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Research Article

Franco Obersnel and Pierpaolo Omari*

Revisiting the sub

- and super-solution

method for the classical radial solutions of

the mean curvature equation

https://doi.org/10.1515/math-2020-0097 received July 31, 2020; accepted October 9, 2020

Abstract: This paper focuses on the existence and the multiplicity of classical radially symmetric solutions of the mean curvature problem:

            − ∇ + |∇ | = ( ∇ ) + ∂ = ∂ ∂ v v f x v v a v a div 1 , , in Ω, 0 on Ω, v ν 2 0 1

with Ω an open ball inN, in the presence of one or more couples of sub- and super-solutions, satisfying or

not satisfying the standard ordering condition. The novel assumptions introduced on the function f allow us to complement or improve several results in the literature.

Keywords: prescribed mean curvature equation, Dirichlet, Neumann, Robin boundary conditions, radial symmetry, classical solution, sub- and super-solutions

MSC 2020: 35J62, 35J93, 35J25, 34C25

1 Introduction and main results

This paper deals with the existence of classical solutions of the mean curvature problem:            − ′ + ′ = ( ′) ] [ ′( ) = ( ) + ′( ) = − ′ − t u u t f t u u R u a u R a u R 1 , , in 0, , 0 0, 0, N N 1 2 1 0 1 (1.1) in the presence of couples of sub- and super-solutions, satisfying or not satisfying the standard ordering condition, that is, the sub-solution is smaller than the super-solution. It is assumed throughout this work that (h1) >R 0,N∈+,a ,a 

0 1 are given constants, witha0⋅a1≥0,a0+a1>0,

and

(h2)f: 0,[ R] × × → is continuous.

Franco Obersnel: Dipartimento di Matematica e Geoscienze, Università degli Studi di Trieste, via A. Valerio 12/1, 34127 Trieste, Italy, e-mail: obersnel@units.it



* Corresponding author: Pierpaolo Omari, Dipartimento di Matematica e Geoscienze, Università degli Studi di Trieste, via A. Valerio 12/1, 34127 Trieste, Italy, e-mail: omari@units.it

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Definition 1.1. (Notion of solution) By a classical solution of (1.1) we mean a function ∈ ([u C1 0,R]) ∩C2(]0,R[) which satisfies the differential equation for all ∈ ]t 0,R[and the boundary condition.

It is easy to verify, arguing like, e.g., in[1, Lemma 3.1], that a classical solution u of (1.1) actually belongs to

([ ])

C2 0,R and satisfies the equation:       − ′ + ′ − − ′ + ′ = ( ′) ∈ [ ] ′ u u N t u u f t u u t R 1 1 1 , , for all 0, , 2 2

provided that the convention u′ ( )t = ″( )u 0

t ift=0 is set. Accordingly, solutions of (1.1) will always be

assumed to belong toC2([0,R]).

From[1, Remark 3.2] it also follows that if u is a solution of (1.1), then, setting ( ) = (| |)v x u x for allx∈Ω, with Ω the open ball inN of center 0 and radius R, the function v belongs toC Ω2( )and is a classical

solution of            

(

)

− ∇ + |∇ | = | | ∇ ⋅ + ∇ ⋅ = ∂ | | | | v v f x v v a v a v div 1 , , in Ω, 0 on Ω. x x x x 2 0 1 (1.2)

The existence of classical solutions of boundary value problems associated with the mean curvature equation       − ∇ + |∇ | = ( ∇ ) v v f x v v div 1 2 , , , (1.3)

in a bounded domain Ω, is a central and delicate topic in the field of the quasilinear elliptic partial differential equations and has been largely discussed in the literature, also due to the applications of (1.3) in numerous geometrical and physical issues. Although breakthrough results were proven between the late sixties and the early seventies, starting with[2–8], this study has been carried out further in the subsequent decades by many authors; for a wide, but far from exhaustive, overview we refer to the mono -graphs[9–12] and references therein.

However, in spite of such a large amount of work, a general sub- and super-solution method – a quite powerful technique for the study of semilinear and quasilinear elliptic problems [13–17] – has not been established yet for the classical solutions of boundary value problems associated with Eq.(1.3). Actually, some partial results have been formulated in[18, Theorems 2.1 and 2.2] and [19, Theorem 1.1], but unfortu-nately the given proofs do not seem complete, as all of them rely on the unproven Lemma 3.2 in[20]. This substantial lack of results within the existing literature might be attributed to the fact that mean curvature problems are fraught with a number of difficulties which do not arise in dealing with other quasilinear problems, such as the possible occurrence of gradient blow up phenomena and the corresponding devel -opment of singularities for the solutions, up to the formation of discontinuities. In order to overcome these drawbacks and to include in the analysis such kind of singular solutions, it appeared natural, starting with [21–26], to settle the problem in the space of bounded variation functions. In particular, this approach allowed to develop in [27–29] a sub- and super-solution method for the bounded variation solutions of various boundary value problems associated with Eq.(1.3), according to the notion of solution introduced in [30]. The likely most relevant feature of these results is that, assuming the sub- and super-solutions satisfy the standard ordering condition, no bound and no monotonicity are imposed on the function f, contrary to what was required in all preceding studies.

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sub- and the super-solutions are regular and satisfy the boundary conditions. A complementary result for the same problem was obtained by a different approach – monotone iteration versus variational techni-ques– in [19, Theorem 3.1], assuming that f is gradient independent, continuous, but non-decreasing with respect to the state variable, and again the sub- and the super-solutions are regular and fulfill the Dirichlet boundary conditions.

Since these results require that f satisfies some monotonicity property, it seems interesting to investi-gate the situation where such assumptions fail. Nevertheless, simple one-dimensional examples, given, e.g., in[37,38], show that the sole existence of a couple of sub- and super-solutions, although yielding the existence of a bounded variation solution, cannot guarantee the existence of a classical one. Therefore, as some supplementary assumption should be added, in this paper we adopt an alternative approach that allows us to remove any monotonicity requirement on f at the expense of imposing a local growth condition, which is independent of the size of the interval [0,R]and, therefore, completely different from those considered in the literature, and in turn is almost necessary for guaranteeing the regularity of the solutions of the associated Cauchy problems. Namely, we introduce the following assumption:

(h3) there exist an interval I⊆ and a continuous functiong:I→ such that, for allt∈ [0,R],sI andξ∈, ( )⋅ ( ) ≥ − ( ) f t s ξ, , sgnξ g s (1.4) and

g s( )ds<1. I (1.5) It is apparent that the condition(1.4) in ( )h3 impliesg s( ) ≥0 for allsI .

Remark 1.1. One sample case where ( )h3 holds is the following. Suppose that the function f admits the decomposition

( ) = ( ) + ( )⋅

f t s ξ, , f t s1 , f ξ2 ξ,

withf ξ2( ) ≥0for allξ∈. Then, f satisfies ( )h3 provided thatf1does. This means that no growth restriction

with respect to the gradient variable is required here.

It is worth observing that the condition ( )h3 guarantees that, for everyu0∈I, any non-extendible solution u of the Cauchy problem:

           − ′ + ′ = ( ′) ( ) = ′( ) = − ′ − t u u t f t u u u u u 1 , , , 0 , 0 0, N N 1 2 1 0

withrange uI, exists on the whole of[0,R]. This is a direct consequence of the inequality        

− + ′( ) = ′( ) + ′( ) − ′( ) ″( ) + ′( ) = − − ′( ) + ′( ) − ( ( ) ′( )) ′( ) ≤ ( ( ))| ′( )| ≤ ( ) ∈ u t u τ u τ u τ u τ u τ τ N τ u τ u τ τ f τ u τ u τ u τ τ g u τ u τ τ g s s t u 1 1 1 1 1 d 1 1 d , , d

d d , for all dom ,

t t t t t t t t t t u 2 2 2 2 2 2 range 0 0 0 0 0

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Conversely, in the one-dimensional case, the condition (1.5) in ( )h3 turns out to be necessary for the existence of solutions. Namely, ifg : →is an arbitrary continuous function, any solutionuC2([a b, ]) of the equation       − ′ + ′ = ( ) ′ u u g u 1 2

satisfies the identity

+ ′( ) − + ′( ) = ( ) ∈ [ ] ( ) ( ) u t u t g s s t t a b 1 1 1 1 d , for all , , , u t u t 12 22 1 2 2 1

and hence, settingI=range u, condition(1.5) must hold.

On the other hand, it is clear that assumptions ( )h1, ( )h2 , ( )h3 alone do not imply the solvability of the problem(1.1). A very simple example is given by

           − ′ + ′ = ] [ ′( ) = ( ) + ′( ) = − ′ − t u u Nt u a u a u 1 in 0, 1 , 0 0, 1 1 0. N N 1 2 1 0 1 (1.6) Indeed, integrating the equation on [0, 1 shows that] (1.6) has no classical solutions although the constant function (f t s ξ, , ) =N satisfies ( )h3, for any interval I such that | | <I N

1

and forg=N .

The solvability of(1.1) can actually be achieved assuming the existence of couples of sub- and super-solutions, whose definition is given below taking into account the previously set convention:u′ ( )t = ″( )u 0

t if

=

t 0, for alluC2([0,R])such that ′( ) =u 0 0. We refer to[28,29,35,37,39–41] for the concrete construction of sub- and super-solutions in the frame of the mean curvature equation.

Definitions 1.1. (Notion of sub- and super-solution).

– A functionα: 0,[ R] → is a sub-solution of the problem (1.1) if there existα1,…,αpC2([0,R])such thatα=max{α1,…,αp}and for each ∈ { …i 1, ,p}

              − ′( ) + ′( ) ≤ ( ( ) ′( )) ∈ ] [ ′( ) ≥ ( ) + ′( ) ≤ − ′ − t α t α t t f t α t α t t R α a α R a α R 1 , , for all 0, , 0 0, 0. N i i N i i i i i 1 2 1 0 1

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              − ′( ) + ′( ) < ( ( ) ′( )) ∈ ] ] ′( ) > ( ) + ′( ) = − ′ − t α t α t t f t α t α t t R α a α R a α R 1 , , for all 0, , 0 0, 0, N i i N i i i i i 1 2 1 0 1 or               − ′( ) + ′( ) − − ′( ) + ′( ) < ( ( ) ′( )) ∈ [ ] ′( ) = ( ) + ′( ) = ′ α t α t N t α t α t f t α t α t t R α a α R a α R 1 1 1 , , for all 0, , 0 0, 0. i i i i i i i i i 2 2 0 1

– A functionβ: 0,[ R] →is a super-solution of the problem (1.1) if there existβ ,1…,βqC2([0,R])such thatβ= min{β1,…,βq}and for eachj∈ { …1, ,q}

               − ′( ) + ′( ) ≥ ( ( ) ′( )) ∈ ] [ ′( ) ≤ ( ) + ′( ) ≥ − ′ − t β t β t t f t β t β t t R β a β R a β R 1 , , for all 0, , 0 0, 0. N j j N j j i j j 1 2 1 0 1

– A super-solution β is strict if for each ∈ { … }j 1, ,q either                − ′( ) + ′( ) > ( ( ) ′( )) ∈ ] [ ′( ) < ( ) + ′( ) > − ′ − t β t β t t f t β t β t t R β a β R a β R 1 , , for all 0, , 0 0, 0, N j j N j j j j j 1 2 1 0 1 or                − ′( ) + ′( ) − − ′( ) + ′( ) > ( ( ) ′( )) ∈ [ [ ′( ) = ( ) + ′( ) > ′ β t β t N t β t β t f t β t β t t R β a β R a β R 1 1 1 , , for all 0, , 0 0, 0, j j j j j j j j j 2 2 0 1 or                − ′( ) + ′( ) > ( ( ) ′( )) ∈ ] ] ′( ) < ( ) + ′( ) = − ′ − t β t β t t f t β t β t t R β a β R a β R 1 , , for all 0, , 0 0, 0, N j j N j j j j j 1 2 1 0 1 or                − ′( ) + ′( ) − − ′( ) + ′( ) > ( ( ) ′( )) ∈ [ ] ′( ) = ( ) + ′( ) = ′ β t β t N t β t β t f t β t β t t R β a β R a β R 1 1 1 , , for all 0, , 0 0, 0. j j j j j j j j j 2 2 0 1

Notation. For allu v, ∈C1([0,R]), we writeu v if ( ) ≤ ( )u t v t for allt∈ [0,R];u<v ifuv anduv;

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The following existence and localization result holds in the presence of one couple of sub- and super-solutions satisfying the standard ordering condition.

Theorem 1.1. Assume ( )h1, ( )h2, ( )h3 and

( )h4 there exist a sub-solution α and a super-solution β of the problem (1.1) satisfying

≤ ⊆

α β and range α, range β I.

Then, the problem(1.1) has at least one classical solution u, with

≤ ≤

α u β.

Furthermore, the multiplicity of solutions can be detected in the presence of two couples of sub- and super-solutions, if one of them does not satisfy the standard ordering condition. Indeed, the following version of the celebrated Amann three-solution theorem [13,16,42] holds true for the problem (1.1).

Theorem 1.2. Assume ( )h1, ( )h2, ( )h3 and ( )h4 . Suppose further that

( )h5 there exist a strict sub-solution α1and a strict super-solution β1of the problem(1.1) satisfying

≤ ≤ ≤ ≤ ≰

α α1 β, α β1 β and α1 β1.

Then, the problem(1.1) has at least three classical solutions u u u1, 2, 3, with

< < ≤ ≪ ≪ ≤ ≱ ≰

u1 u3 u2, α u1 β1, α1 u2 β, u3 α and u1 3 β1.

Remark 1.2. Although not explicitly stated here, the conclusions of Theorems 1.1 and 1.2 can be completed with the existence of extremal solutions. This easily follows from the adopted notion of sub- and super-solutions arguing like, e.g., in[43].

Theorems 1.1 and 1.2 are the main results obtained in this work and the entire Section 2 is devoted to their proofs. Finally, in Section 3 we provide some additional results for the Neumann and the periodic problems, where some alternative boundedness conditions are imposed on the function f(see ( )h6, or ( ′)h6 , below) that

unlike ( )h3 involve the length of the interval [0,R], or, respectively, the period T. It is clear that the complementary nature of the assumptions introduced in this paper allows us to deal with manifold situa -tions, which can be treated by suitably combining our statements, but are not covered by other results available in the existing literature, such as those in[29,34,37,40,44–49].

2 Proofs

In this section, we deliver the proofs of Theorems 1.1 and 1.2. Wefirst introduce a modified version of the problem(1.1), inspired from [16], and show that any solution of the modified problem is actually a solution of the original one. Next, we prove the existence of a solution of the modified problem, thus completing the proof of Theorem 1.1. We treat separately the Neumann case and the Dirichlet and the Robin cases since the proof, although basically the same, requires more care in the former situation. We also provide some information on the topological degree which will be crucial in detecting the existence of multiple solutions. The proof of Theorem 1.2 is then given in the final part of this section, relying on the previous degree calculations. For convenience, we henceforth set

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2.1 A modi

fied problem

In this section, we introduce a modified version of the problem (1.1) and prove that any solution of the modified problem is actually a solution of the original one. Assume ( )h1, ( )h2 , ( )h3 and ( )h4. Let us set

= ( ) = ( − ) − K g s s L K K K d and 2 1 . I (2.1) For anyM>0, we define

     ( ) = ( ) ≤ ≤ ( ) + ′( )( − ) > − (− ) < φ ξ φ ξ ξ M φ M φ M ξ M ξ M φ ξ ξ if 0 , if , if 0. M M (2.2) We also write

( ) =ξ φ σ( ) σ ΦM d . ξ M 0 Setting ( ) = ( ) − ( ) − + + ψ ξ φ ξ ξ ξ ξ Φ 1 1 1 M M 2

and observing thatψ is even,ψ 0( ) =0 andψ ξ′( ) ≥0 for allξ≥0, we see that  ( ) − ( ) ≥ − + ∈ φ ξ ξ ξ ξ ξ Φ 1 1 1 for all . M M 2 (2.3)

Take∈ such that

> + {∥ ′∥∞ ∥ ′∥∞ = … = … }

L˜ L max αi , βj :i 1, , ,p j 1, ,q. (2.4)

We introduce the following truncation of the function f:      ( ) = ( − ) < − ( ) | | ≤ ( ) > h t s ξ f t s L ξ L f t s ξ ξ L f t s L ξ L , , , , ˜ if ˜, , , if ˜, , , ˜ if ˜. (2.5) We set, for each =i 1,…, ,p

   ( ) = ( ) < ( ) ≥ ( ) γ t s α t s α t s s α t , if , if i i i i

and, for eachj=1,…, ,q

     ( ) = ( ) > ( ) ≤ ( ) δ t s β t s β t s s β t , if , if . j j j j

Define, for each = …i 1, , , the operatorp  :C([0,R]) →L∞(0,R)

i 1 by        ( )( ) =u t h t γ t u t( ( )) ( ( )) ∈ [ ] tγ t u t t R , , , d d , for a.e. 0, i i i

and, for eachj=1,…, , the operatorq  :C([0,R]) →L∞(0,R)

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Finally, we introduce the operator  : C1([0,R]) →L∞(0,R) by setting, for all uC1([0,R]) and a.e. ∈ [ ] t 0,R,          ( )( ) = ( )( ) + ( ( ) − ( )) ( ) < ( ) ( ( ) ′( )) ( ) ≤ ( ) ≤ ( ) ( )( ) − ( ( ) − ( )) ( ) > ( ) = … = … u t u t α t u t u t α t h t u t u t α t u t β t u t u t β t u t β t max arctan if , , , if , min arctan if . i p i j q j 1, , 1, , (2.6)

Let us consider the modified problem     − ( ( ′) )′ = ( ) ] [ ′( ) = ( ) + ′( ) = − − φ u t t u R u a u R a u R in 0, , 0 0, 0. M N 1 N 1 0 1 (2.7) By a solution of(2.7), we mean a function ∈u W2,∞(0,R)which satisfies the equation a.e. in ]0,R[as well as the boundary conditions.

We prove that, takingM>L˜, any solution u of(2.7) is a solution of (1.1) too. We first verify that ≥u α by showing thatuαifor each ∈ { …i 1, ,p}. Let usfix ∈ { … }i 1, ,p. By the monotonicity ofφM we have that

≤ ( ( ′) − ( ′))( ′ − ′) = − ( ( ′) − ( ′))(( − ) )′ { ( )< ( )} − − − t φ u φ α u α t t φ u φ α u α t 0 d d . t u t α t N M M i i R N M M i i : 1 0 1 i

Integrating by parts and observing that, from Definitions 1.1 and from (2.7), [(tN−(φ u t( ′( )) −φ α t( ′( ))))( ( ) −u t α t( )) ] ≤− 0, M M i i R 1 0 we get

≤ (t −(φ u( ′) −φ α( ′)))′( −u α)− t 0 d . R N M M i i 0 1 (2.8)

Note that, asM>L˜, αisatisfies the inequality

− (φ α t( ′) −)′ ≤tf t α α(, , ′) in 0,] R[.

M i N 1 N 1 i i (2.9)

Hence we obtain, from(2.8), (2.7), (2.9) and from the definitions of  and h,

 

(

(

)

)

≤ (− ( ) + ( ′))( − ) ≤ (− ( ) − ( − ) + ( ′))( − ) = ( ) ( ) + ( − ) − ( ′) ( − ) = ( − )( − ) ≤ ( − )( − ) ≤ − − − − { ( )< ( )} − { ( )< ( )} − { ( )< ( )} − t u f t α α u α t t u α u f t α α u α t t h t γ t u γ t u α u f t α α u α t t α u u α t t α u u α t 0 , , d arctan , , d , , , , arctan , , d arctan d arctan d 0. R N i i i R N i i i i t u t α t N i d t i i i i t u t α t N i t u t α t N i i 0 1 0 1 : 1 d : 1 : 1 i i i

Thus, we infer that ( −u α) =− 0

i , that is,uαi. Since this is valid for each ∈ { …i 1, ,p}, we conclude that

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To prove that u is a solution of(1.1) it remains to show that∥ ′∥ ≤uL˜. Assumefirst thatmax[0,R]u′ = ′( ) >u t1 0 for somet1∈ ]0,R]and taket0=max{ ∈ [t 0,t1]:u t′( ) = }0. We have ′( ) >u t 0 for allt∈ ]t0,t1]and hence, using also(2.3),

− + ′( ) ≤ ( ′( )) ′( ) − ( ′( )) = [ ( ′( )) ′( )] − ( ′( )) ″( ) = ( − ) ′( ) ( ′( )) − ( ( ) ′( )) ′( ) ≤ ( ( )) ′( ) = ( ) ≤ ( ) ( ) u t φ u t u t u t φ u t u t φ u t u t t N u t t φ u t t h t u t u t u t t g u t u t t g s s K 1 1 1 Φ d 1 d , , d d d . M M M tt t t M t t M t t t t u t u t 12 1 1 1 0 1 0 1 0 1 0 1 0 1 0 1

Recalling (2.1) we find that ′( ) ≤ <u t1 L . Similarly, we show that if min[0,R]u′ = ′( ) <u t1 0 for some

∈ ] ]

t1 0,R, then we have | ′( )| ≤u t1 L< .

2.2 Existence of a solution

In this section, we prove the existence of a solution of the modified problem (2.7) for any givenM>0, treating separately the Neumann case and the Dirichlet and Robin cases.

2.2.1 The Neumann problem

Assumea0=0in(2.7), i.e., consider the Neumann problem:     −( ( ′) )′ = ( ) ] [ ′( ) = ′( ) = − − φ u t t u R u u R in 0, , 0 0, 0, M N 1 N 1 (2.11) with defined by (2.6). Existence of solutions Set = { ∈ ([ ]) ′( ) = ′( ) = } X u C1 0,R :u 0 0,u R 0 and = { ∈ ( ) = } X˜ w X:w R 0 .

We endow X with the norm

∥ ∥ = | ( )| + ∥ ′∥∞uX u R u . (2.12)

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  :L∞(0,R) → by v=NRN

tv t( )d ,t R N 0 1 ˜ :XX˜ by              ( )( ) =

( ( )( ) − ( )) ∈ [ ] − u t φ σ s u σ u σ s t R ˜ d d for all 0, , t R M s N 1 0 1 and:XXby ( +r w) =r+( +r w) +˜( +r w).

It is easy to verify thatu=r+wX is afixed point of if and only if u is a solution of (2.11). Compute, for a.e.t∈ [0,R],              ( )( ) = − −

−( ( )( ) − ( )) d t u t φ σ t u σ u σ d ˜ d M t N 1 0 1 and                            

( )( ) = −( )′ ( ( )( ) − ( )) ⋅ ( − ) ( ( )( ) − ( )) + ( ( )( ) − ( )) − − − − t u t φ σ t u σ u σ N t σ u σ u σ u t u d d ˜ d 1 d . M t N N t N 2 2 1 0 1 0 1

Since the operator  :C1([0,R]) →L∞(0,R) is continuous and has a bounded range, the operator 

 :L∞(0,R) → is linear and continuous, and the functionφ: 

M

1 is of classC1and has a bounded

derivative, it follows that˜ :XX˜ is continuous and there exists a constantC>0 such that 

∥ ( )∥˜ u W2,∞<C for all uX.

For later use, it is convenient to assume further thatC>L˜, where L˜ is the constant introduced in(2.4). By the Ascoli-Arzelà theorem,˜ has a relatively compact range in X˜, i.e.,range ˜ is compact in X˜ . Conse -quently, the operator:XXis completely continuous, i.e., is continuous and maps bounded subsets of X onto relatively compact subsets of X.

Choose a constant

> ∥ ∥ + ∥ ∥ +

= … ∞ = … ∞

D max α max β RC

i 1, ,p i j 1, ,q j

and define in X the open bounded set

 = { = +u r wX:| | <r D and ∥ ∥ < }wX C . (2.13) We prove that, for allλ∈ [0, 1 , any] u∈¯ such that

 = ( ) u λ u , or equivalently   = ( ) ( − ) = ( ) w λ˜ u and 1 λ r λ u,

satisfiesu∈. Since, for allλ∈ [0, 1 , we have that]

 

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it is enough to show that | | <r D. Assume, by contradiction, that there isλ∈ ]0, 1 such that, e.g.,] r≤ −D. We have, for all ∈ [t 0,R],

( ) = + ( ) ≤ − + ∥ ∥∞ < − ∥ ∥ − ∥ ∥ − + ≤ ( )

= … ∞ = … ∞ = … [ ]

u t r w t D w max α max β RC RC min minα

i 1, ,p i j 1, ,q j i 1, ,p 0,R i and hence ( ) =

−( )( ) =

− ( ( )( ) + ( ( ) − ( ))) >

( )( ) = … − = …

u t u t dt t max u t arctanα t u t dt t max u t d .t

R N R N R N i p i R N i p i 0 1 0 1 1, , 0 1 1, , N

Thus we get, for each =i 1,…, ,p       

( ) > ( ( )) ( ( )) = ( ( ) ′( )) ≥ − ( ( ′( )))′ = − ( ′( ) ≥ − − − − u t h t γ t u t tγ t u t t t f t α t α t t t φ α t t R φ α R , , , d d , d , , d d 0, R N R N i i R N i i R N M i N M i 0 1 0 1 0 1 1 N

asαisatisfies (2.9) and ′( ) ≤α Ri 0, thus yielding the contradiction

 ≥ ( − ) =λ r λ ( ) >u

0 1 0.

Similarly, we can verify thatr<D. Consequently, the homotopy invariance of the degree implies that  

( − ) =

deg ,O 1.

In particular, has a fixed point ∈u X which in turn is a solution of(2.11).

Degree calculations

Assume now that the sub-solution α and the super-solution β are strict. We first show that if u is a solution of(1.1) with ≤ ≤α u β, then u satisfies ≪ ≪α u β. We already know thatαuβ and, in particular, we haveφ uM( ′) =φ u( ′)as M>L˜> ∥ ′∥u ∞. We shall limit ourselves to prove that u satisfies ≫u αi for each

∈ { … }

i 1, ,p, as showing thatuβjfor eachj∈ { …1, ,q}is totally similar. Fix ∈ { …i 1, ,p}and setv=uαi.

Let us suppose, by contradiction, that . Hence, there existst0∈ [0,R]such that ( ) =v t0 minv=0. By definition of sub-solution, we immediately realize that ′( ) =v t0 0 and thus α ti( ) = ( )0 u t0 and

′( ) = ′( )

α ti 0 u t0. In particular, if t0=0 or t0=R, the definition of solution yields ′( ) =α ti 0 0. Recalling

now the definition of strict sub-solution we observe that, ift0=R, only the third or the fourth alter -native in Definition 1.1 may occur. Hence, if <0 t0≤R, we get

− ′ ( ′( )) ″( ) = − ′ ( ′( )) ″( ) − − ( ′( )) + ′ ( ′( )) ″( ) + − ( ′( )) > ( ( ) ′( )) − ( ( ) ′( )) = φ u t v t φ u t u t N t φ u t φ α t α t N t φ α t f t u t u t f t α t α t 1 1 , , , , 0. M M M M i i M i i i 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

On the other hand, ift0=0, only the second or the fourth alternative in Definition 1.1 may occur and we find −Nv″( ) = −0 M′ ( ) ″( ) > (0 v 0 f 0,u( )0 , 0) − (f 0,αi( )0 , 0) =0.

In all cases we infer ″( ) <v t0 0, but this is impossible at a minimum point which is simultaneously a critical point. Next we define in X the open bounded set

= { ∈u X:αuβ,∥ ′∥u ∞ < }C . U

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≪ ≪ ∥ ′∥∞ <

α u β, u C,

and hence\ ¯U is fixed point free. The excision property of the degree thus yields

    

( − ) = ( − ) =

deg ,U deg , 1.

2.2.2 The Dirichlet and the Robin problems

Assume nowa0>0in(2.7), i.e., consider either the Dirichlet problem     −( ( ′) )′ = ( ) ] [ ′( ) = ( ) = − − φ u t t u R u u R in 0, , 0 0, 0, M N 1 N 1

in casea1=0, or the Robin problem     −( ( ′) )′ = ( ) ] [ ′( ) = ( ) + ′( ) = − − φ u t t u R u a u R a u R in 0, , 0 0, 0, M N 1 N 1 0 1 in casea1>0. Existence of solutions

Much like in the Neumann case, we set

= { ∈ ([ ]) ′( ) = }

X u C1 0,R :u 0 0

and we endow X with the norm defined by (2.12). We also introduce the operator :XXby setting

                          ( )( ) = −

( )( ) +

( )( ) ∈ [ ] − − − u t a a φ σ R u σ σ φ σ s u σ σ s t R d d d for all 0, . M R N t R M s N 1 0 1 0 1 1 0 1 (77) The fixed points ∈u X of  are precisely the solutions of (2.7). It is easy to see that  is completely continuous and there exists a constantC> L˜ such that



∥ ( )∥u W2,∞ <C, for all uX. Define in X the open ball

 = { ∈u X:∥ ∥ < }u X C .

Since ( ) ⊆¯ , the Schauderfixed point theorem implies the existence of a fixed pointu∈, which in turn is a solution of(2.7). Moreover, a simple homotopy argument yields

 

( −I ) =

deg , 1.

Degree calculations

Wefirst show that if u is a solution of (1.1) with ≤ ≤α u β, then u satisfies ≪ ≪α u β, keeping the notation of Section 2.2.1. From the definition of strict sub-solution, we infer that ( ) >v t 0 for all ∈ [t 0,R[. Assume that ( ) =v R 0. Since ( ) =v R min , we get either ′( ) =v v R 0 or ′( ) <v R 0. In the former situation, a contra -diction follows arguing as we did in the Neumann case. In the latter situation, we automatically have that

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2.3 Multiplicity of solutions

2.3.1 Degree calculations

In this section, we consider the Neumann case and the Dirichlet and Robin cases simultaneously. In what follows, the symbols X, C, refer to the symbols introduced in Section 2.2.1, in the case of the Neumann problem, or in Section 2.2.2, in the case of the Dirichlet and the Robin problems. Let us consider the modified problem (2.7). Since all solutions u of (2.7) satisfy ≤ ≤α u β, it is clear thatα−1≪uβ+1. Let us introduce the following open bounded subsets of X

= { ∈ − ≪ ≪ + ∥ ′∥ < } = { ∈ − ≪ ≪ ∥ ′∥ < } = { ∈ ≪ ≪ + ∥ ′∥ < } ∞ ∞ ∞ u X α u β u C u X α u β u C u X α u β u C : 1 1 and , : 1 and , : 1 and . 0 1 1 2 1 U U U

Note thatU1⊂U ,0 U2⊂ U , and, as0 α1≰β1,U1∩U2= ∅.LetuX be afixed point of . Then it satisfies

− ≪ ≪ +

α 1 u β 1. Suppose further thatuβ1. Sinceβ1is a strict super-solution of the problem (1.1), we actually haveuβ1, as shown in Sections 2.2.1 and 2.2.2 when computing the degree. Similarly, ifuα1, sinceα1is a strict sub-solution of the problem (1.1), we actually have ≫u α1. Therefore, we conclude that

 

∉ ( − )(∂ ∪ ∂ ∪ ∂ )

0 U0 U1 U2 . (2.14)

Define now the open bounded subset of X

= \ ∪ .

3 0 1 2

U U U U

By(2.14), using the excision property of the degree, we get

   

( − ) = ( − (∂ ∪ ∂ ))

deg ,U0 deg ,U0\ U1 U2

and, hence, the additivity property of the degree implies that

       

( − ) = ( − ) + ( − ) + ( − )

deg ,U0 deg ,U1 deg ,U2 deg ,U3 .

Since we have that

     

( − ) = ( − ) = ( − ) =

deg ,U0 deg ,U1 deg ,U2 1,

wefinally get

 

( − ) = −

deg ,U3 1.

2.3.2 Existence of solutions

SinceU ,1 U ,2 U are mutually disjoint, the previous degree calculations imply that there are three distinct3 fixed points u1,u2,u3of the operator , with

∈ ∈ ∈

u1 U1, u2 U2, u3 U3.

Since anyfixed point u of satisfies ≤ ≤α u β, u u u1, 2, 3are all solutions of(1.1). Moreover, we have that

≤ ≪ ≪ ≤ ≱ ≰

α u1 β1, α1 u2 β, u3 α1, u3 β1.

Sincemin{u β3, 1}is a super-solution of (1.1), there exists a solution u˜1of(1.1) satisfying

≤ ≤ { } <

α u˜1 minu β3, 1 u3.

Similarly, asmax{u α3, 1}is a sub-solution of (1.1), there exists a solution u˜2of(1.1) satisfying

< { } ≤ ≤

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Then, replacing u1with1andu2with2, we can conclude that(1.1) has three solutions such that < <

u1 u3 u .2

3 Complementary results

In this section, we provide some additional results, complementing Theorems 1.1 and 1.2, where alternative bounds are imposed on the function f, involving, unlike assumption ( )h3, the length of the interval[0,R]. We shall mainly focus on the Neumann problem as, in the case of Dirichlet or Robin boundary conditions, a suitable boundedness assumption on f is enough to guarantee alone the existence of solutions. In the last part, we briefly discuss also the periodic problem, which is closely related to the one-dimensional Neumann problem.

3.1 The Neumann problem

In this section, we consider the Neumann problem:            − ′ + ′ = ( ′) ] [ ′( ) = ′( ) = − ′ − t u u t f t u u R u u R 1 , , in 0, , 0 0, 0. N N 1 2 1 (3.1) It has already been observed in[46, Lemma 2.5] that a global bound on f yields an estimate on the derivative of any solution of(3.1) and eventually, in [46, Theorem 2.6], the solvability of (3.1) if an asymptotic sign condition holds. The next result provides instead the existence of a solution of(3.1) assuming a local bound on f with a sharper constant, together with the existence of a couple of sub- and super-solutions satisfying the standard ordering condition. In the light of the non-existence results in [50] the bound on the function f is optimal in dimensionN= 1.

Proposition 3.1. Assume ( )h1, ( )h2

( )h6 there exist an intervalI⊆ and a constant K, with

<K< N R

0 2,

N

such that, for all ∈ [t 0,R],sI andξ∈,

| (f t s ξ, , )| ≤K (3.2)

and

( )h7 there exist a sub-solution α and a super-solution β of the problem (3.1) satisfying

≤ ⊆

α β andrange , rangeα β I. Then, the problem(3.1) has at least one classical solution u, with

≤ ≤

α u β.

Proof. Suppose u is a solution of the problem (3.1) withrange uI. If ∈ ]t 0, R ] 2

N , integrating the equation

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whereas, if ∈ ]t R ,R[ 2

N , integrating between t and R, we get

| ( ′( ))| ≤ − −| ( ( ) ′( ))| ≤ − ≤ < − φ u t t s f s u s u s s KR t Nt K R N , , d 2 1. N t R N N N N 1 1 1 N Set

(

)

= − L φ K R . N 1 2 N (3.3)

Then any solution u of the problem(3.1), withrange uI, satisfies the estimate

∥ ′∥u ∞ ≤L. (3.4)

Next we proceed as in the proof of Theorem 1.1, by introducing, for anyM>0, the functionφMas in(2.2), the constantL˜ as in(2.4), the truncated function h as in (2.5), the operator as in (2.6) and the modified problems(2.10) and (2.7) witha0=0. As in that proof we show that a solution u of the modified problem (2.7) exists and is a solution of the problem (2.10) too. To conclude we observe that the function h satisfies assumption ( )h6 and therefore, arguing as in thefirst part of this proof, the estimate (3.4) holds. Hence, u is a

solution of(3.1), provided thatM>L˜.

In the following result, we consider the case where α andβ do not satisfy the standard ordering condition. In this situation, we need to ask the bound in(3.2) to be global, in the sense thatI=.

Proposition 3.2. Assume ( )h1, ( )h2 , ( )h6 withI=, and

( )h8 there exist a sub-solution α and a super-solution β of the problem (3.1) satisfying

α β.

Then, the problem(3.1) has at least one classical solution u, with

( ) ≤ ( ) ( ) ≥ ( ) ∈ [ ]

u t1 α t1 and u t2 β t2 for some t t1, 2 0,R. Proof. We define L as in (3.3) and

= {∥ ∥ ∥ ∥ = … = … } + ( + )

D max αi C1, βj C1 : i 1, , ,p j 1, ,q L1 R.

As noted in the proof of Proposition 3.1, condition ( )h6 implies that any solution u of the problem (3.1)

satisfies (3.4). Therefore, all solutions u of (3.1) such that ( ) ≤ ( )u t1 α t1 and ( ) ≥ ( )u t2 β t2 , for somet t1, 2∈ [0,R], satisfy

∥ ∥uC1 ≤ ∥ ∥α∞ + ∥ ∥β∞ + ( + )∥ ′∥1 R u ∞ ≤D. (3.5)

ChooseL˜>Dand define a continuous functionη : → [2 0, 1]satisfying

      ( ) = ( ) ∈ [− ] ( ) ∈ [− ] η s ξ s ξ D D s ξ L L , 1 if , , , 0 if , \ ˜, ˜ . 2 2 2

Choose any ε, with0< ε< K

π

2

, and define the functionh: 0,[ R] ×2→ by

( ) = ( ) ( ) − +( − ) + −( + )

h t s ξ, , η s ξ f t s ξ, , , εarctan s L˜ εarctan s L˜ . Consider the Neumann problem

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Since for all ∈ [t 0,R]

( ( ) ′( )) = ( ( ) ′( )) ( ( ) ′( )) = ( ( ) ′( ))

h t α t, ,α t f t α t, ,α t and h t β t, ,β t f t β t, ,β t ,

α is a sub-solution and β is a super-solution of the problem (3.6). Furthermore, as for all ∈ [t 0,R]:

( − − ) = ( + ) = −

h t, L˜ 1, 0 επ h t L επ

4 and , ˜ 1, 0 4,

the constant functionα0= − − 1is a strict sub-solution and the constant functionβ0=+1is a strict super-solution of the problem (3.6). We have that

≤ ≤ ≤ ≤ ≰

α0 α β0, α0 β β0 and α β.

Let us distinguish three possible cases.

Wefirst suppose that there exists a solution u of (3.6) such that ≥u α but . In this case, there existst1∈ [0,R]with ( ) = ( )u t1 α t1. Asαβ there exists alsot2∈ [0,R]with ( ) ≥ ( )u t2 β t2.

Next, we suppose that there exists a solution u of(3.6) such that ≤u β but . In this case, there existst2∈ [0,R]with ( ) = ( )u t2 β t2. Asαβ there exists alsot1∈ [0,R]with ( ) ≤ ( )u t1 α t1.

Finally, we suppose that, for any solution u of(3.6), if ≥u α, thenuα, and ifuβ, thenuβ. In this case, degree calculations similar to those performed in Sections 2.2 and 2.3 yield the existence of a solution u of(3.6) such that ≱u α anduβ, that is, ( ) < ( )u t1 α t1 and ( ) > ( )u t2 β t2 for somet t1, 2∈ [0,R].

To conclude we observe that the function h satisfies assumption ( )h6 withI= and, therefore, arguing

as in thefirst part of this proof, the estimate (3.5) holds and, hence, u is a solution of (3.1) as well. □

3.2 The Dirichlet and the Robin problems

In this section, we consider the Dirichlet and the Robin problems:            − ′ + ′ = ( ′) ] [ ′( ) = ( ) + ′( ) = − ′ − t u u t f t u u R u a u R a u R 1 , , in 0, , 0 0, 0, N N 1 2 1 0 1 (3.7) witha0>0. In this case, if f satisfies a bound similar to (3.2), which yields an estimate on the derivative, the solvability of(3.7) is always guaranteed, without requiring the existence of sub- and super-solutions. Proposition 3.3. Assume ( )h1, witha0>0, ( )h2 and

( )h9 there exists a constant K, with

<K< N R 0

such that, for all ∈ [t 0,R],s∈ andξ∈,

| (f t s ξ, , )| ≤K. Then, the problem(3.7) has at least one classical solution u. Proof. Let us consider, depending on ∈ [λ 0, 1 , the problem]

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witha0>0. Suppose u is a solution of(3.8) for some ∈ [λ 0, 1 . As in the proof of Proposition 3.1, integrating] the equation in(3.8) between 0 and any ∈ ]t 0,R], wefind

| ( ′( ))| ≤φ u t KR <

N 1.

Using the boundary condition we then infer the existence of a constantC>0 such that ∥ ∥uC1 <C,

for any such u and λ. A simple degree argument then yields the existence of a solution of the problem(3.7). □

3.3 The periodic problem

We discuss in this section the existence of classical periodic solutions of the equation:       − ′ + ′ = ( ′) ′ u u f t u u 1 , , . 2 (3.9)

For the sake of simplicity and to point out the symmetry with the previous sections, we shall denote the assumptions considered in this part with the symbols ( ′)hi , =i 1, 2, 3…. It is assumed throughout this section that

( ′)h1 T>0 is a given period and

( ′)h2 f :  × × → is continuous and T-periodic with respect to the first variable.

Definition 3.1. (Notion of solution) A classical T-periodic solution of (3.9) is a functionuC2( ) , which is T-periodic and satisfies the equation for allt∈.

We note that, ifuC1( ) is any T-periodic function, then there exists ∈ [t 0,T[

0 such that ′( ) =u t0 0.

Therefore, if u is any T-periodic solution of (3.9), then, for some t0, u satisfies the Neumann boundary

conditions ′( ) = ′(u t0 u t0+T) =0. Consequently, all gradient estimates established in the previous sections for the one-dimensional Neumann problem extend to the periodic case. Taking this fact into account, we omit the proofs of the following propositions, as they parallel those of Theorems 1.1, 1.2 and Propositions 3.1, 3.2. Before stating our results we need to introduce the notion of sub-solution and super-solution for the periodic problem.

Definitions 3.1. (Notion of sub- and super-solution).

– A functionα: 0,[ T] →is a sub-solution of the T-periodic problem for (3.9) if there exist …α1, ,αpC2([0,T]) such thatα=max{α1,…,αp}and for each ∈ { …i 1, ,p}

              − ′( ) + ′( ) ≤ ( ( ) ′( )) ∈ ] [ ( ) = ( ) ′( ) ≥ ′( ) ′ α t α t f t α t α t t T α α T α α T 1 , , for all 0, , 0 , 0 . i i i i i i i i 2

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or         − ′( ) + ′( ) < ( ( ) ′( )) ∈ [ ] ′ α t α t f t α t α t t T 1 , , for all 0, . i i i i 2

– A functionβ: 0,[ T] →is a super-solution of the T-periodic problem for (3.9) if there existβ1,…,βq

([ ])

C2 0,T such that β=min{β,,β }

q

1 and for eachj∈ { …1, ,q}                − ′( ) + ′( ) ≥ ( ( ) ′( )) ∈ ] [ ( ) = ( ) ′( ) ≤ ′( ) ′ β t β t f t β t β t t T β β T β β T 1 , , for all 0, , 0 , 0 . j j j j i i i i 2

– A super-solution β is strict if for each ∈ { … }j 1, ,q either                − ′( ) + ′( ) > ( ( ) ′( )) ∈ ] [ ′( ) < ′( ) ′ β t β t f t β t β t t T β β T 1 , , for all 0, , 0 j j j j i i 2 or         − ′( ) + ′( ) > ( ( ) ′( )) ∈ [ ] ′ β t β t f t β t β t t T 1 , , for all 0, . j j j j 2

Notation. For allu v, ∈C1([0,T])satisfying ( ) = ( )u0 u T and ( ) = ( )v0 v T, we writeuv if ( ) ≤ ( )u t v t for all

∈ [ ]

t 0,T,u<v ifuv anduv,uv if ( ) < ( )u t v t for all ∈ [t 0,T]. Proposition 3.4. Assume ( ′)h1 , ( ′)h2 and

( ′)h3 there exist an intervalI⊆and a continuous functiong:I→such that, for allt∈,sI andξ∈,

( )⋅ ( ) ≥ − ( ) ( ) <

f t s ξ, , sgnξ g s and g s ds 1

I

and

( ′)h4 there exist a sub-solution α and a super-solution β of the T-periodic problem for (3.9) satisfying

≤ ⊆

α β and range , rangeα β I.

Then, Eq.(3.9) has at least one classical T-periodic solution u, with

≤ ≤

α u β.

Proposition 3.5. Assume ( ′)h1 , ( ′)h2 , ( ′)h3, ( ′)h4 and

( ′)h5 there exist a strict sub-solution α1and a strict super-solution β1of the T- periodic problem for (3.9) satisfying

≤ ≤ ≰

α α1, β1 β and α1 β1. Then, Eq.(3.9) has at least three classical T-periodic solutions u u u1, 2, 3, with

< < ≤ ≪ ≪ ≤ ≱ ≰

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Proposition 3.6. Assume ( ′)h1 , ( ′)h2 ,

( ′)h6 there exist an intervalI⊆ and a constant K, with

<K< T

0 2,

such that, for all ∈ [t 0,R],sI andξ∈,

| (f t s ξ, , )| ≤K and

( ′)h7 there exist a sub-solution α and a super-solution β of the -T periodic problem for(3.9) satisfying

≤ ⊆

α β and range , rangeα β I.

Then, Eq.(3.9) has at least one classical T-periodic solution u, with

≤ ≤

α u β.

Proposition 3.7. Assume ( ′)h1, ( ′)h2 , ( ′)h6, withI=, and

( ′)h8 there exist a sub-solution α and a super-solution β of the -T periodic problem for(3.9) satisfying ≰

α β.

Then, Eq.(3.9) has at least one classical T-periodic solution u, with

( ) ≤ ( ) ( ) ≥ ( ) ∈ [ ]

u t1 α t1 and u t2 β t2 for some t t1, 2 0,R.

Propositions 3.4–3.7, also in combination with Remark 1.1, complete and extend several results pre-viously obtained in[29,37,40,44,45,48,49].

Acknowledgments: This research has been performed under the auspices of“Gruppo Nazionale per l’Analisi Matematica, la Probabilità e le loro Applicazioni– Istituto Nazionale di Alta Matematica.” The authors have been supported by “Università degli Studi di Trieste – Finanziamento di Ateneo per Progetti di Ricerca Scientifica – FRA 2018, FRA 2020.”

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