state variable, like (a(x)x')' for some sufficiently regular function a(x), which can be everywhere positive [non-negative] (as in the diffusion [degenerate] processes), or a changing sign function, as in the diffusion-aggregation models (see [7], [1113]).

        So, it naturally arises the interest for mixed nonlinear differential operators of the type (a(x)Φ(x'))'. In this context, in [11] we studied boundary value problems on the whole real line
        ( a ( x ( t ) ) Φ ( x ( t ) ) ) = f ( t , x ( t ) , x ( t ) ) x ( - ) = ν 1 , x ( + ) = ν 2 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equc_HTML.gif

        obtaining results on both existence and non-existence of heteroclinic solutions. Such criteria are based on the comparison between the behavior of the right-hand side f(t, x, x') as |t| → +∞ and x' → 0, combined to the infinitesimal order of the differential operator Φ(x') as x' → 0. Rather surprisingly, the presence of the state variable x inside the right-hand side and the differential operator does not influence in any way the existence or the non-existence of solutions, but it only entails a more technical proof and a sligthly stronger set of assumptions on the operator Φ. Roughly speaking, if a(x) is positive and f(t, x, x') = g(t, x')h(x) for some positive continuous function h, then the solvability of the boundary value problem depends neither on a, nor on h. Moreover, even the prescribed boundary values ν1, ν2 are not involved on the existence of solutions.

        A crucial assumption in [11] is a limitation on the rate of the possible decay of f(·, x, x') as |t| → +∞; precisely, we assumed that f(t, x, x') ≈ |t| δ for some δ > -1 (possibly positive).

        In the present article we focus our attention on right-hand sides having the critical rate of decay δ = -1 and show that, contrary to the situation studied in [11], now the solvability of the boundary value problem is influenced by the behavior of the right-hand side and of the differential operator with respect to the state variable x. For instance, when f(t, x, x') = g(x)h(t, x') the existence of solutions depends on the amplitude of the range of the values assumed by the functions a and g in the interval [ν1, ν2] determined by the prescribed boundary values.

        In Section 2 we study the existence/non-existence of solutions for general right-hand sides f(t, x(t), x'(t)) (see Theorems 2.3-2.5); more operative criteria are stated in the subsequent section for f of product type.

        We conclude the article with some examples (see Examples 3.8-3.10), useful to have a quick glance on the role played by the behavior with respect to x.

        The study of the solvability of the boundary value problem for rates of decay δ < - 1 is still open.

        2. Existence and non-existence theorems

        Let us consider the equation
        ( a ( x ( t ) ) Φ ( x ( t ) ) ) = f ( t , x ( t ) , x ( t ) ) f o r a . e . t , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ1_HTML.gif
        (2.1)

        where a : ℝ → ℝ is a positive continuous function, and f : ℝ3 → ℝ is a given Carathéodory function. From now on we will take into consideration increasing homeomorphisms Φ : ℝ → ℝ, with Φ(0) = 0.

        Our approach is based on fixed point techniques suitably combined to the method of upper and lower solutions, according to the following definition.

        Definition 2.1. A lower [upper] solution to equation (2.1) is a bounded function αC1(ℝ) such that (aα)(Φ ○ α') ∈ W1,1(ℝ) and
        ( a ( α ( t ) ) Φ ( α ( t ) ) ) [ ] f ( t , α ( t ) , α ( t ) ) , f o r a . e . t . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equd_HTML.gif
        Throughout this section we will assume the existence of an ordered pair of lower and upper solutions α, β, i.e., satisfying α(t) ≤ β(t) for every t ∈ ℝ, and we will adopt the following notations:
        I : = [ inf t α ( t ) , sup t β ( t ) ] , ν : = I = sup t β ( t ) - inf t α ( t ) m : = min x I a ( x ) > 0 , M : = max x I a ( x ) , d : = max { α ( t ) + β ( t ) : t } . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Eque_HTML.gif

        Note that the value d is well-defined, in fact lim t + α ( t ) = lim t + β ( t ) = 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq2_HTML.gif, since (aα)(Φ ○ α'), (aβ)(Φ ○ β') belong to W1,1(ℝ) and m > 0.

        Moreover, in what follows [x]+ and [x]- will respectively denote the positive and negative part of the real number x, and we set xy := min{x, y}, xy := max{x, y}.

        The next result proved in [11] concerns the convergence of sequences of functions correlated to solutions of the previous equation.

        Lemma 2.2. For all n ∈ ℕ let I n := [-n, n] and let u n C1(I n ) be such that: ( a u n ) ( Φ u n ) W 1 , 1 ( I n ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq3_HTML.gif, the sequences (u n (0)) n and ( u n ( 0 ) ) n http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq4_HTML.gif are bounded and finally
        ( a ( u n ( t ) ) Φ ( u n ( t ) ) ) = f ( t , u n ( t ) , u n ( t ) ) f o r a . e . t I n . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equf_HTML.gif
        Assume that there exist two functions H, γL1(ℝ) such that
        u n ( t ) H ( t ) a n d a ( u n ( t ) ) Φ ( u n ( t ) ) γ ( t ) a . e . o n I n , f o r a l l n . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equg_HTML.gif
        Then, the sequence (x n ) n C1(ℝ) defined by
        x n ( t ) : = u n ( t ) f o r t I n u n ( n ) f o r t > n u n ( - n ) f o r t < - n http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equh_HTML.gif

        admits a subsequence uniformly convergent into a function xC1(ℝ), with (ax) (Φ ○ x') ∈ W1,1(ℝ), solution to equation (2.1).

        Moreover, if lim n + u n ( - n ) = u - http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq5_HTML.gif and lim n + u n ( n ) = u + http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq6_HTML.gif, then we have that
        lim t - x ( t ) = u - lim t + x ( t ) = u + . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equi_HTML.gif

        The first existence result concerns differential operators growing at most linearly at infinity.

        Theorem 2.3. Assume that there exists a pair of lower and upper solutions α, βC1(ℝ) of the equation (2.1), satisfying α(t) ≤ β(t), for every t ∈ ℝ, with α increasing in (-∞, -L), β increasing in (L, +∞), for some L > 0.

        Let Φ be such that
        lim sup y + Φ ( y ) y < + http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ2_HTML.gif
        (2.2)
        and
        lim inf y 0 + Φ ( y ) y μ > 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ3_HTML.gif
        (2.3)

        for some positive constant μ.

        Assume that there exist a constant H > 0, a continuous function θ : ℝ+ → ℝ+ and a function λL q ([-L, L]), with 1 ≤ q ≤ ∞, such that
        f ( t , x , y ) λ ( t ) θ ( a ( x ) Φ ( y ) ) f o r a . e . t L , e v e r y x I , y H http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ4_HTML.gif
        (2.4)
        + τ 1 - 1 q θ ( τ ) d τ = + http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ5_HTML.gif
        (2.5)

        (with 1 q = 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq7_HTML.gif if q = +∞).

        Finally, suppose that for every C > 0 there exist a function η C L1(ℝ) and a function K C W l o c 1 , 1 ( [ 0 , + ) ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq8_HTML.gif, null in [0, L] and strictly increasing in [L, +∞),

        such that:
        + e - 1 μ M K C ( t ) d t < + http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ6_HTML.gif
        (2.6)
        and put
        N C ( t ) : = Φ - 1 M m Φ ( C ) e - 1 M K C ( t ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ7_HTML.gif
        (2.7)
        we have
        f ( t , x , y ) - K C ( t ) Φ ( y ) f o r a . e . t L , e v e r y x I , y N C ( t ) , f ( - t , x , y ) K C ( t ) Φ ( y ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ8_HTML.gif
        (2.8)
        f ( t , x , y ) η C ( t ) i f x I , y N C ( t ) + α ( t ) + β ( t ) , f o r a . e . t . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ9_HTML.gif
        (2.9)
        Then, there exists a function xC1(ℝ), with (ax)(Φ ○ x') ∈ W1,1(ℝ), such that
        ( a ( x ( t ) ) Φ ( x ( t ) ) ) = f ( t , x ( t ) , x ( t ) ) f o r a . e . t α ( t ) x ( t ) β ( t ) f o r e v e r y t x ( - ) = α ( - ) , x ( + ) = β ( + ) . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equj_HTML.gif

        Proof. In some parts the proof is similar to that of Theorem 3.2 [11]. So, we provide here only the arguments which differ from those used in that proof.

        By (2.2), without loss of generality we assume H > ν 2 L http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq9_HTML.gif and
        Φ ( y ) K y w h e n e v e r y > H , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ10_HTML.gif
        (2.10)

        for some constant K > 0.

        Moreover, by (2.5), there exists a constant C > Φ - 1 ( M m Φ ( H ) ) H http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq10_HTML.gif such that
        M Φ ( H ) m Φ ( C ) τ 1 1 q θ ( τ ) d τ > ( K M ν ) 1 1 q λ q . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ11_HTML.gif
        (2.11)

        Fix n ∈ ℕ, n > L, and put I n := [-n, n].

        Let us consider the following auxiliary boundary value problem on the compact interval I n :
        ( P n * ) ( a ( T x ( t ) ) Φ ( x ( t ) ) ) = f ( t , T x ( t ) , Q x ( t ) ) + arctan ( w ( t , x ( t ) ) ) , a . e . t I n x ( - n ) = α ( - n ) , x ( n ) = β ( n ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equk_HTML.gif
        where T : W1,1(I n ) → W 1,1(I n ) is the truncation operator defined by
        T x ( t ) : = [ β ( t ) x ( t ) ] α ( t ) ; Q x ( t ) : = - ( N C ( t ) + α ( t ) + β ( t ) ) [ T x ( t ) ( N C ( t ) + α ( t ) + β ( t ) ) ] ; http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equl_HTML.gif

        and finally w : ℝ2 → ℝ is the penalty function defined by w(t, x) := [x - β(t)]+ - [x -α(t)]-.

        By the same argument used in the proof of Theorem 3.2 [11], one can show, using only assumption (2.9), that for every n > L problem ( P n * ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq11_HTML.gif admits a solution u n such that
        α ( t ) u n ( t ) β ( t ) f o r a l l t I n , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ12_HTML.gif
        (2.12)
        hence T u n ( t ) u n ( t ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq12_HTML.gif and w(t, u n (t)) ≡ 0. Moreover, it is possible to prove that
        u n ( t ) 0 w h e n e v e r L t n http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ13_HTML.gif
        (2.13)
        u n ( t 0 ) = 0 f o r s o m e t 0 [ L , n ) u n ( t ) 0 i n [ t 0 , n ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ14_HTML.gif
        (2.14)

        (see Steps 3 and 4 in the proof of Theorem 3.2 [11]).

        Now our goal is to prove an a priori bound for the derivatives, that is u n ( t ) N C ( t ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq13_HTML.gif for a.e. tI n . We split this part into two steps.

        Step 1. We have u n ( t ) < C N C ( t ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq14_HTML.gif for every t ∈ [-L, L].

        Indeed, since u n C1(I n ) and u n ( [ - L , L ] ) I http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq15_HTML.gif, we can apply Lagrange Theorem to deduce that for some τ0 ∈ [-L, L] we have
        u n ( τ 0 ) = 1 2 L u n ( L ) - u n ( - L ) sup β - inf α 2 L = ν 2 L < H < C . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equm_HTML.gif

        Assume, by contradiction, the existence of an interval (τ1, τ2) ⊂ (-L, L) such that H < u n ( t ) < C http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq16_HTML.gif in (τ1, τ2) and u n ( τ 1 ) = H http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq17_HTML.gif, u n ( τ 2 ) = C http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq18_HTML.gif or viceversa.

        Since N C ( t ) = Φ - 1 ( M m Φ ( C ) ) C http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq19_HTML.gif for every t ∈ (τ1, τ2), we have u n ( t ) < N C ( t ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq20_HTML.gif for every t ∈ (τ1, τ2). Then, by the definition of ( P n * ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq11_HTML.gif and assumption (2.4), for a.e. t ∈ (τ1, τ2) we have
        ( a ( u n ( t ) ) Φ ( u n ( t ) ) ) = ( a ( T u n ( t ) ) Φ ( u n ( t ) ) ) = f ( t , T u n ( t ) , Q u n ( t ) ) = f ( t , u n ( t ) , u n ( t ) ) λ ( t ) θ ( a ( u n ( t ) ) Φ ( u n ( t ) ) ) . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equn_HTML.gif
        Therefore, using a change of variable and the Hölder inequality, we get
        M Φ ( H ) m Φ ( C ) τ 1 1 q θ ( τ ) d τ τ 1 τ 2 | a ( u n ( t ) ) Φ ( u n ( t ) ) | 1 1 q θ ( | a ( u n ( t ) ) Φ ( u n ( t ) ) | ) | ( a ( u n ( t ) ) Φ ( u n ( t ) ) ) | d t τ 1 τ 2 λ ( t ) | a ( u n ( t ) ) Φ ( u n ( t ) ) | 1 1 q d t λ q ( M τ 1 τ 2 | Φ ( u n ( t ) ) | d t ) 1 1 q . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ15_HTML.gif
        (2.15)
        Moreover, since u n http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq21_HTML.gif has constant sign in (τ1, τ2), using (2.12) we have
        τ 1 τ 2 u n ( t ) d t = u n ( τ 2 ) - u n ( τ 1 ) ν . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equo_HTML.gif
        Therefore, by (2.10), from the previous chain of inequalities we deduce
        M Φ ( H ) m Φ ( C ) τ 1 - 1 q θ ( τ ) d τ λ q K M τ 1 τ 2 u n ( t ) d t 1 - 1 q λ q ( K M ν ) 1 - 1 q http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ16_HTML.gif
        (2.16)

        in contradiction with (2.11). Thus, we get u n ( t ) < C http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq22_HTML.gif for every t ∈ [-L, L] and the claim is proved.

        Step 2. We have u n ( t ) < N C http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq23_HTML.gif(t) for every tI n \ [-L, L].

        Define t ^ : = sup { t > L : u n ( τ ) < N C ( τ ) f o r e v e r y τ [ L , t ] } http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq24_HTML.gif, and assume by contradiction that t ^ < n http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq25_HTML.gif. Hence, u n ( t ^ ) = N C ( t ^ ) > 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq26_HTML.gif and by (2.13), (2.14) we deduce that u n ( t ) > 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq27_HTML.gif in [ L , t ^ ] http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq28_HTML.gif. Moreover, by (2.12) and the definition of Q u n http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq29_HTML.gif we get
        ( a ( u n ( t ) ) Φ ( u n ( t ) ) ) = f ( t , u n ( t ) , u n ( t ) ) i n [ L , t ^ ] , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equp_HTML.gif
        so, by (2.8) we have
        ( a ( u n ( t ) ) Φ ( u n ( t ) ) ) - K C ( t ) Φ ( u n ( t ) ) - K C ( t ) M a ( u n ( t ) ) Φ ( u n ( t ) ) , a . e . i n [ L , t ^ ] . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equq_HTML.gif
        Then, recalling that K C (L) = 0 and u n ( t ) > 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq27_HTML.gif for every t [ L , t ^ ] http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq30_HTML.gif, we infer
        a ( u n ( t ) ) Φ ( u n ( t ) ) a ( u n ( L ) ) Φ ( u n ( L ) ) = e L t ( a ( u n ( s ) ) Φ ( u n ( s ) ) ) a ( u n ( s ) ) Φ ( u n ( s ) ) d s e - 1 M K C ( t ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equr_HTML.gif
        implying
        a ( u n ( t ) ) Φ ( u n ( t ) ) a ( u n ( L ) ) Φ ( u n ( L ) ) e - 1 M K C ( t ) < M Φ ( C ) e - 1 M K C ( t ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equs_HTML.gif

        since u n ( L ) < C http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq31_HTML.gif. Therefore, u n ( t ) N C ( t ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq32_HTML.gif for every t [ L , t ^ ] http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq30_HTML.gif, in contradiction with the definition of t ^ http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq33_HTML.gif. The same argument works in the interval [-n, -L] and the claim is proved.

        Summarizing, since u n ( t ) N C ( t ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq13_HTML.gif for every tI n , by the definition of Q u n http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq29_HTML.gif we have
        ( a ( u n ( t ) ) Φ ( u n ( t ) ) ) = f ( t , u n ( t ) , u n ( t ) ) f o r a . e . t I n . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equt_HTML.gif

        Observe now that condition (2.3) implies that limsup ξ 0 + Φ - 1 ( ξ ) ξ 1 μ < + http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq34_HTML.gif. Hence, by assumption (2.6) we get N C L1(ℝ) and applying Lemma 2.2 with H(t) = N C (t) and γ(t) = η C (t) we deduce the existence of a solution x to problem (P). □

        In order to deal with differential operators having superlinear growth at infinity, we need to strengthen condition (2.5), taking a Nagumo function with sublinear growth at infinity, as in the statement of the following result.

        Theorem 2.4. Suppose that all the assumptions of Theorem 2.3 are satisfied, with the exception of (2.2), and with (2.5) replaced by
        lim y + θ ( y ) y = 0 . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ17_HTML.gif
        (2.17)

        Then, the assertion of Theorem 2.3 follows.

        Proof. The proof is quite similar to that of the previous Theorem. Indeed, notice that assumptions (2.2) and (2.5) of Theorem 2.3 have been used only in the choice of the constant C (see (2.11)) and in the proof of Step 1. Hence, we now present only the proof of this part, the rest being the same.

        Notice that by assumption (2.17), we have
        lim ξ + M Φ ( H ) m ξ τ 1 - 1 q θ ( τ ) d τ ξ 1 - 1 q = + http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equu_HTML.gif
        hence, there exists a constant C > Φ - 1 ( M m Φ ( H ) ) H http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq10_HTML.gif such that
        M Φ ( H ) m Φ ( C ) τ 1 1 q θ ( τ ) d τ > ( 2 M L Φ ( C ) ) 1 1 q λ q . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ18_HTML.gif
        (2.18)
        With this choice of the constant C, the proof proceeds as in Theorem 2.3. The only modification concerns formula (2.16), which becomes, taking (2.15) into account:
        M Φ ( H ) m Φ ( C ) τ 1 - 1 q θ ( τ ) d τ λ q M τ 1 τ 2 Φ ( u n ( t ) ) d t 1 - 1 q λ q ( 2 M L Φ ( C ) ) 1 - 1 q http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equv_HTML.gif

        in contradiction with (2.18). From here on, the proof proceeds in the same way. □

        In the particular case of p-Laplacian operators, one can use the positive homogeneity for weakening assumption (2.17) of Theorem 2.4 and widening the class of the admissible Nagumo functions, as we show in the following result.

        Theorem 2.5. Let Φ : ℝ → ℝ, Φ(y) = |y|p-2y, and assume that there exists a pair of lower and upper solutions α, βC1(ℝ) to equation (2.1), satisfying α(t) ≤ β(t), for every t ∈ ℝ, with α increasing in (-∞, -L), β increasing in (L, +∞), for some constant L > 0.

        Moreover, assume that there exist a positive constant H, a continuous function

        θ : ℝ+ → ℝ+ and a function λL q ([-L, L]), with 1 ≤ q ≤ +∞, such that
        f ( t , x , y ) λ ( t ) θ ( a ( x ) y p - 1 ) f o r a . e . t L , e v e r y x I , y H http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ19_HTML.gif
        (2.19)
        + τ 1 p - 1 ( 1 - 1 q ) θ ( τ ) d τ = + . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ20_HTML.gif
        (2.20)
        Finally, suppose that for every C > 0 there exist a function η C L1(ℝ) and a function K C W l o c 1 , 1 ( [ 0 , + ) ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq8_HTML.gif, null in [0, L] and strictly increasing in [L, +∞), such that:
        + e - 1 M ( p - 1 ) K C ( t ) d t < + , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ21_HTML.gif
        (2.21)
        and put
        N C ( t ) : = C M m 1 p - 1 e - 1 M ( p - 1 ) K C ( t ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equw_HTML.gif
        we have
        f ( t , x , y ) - K C ( t ) y p - 1 f o r a . e . t L , e v e r y x I , y N C ( t ) , f ( - t , x , y ) K C ( t ) y p - 1 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ22_HTML.gif
        (2.22)
        f ( t , x , y ) η C ( t ) i f x I , y N C ( t ) + α ( t ) + β ( t ) , f o r a . e . t . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ23_HTML.gif
        (2.23)
        Then, there exists a function xC1(ℝ), with (ax)(Φ ○ x') ∈ W1,1(ℝ), such that
        ( a ( x ( t ) ) Φ ( x ( t ) ) ) = f ( t , x ( t ) , x ( t ) ) f o r a . e . t α ( t ) x ( t ) β ( t ) f o r e v e r y t x ( - ) = α ( - ) , x ( + ) = β ( + ) . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equx_HTML.gif

        Proof. The proof is quite similar to that of Theorem 2.3. Indeed, notice that the present statement has the same assumptions of Theorem 2.3, written for Φ(y) = |y|p-2y, with the exception of conditions (2.2) and (2.5), which were used only in the proof of Step 1. Hence, as in the proof of the previous Theorem 2.4, we now provide only the proof of Step 1, the rest being the same.

        At the beginning of the proof, without loss of generality we assume H > ν 2 L http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq9_HTML.gif and we choose C > M m 1 p - 1 H H http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq35_HTML.gif, in such a way that
        M H p - 1 m C p - 1 τ 1 p - 1 ( 1 - 1 q ) θ ( τ ) d τ > λ q ν M 1 p - 1 1 - 1 q . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ24_HTML.gif
        (2.24)
        The proof of Step 1 begins as previously, determining an interval J = (τ1, τ2) ⊂ (-L, L) such that u n ( τ 0 ) = 1 2 L u n ( L ) - u n ( - L ) sup β - inf α 2 L = ν 2 L < H < C . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equm_HTML.gif in J, and u n ( τ 1 ) = H http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq17_HTML.gif, u n ( τ 2 ) = C http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq18_HTML.gif or vice versa. Then, as in the proof of Theorem 2.3, assumption (2.19) implies that for a.e. tJ we have
        ( a ( u n ( t ) ) Φ ( u n ( t ) ) ) = ( a ( T u n ( t ) ) Φ ( u n ( t ) ) ) = f ( t , T u n ( t ) , Q u n ( t ) ) = = f ( t , u n ( t ) , u n ( t ) ) λ ( t ) θ ( a ( u n ( t ) ) u n ( t ) p - 1 ) . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equy_HTML.gif
        Therefore, put
        α 1 : = a ( x ( τ 1 ) ) x ( τ 1 ) p - 1 , α 2 : = a ( x ( τ 2 ) ) x ( τ 2 ) p - 1 , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equz_HTML.gif
        we get
        M H p 1 m C p 1 τ 1 p 1 ( 1 1 q ) θ ( τ ) d τ | α 1 α 2 τ 1 p 1 ( 1 1 q ) θ ( τ ) d τ | = | τ 1 τ 2 ( a ( u n ( t ) ) | u n ( t ) | p 1 ) 1 p 1 ( 1 1 q ) θ ( a ( u n ( t ) ) | u n ( t ) | p 1 ) | ( a ( u n ( t ) ) | u n ( t ) | p 1 ) | d t | τ 1 τ 2 λ ( t ) ( a ( u n ( t ) ) 1 p 1 | u n ( t ) | ) 1 1 q d t λ q M 1 p 1 ( 1 1 q ) ( τ 1 τ 2 | u n ( t ) | d t ) 1 1 q λ q ( ν M 1 p 1 ) 1 1 q http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equaa_HTML.gif

        in contradiction with (2.24). Thus, we get u n ( t ) < C http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq22_HTML.gif for every t ∈ [-L, L] and Step 1 is proved. □

        As we mentioned in Section 1, the assumptions of the previous existence Theorems are not improvable in the sense that if conditions (2.3) and (2.8) are satisfied with the reversed inequalities and the summability condition (2.6) [respectively (2.21) for the case of p-Laplacian] does not hold, then problem (P) does not admit solutions, as the following results state.

        Theorem 2.6. Suppose that
        lim sup y 0 + Φ ( y ) y μ < + http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ25_HTML.gif
        (2.25)
        for some positive constant μ. Moreover, assume that there exist two constants L ≥ 0, ρ > 0 and a positive strictly increasing function K W l o c 1 , 1 ( [ L , + ) ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq36_HTML.gif satisfying
        + e - 1 μ m ̃ K ( t ) d t = + http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ26_HTML.gif
        (2.26)
        where m ̃ : = min x [ ν - , ν + ] a ( x ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq37_HTML.gif, such that one of the following pair of conditions holds:
        f ( t , x , y ) K ( t ) Φ ( | y | ) f o r a . e . t L , e v e r y x [ ν , ν + ], | y | < ρ http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ27_HTML.gif
        (2.27)
        or
        f ( t , x , y ) K ( - t ) Φ ( y ) f o r a . e . t - L , e v e r y x [ ν - , ν + ] , y < ρ . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ28_HTML.gif
        (2.28)
        Moreover, assume that
        t f ( t , x , y ) 0 f o r a . e . t L , e v e r y x , y < ρ . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ29_HTML.gif
        (2.29)

        Then, problem (P) can only admit solutions which are constant in [L, +∞) (when (2.27) holds) or constant in (-∞, -L] (when (2.28) holds). Therefore, if both (2.27) and (2.28) hold and L = 0, then problem (P) does not admit solutions. More precisely, no function xC1(ℝ), with (ax)(Φ○x') almost everywhere differentiable, exists satisfying the boundary conditions and the differential equation in (P).

        Proof. Suppose that (2.27) holds (the proof is the same if (2.28) holds).

        Let xC1(ℝ), with (ax)(Φ○x') almost everywhere differentiable (not necessarily belonging to W1,1(ℝ)), be a solution of problem (P). First of all, let us prove that lim t + Φ ( x ( t ) ) = 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq38_HTML.gif.

        Indeed, since x(+∞) = ν+ ∈ ℝ, we have lim sup t + x ( t ) 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq39_HTML.gif and lim inf t + x ( t ) 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq40_HTML.gif.

        Taking into account that Φ is an increasing homeomorphism with Φ(0) = 0, if lim inf t + x ( t ) < 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq41_HTML.gif, then there exists an interval [t1, t2] ⊂ [L, +∞) such that -ρ < Φ (x'(t)) < 0 in [t1, t2], Φ ( x ( t 2 ) ) > m M Φ ( x ( t 1 ) ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq42_HTML.gif. But by virtue of assumption (2.29)

        we deduce that a(x(t))Φ(x'(t)) is decreasing in [t1, t2] and then
        Φ ( x ( t 2 ) ) 1 M a ( x ( t 2 ) ) Φ ( x ( t 2 ) ) 1 M a ( x ( t 1 ) ) Φ ( x ( t 1 ) ) m M Φ ( x ( t 1 ) ) , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equab_HTML.gif

        a contradiction. Hence, necessarily lim inf t + x ( t ) = 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq43_HTML.gif. We can prove in a similar way that lim sup t + x ( t ) = 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq44_HTML.gif. So, lim t + x ( t ) = 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq45_HTML.gif and we can define t* := inf{tL : |x'(τ)| < ρ in [t, +∞)}.

        We claim that x'(t) ≥ 0 for every tt*. Indeed, if x ( t ^ ) < 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq46_HTML.gif for some t ^ t * http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq47_HTML.gif, since a(x(t))Φ(x'(t)) is decreasing in [t*, +∞) by (2.29), we get
        a ( x ( t ) ) Φ ( x ( t ) ) a ( x ( t ^ ) ) Φ ( x ( t ^ ) ) m Φ ( x ( t ^ ) ) < 0 , f o r e v e r y t t ^ . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ30_HTML.gif
        (2.30)
        Since a is positive, then Φ(x'(t)) < 0 for every t t ^ http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq48_HTML.gif. Hence, from (2.30) we get M Φ ( x ( t ) ) m Φ ( x ( t ^ ) ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq49_HTML.gif, and so
        x ( t ) Φ - 1 m M Φ ( x ( t ^ ) ) < 0 f o r e v e r y t t ^ http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equac_HTML.gif

        in contradiction with the boundedness of x. Thus, the claim is proved.

        Let us define t ̃ : = inf { t t * : x ( τ ) ν - i n [ t , + ) } t * http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq50_HTML.gif. We now prove that x'(t) = 0 for every t t ̃ http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq51_HTML.gif.

        Let us assume by contradiction that x ( t ̄ ) > 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq52_HTML.gif for some t ̄ t ̃ http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq53_HTML.gif. Put T : = sup { t t ̄ : x ( τ ) > 0 i n [ t ̄ , t ] } http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq54_HTML.gif; we claim that T = +∞. Indeed, if T < +∞, since 0 < x'(t) < ρ in [ t ̄ , T ] http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq55_HTML.gif, by (2.27) we have
        ( a ( x ( t ) ) Φ ( x ( t ) ) ) = f ( t , x ( t ) , x ( t ) ) - K ( t ) Φ ( x ( t ) ) f o r a . e . t [ t ̄ , T ] . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ31_HTML.gif
        (2.31)
        So, assuming without loss of generality ρ ≤ 1, we get
        ( a ( x ( t ) ) Φ ( x ( t ) ) ) - K ( t ) Φ ( x ( t ) ) - K ( t ) m ̄ a ( x ( t ) ) Φ ( x ( t ) ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equad_HTML.gif
        where m ̄ : = min ξ [ x ( t ̄ ) , x ( T ) ] a ( ξ ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq56_HTML.gif. Then, integrating in [t, T] with t < T we obtain (taking into account that x'(T) = 0)
        a ( x ( t ) ) Φ ( x ( t ) ) t T K ( τ ) m ̄ a ( x ( τ ) ) Φ ( x ( τ ) ) d τ f o r e v e r y t ( t ̄ , T ] http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equae_HTML.gif

        so by the Gronwall's inequality we deduce a(x(t))Φ(x'(t)) ≤ 0, i.e. x'(t) ≤ 0 in the same interval, in contradiction with the definition of T. Hence T = +∞.

        Therefore, since 0 < x'(t) < ρ and ν - x(t) ≤ ν+ in [ t ̄ , + ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq57_HTML.gif, we get
        ( a ( x ( t ) ) Φ ( x ( t ) ) ) = f ( t , x ( t ) , x ( t ) ) - K ( t ) Φ ( x ( t ) ) - K C ( t ) m ̃ a ( x ( t ) ) Φ ( x ( t ) ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equaf_HTML.gif
        for a.e. t t ̄ http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq58_HTML.gif, where m ̃ : = min x [ ν - , ν + ] a ( x ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq59_HTML.gif. The above inequalities imply that for a.e. t t ̄ http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq58_HTML.gif
        log a ( x ( t ) ) Φ ( x ( t ) ) a ( x ( t ̄ ) ) Φ ( x ( t ̄ ) ) = t ̄ t ( a ( x ( s ) ) Φ ( x ( s ) ) ) a ( x ( s ) ) Φ ( x ( s ) ) d s 1 m ̃ ( K ( t ̄ ) - K ( t ) ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equag_HTML.gif
        and then
        Φ ( x ( t ) ) 1 M ̃ a ( x ( t ̄ ) ) Φ ( x ( t ̄ ) ) e 1 m ̃ ( K ( t ̄ ) - K ( t ) ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equah_HTML.gif

        where M ̃ : = max x [ ν - , ν + ] a ( x ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq60_HTML.gif. By virtue of (2.25) and (2.26), since x ( t ̄ ) > 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq61_HTML.gif, we get x ( + ) - x ( t ̄ ) = t ̄ + x ( t ) d t = + http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq62_HTML.gif, in contradiction with the boundedness of x.

        Therefore, x'(t) ≡ 0 in [ t ̃ , + ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq63_HTML.gif and by the definition of t ̃ http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq64_HTML.gif this implies t ̃ = t * http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq65_HTML.gif. So, x'(t) ≡ 0 in [t*, +∞) and by the definition of t* this implies t* = L. □

        Remark 2.7. In view of what observed in Remark 6 [13], if the sign condition in (2.29) is satisfied with the reverse inequality, i.e., if
        t f ( t , x , y ) 0 f o r a . e . t L , e v e r y x , y < ρ , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ32_HTML.gif
        (2.32)

        then it is possible to prove that lim x ± x ( t ) = 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq66_HTML.gif and x'(t) ≤ 0 for |t| ≥ L. So, since ν - < ν+, when L = 0 problem (P) does not admit solutions.

        3. Criteria for right-hand side of the type f(t, x, y) = b(t, x)c(x, y)

        In this section we present some operative criteria useful when the right-hand side has the following product structure
        f ( t , x , y ) = b ( t , x ) c ( x , y ) . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equai_HTML.gif

        As we will show, there is a strict link between the local behaviors of c(x, ·) at y = 0 and of b(·, x) at infinity which plays a key role for the existence or non-existence of solutions.

        In what follows we assume that b is a Carathéodory function and c is a continuous function satisfying
        c ( x , y ) > 0 f o r e v e r y y 0 a n d x [ ν - , ν + ] ; c ( ν - , 0 ) = c ( ν + , 0 ) = 0 . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equaj_HTML.gif
        Notice that in this framework, the constant functions α(t) :≡ ν- and β(t) :≡ ν+ are a pair of well-ordered, monotone, lower and upper solutions. Consequently, according to the notations given after Definition 2.1, in this case we have
        I = [ ν - , ν + ] , ν = ν + - ν - , d = 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equak_HTML.gif
        and again
        m : = min x I a ( x ) > 0 , M : = max x I a ( x ) , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equal_HTML.gif

        According to the results of the previous section, the first three results provide sufficient conditions for the existence of solutions for our special f split in the product of b and c. Then we will deal with sufficient conditions for the non-existence of solutions.

        Theorem 3.1. Let there exists a function λ L l o c q ( ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq67_HTML.gif, 1 ≤ q ≤ +∞, such that
        b ( t , x ) λ ( t ) f o r a . e . t , e v e r y x [ ν - , ν + ] . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ33_HTML.gif
        (3.1)
        Suppose that there exist positive constants h1, h2, k1, k2, ρ, H, L, ε, with ε ≤ 1, and a constant σ [ - 1 , - 1 + h 1 k 1 M ε ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq68_HTML.gif, such that for every x ∈ [ν-, ν+] we have
        t b ( t , x ) 0 f o r a . e . t > L , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ34_HTML.gif
        (3.2)
        h 1 t - 1 b ( t , x ) h 2 t σ , f o r a . e . t > L , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ35_HTML.gif
        (3.3)
        k 1 Φ ( y ) c ( x , y ) k 2 Φ ( y ) ε , w h e n e v e r y < ρ , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ36_HTML.gif
        (3.4)
        c ( x , y ) k 2 Φ ( y ) 2 - 1 q w h e n e v e r y H . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ37_HTML.gif
        (3.5)

        Finally, let conditions (2.2) and (2.3) hold with 0 < μ < h 1 k 1 M http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq69_HTML.gif.

        Then, problem (P) admits solutions.

        Proof. Put θ ( r ) : = k 2 r m 2 - 1 q http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq70_HTML.gif for r > 0, from (3.1) and (3.5) it is immediate to verify the validity of conditions (2.4) and (2.5). Let us now fix a constant C > 0 and put
        Ĉ : = max ρ , Φ - 1 M m Φ ( C ) . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equam_HTML.gif
        Since c(x, y) > 0 for y ≠ 0, denoted by m ^ C : = min { c ( x , y ) : x [ ν - , ν + ] , ρ y Ĉ } http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq71_HTML.gif, we have m ^ C > 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq72_HTML.gif. Finally, put
        ψ C : = min { m ^ C Φ ( Ĉ ) , k 1 } . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equan_HTML.gif
        Consider the following functions:
        γ ( t ) : = min { min x [ ν - , ν + ] b ( - t , x ) , min x [ ν - , ν + ] b ( t , x ) } , t 0 ; H C ( t ) : = 0 f o r 0 t L ; ψ C L t γ ( τ ) d τ f o r t L ; M C ( t ) : = Φ - 1 M m Φ ( C ) e - 1 M H C ( t ) , t . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equao_HTML.gif
        Observe that by assumption (3.3) we have γ(t) > 0 for a.e. tL and lim t + H C ( t ) = + http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq73_HTML.gif, hence lim t + M C ( t ) = 0 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq74_HTML.gif. So, there exists a constant L C * > L http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq75_HTML.gif such that M C (t) ≤ ρ whenever t L C * http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq76_HTML.gif. Let us define
        K C ( t ) : = H C ( t ) f o r 0 t L C * ; H C ( L C * ) + k 1 L C * t γ ( τ ) d τ f o r t > L C * http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equap_HTML.gif

        and let N C be the function defined in (2.7).

        By the positivity of the function γ, K C is strictly increasing for tL. Moreover, by condition (3.1), we have K C W l o c 1 , 1 ( [ 0 , + ) ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq8_HTML.gif. Further, since ψ C k1 we have H C (t) ≤ K C (t) for every t ≥ 0 and then N C (t) ≤ M C (t) for every t ∈ ℝ.

        Observe that by (3.2) and the definition of ψ C , we obtain
        f ( t , x , y ) = b ( t , x ) c ( x , y ) ψ C b ( t , x ) Φ ( y ) - K C ( t ) Φ ( y ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equaq_HTML.gif
        and
        f ( - t , x , y ) = b ( - t , x ) c ( x , y ) ψ C b ( - t , x ) Φ ( y ) K C ( t ) Φ ( y ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equar_HTML.gif
        for a.e. t ( L , L C * ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq77_HTML.gif, every x ∈ [ν-,ν+] and every y N C ( t ) Ĉ http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq78_HTML.gif. Similarly, by (3.4) we have
        f ( t , x , y ) = b ( t , x ) c ( x , y ) k 1 b ( t , x ) Φ ( y ) - K C ( t ) Φ ( y ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equas_HTML.gif
        and
        f ( - t , x , y ) = b ( - t , x ) c ( x , y ) k 1 b ( - t , x ) Φ ( y ) K C ( t ) Φ ( y ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equat_HTML.gif

        for a.e. t L C * http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq79_HTML.gif, every x ∈ [ν-,ν+] and every |y| ≤ N C (t) ≤ M C (t) ≤ ρ. Then, condition (2.8) of Theorem 2.3 holds.

        Now, from (3.3) it follows that h 1 k 1 t - 1 K C ( t ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq80_HTML.gif for a.e. t L C * http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq79_HTML.gif. As a consequence,
        K C ( t ) K C ( L C * ) + h 1 k 1 log t L C * f o r e v e r y t > L C * . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ38_HTML.gif
        (3.6)
        Then, by the upper bound on the exponent μ we get
        + e - 1 μ M K C ( t ) d t C o n s t . + t - h 1 k 1 μ M d t < + , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equau_HTML.gif

        and condition (2.6) follows.

        Finally, let us define
        η C ( t ) : = max x [ ν - , ν + ] b ( t , x ) max ( x , y ) [ ν - , ν + ] × [ - Ĉ , Ĉ ] c ( x , y ) i f t L C * h 2 k 2 t σ Φ ( N C ( t ) ) ε i f t > L C * . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equav_HTML.gif
        By (3.3) and (3.4), for every y ∈ ℝ such that |y| ≤ N C (t) for a.e. t ∈ ℝ and every x ∈ [ν-,ν+], it results
        f ( t , x , y ) = b ( t , x ) c ( x , y ) η C ( t ) , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equaw_HTML.gif
        that is condition (2.9), so it remains to prove that η C L1(ℝ). To this purpose, notice that by (3.1) and the continuity of c we have η C L 1 ( [ - L C * , L C * ] ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq81_HTML.gif. Moreover, when t > L C * http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq82_HTML.gif, by (3.6) we have
        0 < η C ( t ) h 2 k 2 t σ M m Φ ( C ) ε e - ε M K C ( t ) C o n s t . t σ - h 1 k 1 M ε . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equax_HTML.gif

        Since σ < h 1 k 1 M ε - 1 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq83_HTML.gif, we get η C L1(ℝ). Therefore, Theorem 2.3 applies and guarantees the assertion of the present result. □

        For differential operators having superlinear growth at infinity, the following result can be applied, whose proof is a consequence of Theorem 2.4.

        Theorem 3.2. Let all the assumptions of Theorem 3.1 be satisfied with the exception of (2.2) and with (3.5) replaced by
        lim y + max x [ ν - , ν + ] c ( x , y ) Φ ( y ) = 0 . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ39_HTML.gif
        (3.7)

        Then, if (2.3) holds true with a positive μ < h 1 k 1 M http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq84_HTML.gif, problem (P) admits solutions.

        Proof. Set
        θ ( s ) : = max x [ ν - , ν + ] max c x , Φ - 1 s a ( x ) , c x , Φ - 1 - s a ( x ) . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equay_HTML.gif
        Observe that θ is a continuous function on [0, +∞), such that
        θ ( a ( x ) Φ ( y ) ) c ( x , y ) f o r e v e r y x [ ν - , ν + ] , y , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equaz_HTML.gif
        hence (2.4) holds. Moreover, by (3.7), for every ε > 0 there exists a real c ε such that
        c ( x , y ) ε Φ ( y ) f o r e v e r y x [ ν - , ν + ] , y c ε . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equba_HTML.gif
        Hence, for every sM max{Φ(c ε ), -Φ(-c ε )} we have θ ( s ) ε m s http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq85_HTML.gif, that is
        lim s + θ ( s ) s = 0 . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equbb_HTML.gif

        Hence, the proof proceeds as that of Theorem 3.1, applying Theorem 2.4 instead of Theorem 2.3. □

        Finally, in the case of p-Laplacian operators, the following result holds, as a consequence of Theorem 2.5, by the same proof of Theorem 3.1.

        Theorem 3.3. Consider Φ(y) = |y|p-2y, p > 1, and let all the assumptions of Theorem 3.1 be satisfied with the exception of (2.2) and condition (3.5) replaced by
        c ( x , y ) k 2 y p - 1 q , f o r e v e r y y H . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ40_HTML.gif
        (3.8)

        Then, if p < 1 + h 1 k 1 M http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq86_HTML.gif, problem (P) admits solutions.

        Proof. Define θ ( r ) : = k 2 r m 1 + 1 p - 1 ( 1 - 1 q ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq87_HTML.gif. Easy computations allow to verify that assumptions (2.19) and (2.20) are satisfied. The conclusion follows as in the proof of Theorem 3.1, now applying Theorem 2.5. In fact, observe that in this case (2.3) is satisfied for μ = p - 1 and, defined K C and η C as in the proof of Theorem 3.1, conditions (2.22) and (2.23) hold true. Notice that they are the rewriting of conditions (2.8) and (2.9), respectively, in the case of p-Laplacian operators. □

        In the previous results the requirement μ < h 1 k 1 M http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq84_HTML.gif is not merely technical, but it is essential, as it will be clarified by the following non-existence result.

        Theorem 3.4. Suppose that (3.2) holds for a.e. t ∈ ℝ and let there exist a real constant Λ > 0 and a positive function λL1(0, Λ) such that
        b ( t , x ) ( t ) f o r a . e . t Λ , x [ ν - , ν + ] . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ41_HTML.gif
        (3.9)
        Moreover, assume that there exist positive constants h, k, ρ such that
        b ( t , x ) h t - 1 , f o r e v e r y x , a . e . t > Λ http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ42_HTML.gif
        (3.10)
        c ( x , y ) k Φ ( y ) , f o r e v e r y x , 0 < y < ρ . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ43_HTML.gif
        (3.11)

        If (2.25) holds with a positive μ h k m http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq88_HTML.gif, then problem (P) does not have solutions.

        Proof. Put K ( t ) : = k 0 t ( τ ) d τ http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq89_HTML.gif for t ∈ [0, Λ] and K ( t ) : = 0 Λ ( τ ) d τ + h k ( log t - log L ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq90_HTML.gif for t ≥ Λ. Note that assumptions (2.27) and (2.28) are satisfied for L = 0. Moreover,
        + e - 1 μ m K ( t ) d t C o n s t . + t - 1 μ m h k d t = + http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equbc_HTML.gif

        by the lower bound on the exponent μ. So, the assertion follows from Theorem 2.6. □

        The following results are immediate consequences of Theorems 3.1, 3.2, and 3.4.

        Corollary 3.5. Let f(t, x, y) = h(t)g(x)c(y), with h L l o c q ( ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq91_HTML.gif, for 1 ≤ q ≤ +∞, c continuous inand g continuous and positive in [ν-,ν+].

        Assume that t · h(t) ≤ 0 for every t ∈ ℝ and c(y) > 0 for every y ≠ 0. Moreover, suppose that
        lim t + t h ( t ) = : h 1 ( 0 , + ) , lim y 0 c ( y ) Φ ( y ) = : k 1 ( 0 , + ) . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equbd_HTML.gif
        Let (2.2) holds and
        limsup y + c ( y ) Φ ( y ) 2 - 1 q < + . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ44_HTML.gif
        (3.12)

        Then, if (2.3) holds with an exponent μ such that h 1 k 1 min x [ ν - , ν + ] g ( x ) > M μ http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq92_HTML.gif, problem (P) admits solutions; instead if (2.25) holds with an exponent μ satisfying h 1 k 1 max x [ ν - , ν + ] g ( x ) < m μ http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq93_HTML.gif, (P) does not admit solutions.

        Corollary 3.6. Let all the assumptions of Corollary 3.5 be satisfied, apart (2.2) and with (3.12) replaced by the following condition
        lim y + c ( y ) Φ ( y ) = 0 . http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equ45_HTML.gif
        (3.13)

        Then, the same conclusions of Corollary 3.5 hold.

        Finally, for the p-Laplacian operator we can state the following criterium, consequence of Theorems 3.3 and 3.4.

        Corollary 3.7. Let f(t, x, y) = h(t)g(x)c(y), with h L l o c q ( ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq91_HTML.gif, for 1 ≤ q ≤ +∞, b continuous in ℝ, g continuous and positive in [ν - , ν+]. Let Φ(y) = |y|p-2y, for p > 1.

        Assume that t · h(t) ≤ 0 for every t ∈ ℝ and c(y) > 0 for every y ≠ 0. Moreover, suppose that there exist
        lim t + t h ( t ) = : h 1 , lim y 0 c ( y ) y p - 1 = : k 1 , limsup y + c ( y ) y p - 1 q < + , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Eqube_HTML.gif

        for a positive constant h1.

        Then, if h 1 k 1 min x [ ν - , ν + ] g ( x ) > M ( p - 1 ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq94_HTML.gif, problem (P) admits solutions; instead if h 1 k 1 max x [ ν - , ν + ] g ( x ) < m ( p - 1 ) http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq95_HTML.gif, (P) does not have solutions.

        We conclude with some examples in which the previous corollaries apply.

        Example 3.8. Let f ( t , x , y ) : = - h t 1 + t 4 g ( x ) y μ ( 1 + y 2 ) μ - 2 2 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq96_HTML.gif where h is a positive constant and g is a generic continuous function, positive in [ν-,ν+]. Suppose that Φ(y) = y|y|μ-2| arctan y| with μ ≥ 1 for every y ∈ ℝ and a(x) ≡ 1 for every x ∈ ℝ.

        If h ( t ) : = - h t 1 + t 4 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq97_HTML.gif and c ( y ) : = y μ ( 1 + y 2 ) μ - 2 2 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq98_HTML.gif, it is immediate to check that all the assumptions of Corollary 3.5 are satisfied for q := +∞, h1 := h, k1 := 1. Then, if min x [ ν - , ν + ] g ( x ) > μ h http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq99_HTML.gif problem (P) has solutions, instead if max x [ ν - , ν + ] g ( x ) < μ h http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq100_HTML.gif then problem (P) does not have solutions.

        Example 3.9. Let f ( t , x , y ) : = - h t 1 + t 4 g ( x ) y β http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq101_HTML.gif, where h is a positive constant and g is a generic continuous function, positive in [ν-,ν+]. Let Φ(y) := y|y|β-1e|y|and a(x) ≡ 1. Then condition (3.13) is satisfied for every β > 0 and all the assumptions of Corollary 3.6 hold with h1 := h and k1 := 1. Then, if min x [ ν - , ν + ] g ( x ) > μ h http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq102_HTML.gif problem (P) has solutions, instead if max x [ ν - , ν + ] g ( x ) < μ h http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq103_HTML.gif then problem (P) does not have solutions.

        Example 3.10. Let f ( t , x , y ) : = - h arctan t t g ( x ) y p - 1 1 + y 2 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq104_HTML.gif, where h is a positive constant and g is a generic continuous function, positive in [ν-,ν+]. Let Φ(y) := y|y|p-2and a(x) ≡ 1. Then all the assumptions of Corollary 3.7 are satisfied, for q := +∞, h 1 : = h π 2 http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq105_HTML.gif, h2 := 1. Then, if min x [ ν - , ν + ] g ( x ) > 2 ( p - 1 ) π h http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq106_HTML.gif problem (P) has solutions, instead if max x [ ν - , ν + ] g ( x ) < 2 ( p - 1 ) π h http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_IEq107_HTML.gif then problem (P) does not have solutions.

        Remark 3.11. Note that in [11] the existence of heteroclinic solutions was proved when in assumption (2.8) one has Φ(|y|) γ , instead of Φ(|y|), for some γ > 1. Of course, for small |y| we have Φ(|y|) γ < Φ(|y|) for each γ > 1, hence the present condition (2.8) implies the validity of the analogous condition with γ > 1, assumed in [11] (see condition (8)). But, on the other hand, taking γ > 1 one can lose the summability of the function K C required in assumption (7) of [11]. In fact, in the following example the present Theorems 2.3 and 2.4 are applicable, whereas the results established in [11] do not work.

        Consider the problem, already discussed in Example 4 [7]:
        ( Φ ( x ( t ) ) ) = m ( t ) Φ ( x ( t ) ) , a . e . o n x ( - ) = 0 , x ( + ) = 1 , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equbf_HTML.gif
        where a(x) ≡ 1 and m : ℝ → ℝ is the function defined by
        m ( t ) = - α t , t > 1 - α t , t 1 , http://static-content.springer.com/image/art%3A10.1186%2F1687-2770-2011-26/MediaObjects/13661_2011_Article_72_Equbg_HTML.gif

        for some α > 0. As it easy to check, the best function K C satisfying condition (8) in [11] is K C (t) := [α log t]+, but condition (7) of [11] does not hold, whatever γ > 1 may be. Hence, the existence results proved in [11] are not applicable. Instead, notice that condition (2.6) herein considered holds whenever α > μ (see (2.3)) and Theorem 2.3 (or Theorem 2.4) applies, provided that the operator Φ also satisfies the other required assumptions. Similar considerations can be done for the p-Laplacian operator too, using Theorem 2.5.

        Declarations

        Authors’ Affiliations

        (1)
        Dipartimento di Matematica - Università di Bologna
        (2)
        Dipartimento di Scienze Matematiche - Università Politecnica delle Marche

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        © Cupini et al; 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.