Startseite Noncoercive resonant (p,2)-equations with concave terms
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Noncoercive resonant (p,2)-equations with concave terms

  • Nikolaos S. Papageorgiou und Chao Zhang EMAIL logo
Veröffentlicht/Copyright: 11. Oktober 2018

Abstract

We consider a nonlinear Dirichlet problem driven by the sum of a p-Laplace and a Laplacian (a ( p , 2 ) -equation). The reaction exhibits the competing effects of a parametric concave term plus a Caratheodory perturbation which is resonant with respect to the principle eigenvalue of the Dirichlet p-Laplacian. Using variational methods together with truncation and comparison techniques and Morse theory (critical groups), we show that for all small values of the parameter, the problem has as least six nontrivial smooth solutions all with sign information (two positive, two negative and two nodal (sign changing)).

MSC 2010: 35J20; 35J60; 58E05

1 Introduction

Let Ω N be a bounded domain with a C 2 -boundary Ω . In this paper, we study the following parametric ( p , 2 ) -equation:

(1.1) { - Δ p u ( z ) - Δ u ( z ) = λ | u ( z ) | q - 2 u ( z ) + f ( z , u ( z ) ) in  Ω , u | Ω = 0 , λ > 0 ,  1 < q < 2 < p < .

In this problem, for r ( 1 , + ) , by Δ r we denote the r-Laplace differential operator defined by

Δ r u = div ( | D u | r - 2 D u ) for all  u W 0 1 , r ( Ω ) .

When r = 2 , we have the usual Laplacian Δ 2 = Δ defined by

Δ u = div ( D u ) for all  u H 0 1 ( Ω ) .

So, in problem (1.1) the differential operator is the sum of a p-Laplacian and a Laplacian (a ( p , 2 ) -equation). Such an operator is nonhomogeneous and this is a source of difficulties in the analysis of problem (1.1). In the reaction (right-hand side of (1.1)), we have the competing effects of two nonlinearities. One is a concave term u λ | u | q - 2 u (recall 1 < q < 2 < p ) and the other is a Caratheodory perturbation f ( z , x ) (that is, for all x , z f ( z , x ) is measurable and for a.a. z Ω , x f ( z , x ) is continuous). We assume that asymptotically as x ± , f ( z , ) is resonant with respect to the principle eigenvalue λ ^ 1 ( p ) > 0 of ( - Δ p , W 0 1 , p ( Ω ) ) . The resonance occurs from the right of the principal eigenvalue λ ^ 1 ( p ) in the sense that

λ ^ 1 ( p ) | x | p - p F ( z , x ) - uniformly for a.a.  z Ω  as  x ± ,

where F ( z , x ) = 0 x f ( z , x ) 𝑑 s . This makes the energy functional of problem (1.1) indefinite. Our goal is to prove a multiplicity theorem for problem (1.1), providing sign information for all the solutions produced. We show that for all λ > 0 small, problem (1.1) has at least six nontrivial smooth solutions, two positive, two negative and two nodal (that is, sign changing).

Our tools come from critical point theory, together with suitable truncation and comparison techniques and Morse theory (critical groups).

Boundary value problems driven by a combination of differential operators of different nature (such as ( p , 2 ) -equations) arise in the problems of mathematical physics. We mention the works of Benci, Fortunato and Pisani [3] concerning models of particle physics (existence of soliton-type solutions), Cherfils and Ilyason [5] on reaction-diffusion systems, and Wilhelmsson [29] on plasma physics. A survey of recent works on ( p , q ) -equations ( 1 < q < p < ) can be found in the paper of Marano and Mosconi [16]. The particular case of ( p , 2 ) -equations presents special interest and stronger results can be proved for such equations. In this direction we mention the works of Aizicovici, Papageorgiou and Staicu [2], Cingolani and Degiovanni [6], Papageorgiou and Radulescu [21, 22], Papageorgiou, Radulescu and Repovs [23, 24], Sun [27], and Sun, Zhang and Su [28]. From the aforementioned works, only [22] deals with equations which are resonant with respect to λ ^ 1 ( p ) from the right (noncoercive problems), where the existence of two nontrivial solutions with no sign information is proved . Resonant ( p , 2 ) -equations with parametric concave terms were considered by Papageorgiou and Winkert [25]. However, in their equation, the concave term enters with a negative sign and this leads to a coercive energy functional.

2 Mathematical background and hypotheses

Let X be a Banach space and X * its topological dual. By , we denote the duality brackets for the pair ( X * , X ) . Given φ C 1 ( X , ) , we say that φ satisfies the “Cerami condition” (the “C-condition” for short), if the following property holds: Every sequence { u n } n 1 X such that { φ ( u n ) } n 1 is bounded and

( 1 + u n X ) φ ( u n ) 0 in  X *  as  n

admits a strongly convergent subsequence.

This is a compactness-type condition on the functional φ, more general than the usual Palais–Smale condition. It leads to a deformation theorem from which one can deduce the minimax theory of the critical values of φ. One of the main results in that theory is the so-called “mountain pass theorem”, which we recall here.

Theorem 2.1.

If X is a Banach space, φ C 1 ( X , R ) satisfies the C-condition, u 0 , u 1 X , u 1 - u 0 X > ρ > 0 ,

max { φ ( u 0 ) , φ ( u 1 ) } < inf [ φ ( u ) : u - u 0 X = ρ ] = η ρ ,

and

c = inf γ Γ max 0 t 1 φ ( γ ( t ) ) , with  Γ = { γ C ( [ 0 , 1 ] , X ) : γ ( 0 ) = u 0 , γ ( 1 ) = u 1 } ,

then c η ρ and c is a critical value of φ.

In the analysis of problem (1.1), we will use the Sobolev spaces W 0 1 , p ( Ω ) and H 0 1 ( Ω ) . Since 2 < p , we have that W 0 1 , p ( Ω ) H 0 1 ( Ω ) densely. By we denote the norm of W 0 1 , p ( Ω ) . By the Poincaré inequality, we have

u = D u p for all  u W 0 1 , p ( Ω ) .

Also we will use the Banach space C 0 1 ( Ω ¯ ) = { u C 1 ( Ω ¯ ) : u | Ω = 0 } . This is an ordered Banach space with positive (order) cone

C + = { u C 0 1 ( Ω ¯ ) : u ( z ) 0  for all  z Ω ¯ } .

This cone has a nonempty interior given by

int C + = { u C + : u ( z ) > 0  for all  z Ω , u n | Ω < 0 } .

Here u n = ( D u , n ) N , with n ( ) being the outward unit normal on Ω .

For every r ( 1 , + ) , let A r : W 0 1 , r ( Ω ) W - 1 , r ( Ω ) = W 0 1 , r ( Ω ) * ( 1 r + 1 r = 1 ) be the map defined by

A r ( u ) , h = Ω | D u | r - 2 ( D u , D h ) N 𝑑 z for all  u , h W 0 1 , r ( Ω ) .

For this map, we have the following result (see [18, p. 40]).

Proposition 2.2.

The map A r ( ) is bounded (that is, maps bounded sets to bounded sets), continuous, strictly monotone (hence maximal monotone, too) and of type ( S ) + , that is, if u n 𝑤 u in W 0 1 , r ( Ω ) and

lim sup n A r ( u n ) , u n - u 0 ,

then u n u in W 0 1 , r ( Ω ) . If r = 2 , then A 2 = A L ( H 0 1 ( Ω ) , H - 1 ( Ω ) ) .

Suppose that f 0 : Ω × is a Caratheodory function such that

| f 0 ( z , x ) | a 0 ( z ) [ 1 + | x | r - 1 ] for a.a.  z Ω  and all  x ,

with a 0 L ( Ω ) + and

1 < r p * = { N p N - p if  p < N , + if  N p

(the critical Sobolev exponent for p). We set F 0 ( z , x ) = 0 x f 0 ( z , s ) 𝑑 s and we consider the C 1 -functional φ 0 : W 0 1 , p ( Ω ) defined by

φ 0 ( u ) = 1 p D u p p + 1 2 D u 2 2 - Ω F 0 ( z , u ) 𝑑 z for all  u W 0 1 , p ( Ω ) .

The next result can be found in [18, p. 409].

Proposition 2.3.

If u 0 W 0 1 , p ( Ω ) is a local C 0 1 ( Ω ¯ ) -minimizer of φ 0 , that is, there exists ρ 0 > 0 such that

φ 0 ( u 0 ) φ 0 ( u 0 + h ) for all  h C 0 1 ( Ω ¯ ) , with  h C 0 1 ( Ω ¯ ) ρ 0 ,

then u 0 C 0 1 , α ( Ω ¯ ) for some α ( 0 , 1 ) and u 0 is also a local W 0 1 , p ( Ω ) -minimizer of φ 0 , that is, there exists ρ 1 > 0 such that

φ 0 ( u 0 ) φ 0 ( u 0 + h ) for all  h W 0 1 , p ( Ω ) , with  h ρ 1 .

To make an effective use of Proposition 2.3, we need a strong comparison principle, which is provided by the next proposition and is a particular case of [9, Proposition 3].

Given h 1 , h 2 L ( Ω ) we write h 1 h 2 if for every K Ω compact, we can find ε = ε ( K ) > 0 such that h 1 ( z ) + ε h 2 ( z ) for a.a. z K . Evidently, if h 1 , h 2 C ( Ω ) , and h 1 ( z ) < h 2 ( z ) for all z Ω , then h 1 h 2 .

Proposition 2.4.

If ξ , h 1 , h 2 L ( Ω ) , ξ ( z ) 0 for a.a. z Ω , h 1 h 2 , u C 0 1 ( Ω ¯ ) { 0 } , v int C + and

- Δ p u ( z ) - Δ u ( z ) + ξ ( z ) | u ( z ) | p - 2 u ( z ) = h 1 ( z ) for a.a.  z Ω ,
- Δ p v ( z ) - Δ v ( z ) + ξ ( z ) v ( z ) p - 1 = h 2 ( z ) for a.a.  z Ω ,

then v - u int C + .

Next we recall some basic facts about the spectrum of the Dirichlet p-Laplacian. So, we consider the following nonlinear eigenvalue problem:

(2.1) - Δ p u ( z ) = λ ^ | u ( z ) | p - 2 u ( z ) in  Ω , u | Ω = 0 .

We say that λ ^ is an eigenvalue of ( - Δ p , W 0 1 , p ( Ω ) ) if problem (2.1) admits a nontrivial solution u ^ W 0 1 , p ( Ω ) , known as an eigenfunction corresponding to the eigenvalue λ ^ . By σ ^ ( p ) we denote the spectrum of ( - Δ p , W 0 1 , p ( Ω ) ) (that is, the set of eigenvalues). We know that there exists a smallest eigenvalue λ ^ 1 ( p ) , which has the following properties:

  1. λ ^ 1 ( p ) > 0 and it is isolated in the spectrum σ ^ ( p ) (that is, there exists ε > 0 such that ( λ ^ 1 ( p ) , λ ^ 1 ( p ) + ε ) σ ^ ( p ) = ) ,

  2. λ ^ 1 ( p ) is simple (that is, if u ^ 1 , u ^ 2 are eigenfunctions corresponding to λ ^ 1 ( p ) , then u ^ 1 = ξ u ^ 2 with ξ 0 ),

  3. we have

    (2.2) λ ^ 1 ( p ) = inf [ D u p p u p p : u W 0 1 , p ( Ω ) , u 0 ] .

The infimum in (2.2) is realized on the corresponding one-dimensional eigenspace. Evidently, the elements of this eigenspace have constant sign. By u ^ 1 ( p ) we denote the positive L p -normalized (that is, u ^ 1 ( p ) p = 1 ) eigenfunction corresponding to the eigenvalue λ ^ 1 ( p ) > 0 . The nonlinear regularity theory and the nonlinear maximum principle (see [10, pp. 737–738]) imply that u ^ 1 int C + . It is easy to see that σ ^ ( p ) ( 0 , + ) is closed and using the Ljusternik–Schnirelmann minimax scheme, we produce a whole sequence of distinct eigenvalues { λ ^ k ( p ) } k of ( - Δ p , W 0 1 , p ( Ω ) ) such that λ ^ k ( p ) + as k + . These eigenvalues are known as “variational eigenvalues” and we do not know if they exhaust σ ^ ( p ) . This is the case if N = 1 (scalar eigenvalue problem) or if p = 2 (linear eigenvalue problem). All eigenvalues λ ^ λ ^ 1 ( p ) have nodal eigenfunctions.

In what follows, for notational simplicity, we write

λ ^ 1 = λ ^ 1 ( p ) and u ^ 1 = u ^ 1 ( p ) int C + .

We will also encounter a weighted version of the eigenvalue problem (2.1). Namely, let η L ( Ω ) , with η ( z ) 0 for a.a. z Ω , η 0 . We consider the following nonlinear eigenvalue problem:

- Δ p u ( z ) = λ ~ η ( z ) | u ( z ) | p - 2 u ( z ) in  Ω , u | Ω = 0 .

The same results are true for this problem. So, there exists a smallest eigenvalue λ ~ 1 ( p , η ) > 0 , which is isolated, simple and admits the following variational characterization:

λ ~ 1 ( p , η ) = inf [ D u p p Ω η ( z ) | u | p 𝑑 z : u W 0 1 , p ( Ω ) , u 0 ] .

These properties lead to the following monotonicity property for the map η λ ~ 1 ( p , η ) (see [18, p. 250]).

Proposition 2.5.

If η , η ^ L ( Ω ) , 0 η ( z ) η ^ ( z ) for a.a. z Ω , η 0 , η ^ η , then λ ~ 1 ( p , η ^ ) < λ ~ 1 ( p , η ) .

Next let us recall some basic definitions and facts from the theory of critical groups (Morse theory). So, let X be a Banach space, φ C 1 ( X , ) and c . We introduce the following sets:

K φ = { u X : φ ( u ) = 0 } ( the critical set of  φ ) ,
K φ c = { u K φ : φ ( u ) = c } ( the critical set of  φ  at the level  c ) ,
φ c = { u X : φ ( u ) c } ( the sublevel set of  φ  at  c ) .

Let ( Y 1 , Y 2 ) be a topological pair such that Y 2 Y 1 X . For any k 0 , by H k ( Y 1 , Y 2 ) we denote the kth-relative singular homology group with integer coefficients for the pair ( Y 1 , Y 2 ) . Recall that H k ( Y 1 , Y 2 ) = 0 for all k - . If u K φ c is isolated, then the critical groups of φ at u are defined by

C k ( φ , u ) = H k ( φ c U , φ c U { u } ) for all  k 0 ,

with U being a neighborhood of u such that K φ φ c U = { u } . The excision property of singular homology implies that this definition of critical groups above is independent of the choice of the neighborhood U.

If u K φ is isolated and of mountain pass-type (see Theorem 2.1), then C 1 ( φ , u ) 0 . Moreover, if φ C 2 ( X , ) , then from [21] we know that

C k ( φ , u ) = δ k , 1 for all  k 0 ,

with δ k , m being the Kronecker symbol defined by

δ k , m = { 1 if  k = m , 0 if  k m , k , m 0 .

Next we fix our notation. Given x , we set x ± = max { ± x , 0 } . Then for u W 0 1 , p ( Ω ) , we define

u ± ( ) = u ( ) ± .

We know that u ± W 0 1 , p ( Ω ) , u = u + - u - , | u | = u + + u - . Also, given a measurable function g : Ω × (for example, a Caratheodory function), we define

N g ( u ) ( ) = g ( , u ( ) ) for all  u W 0 1 , p ( Ω ) ,

which is the Nemytskii operator corresponding to g. By | | N we denote the Lebesgue measure on N , and if 1 < r < + , then 1 r + 1 r = 1 .

If u , u ^ W 1 , p ( Ω ) and u u ^ , then we define

[ u , u ^ ] = { y W 0 1 , p ( Ω ) : u ( z ) y ( z ) u ^ ( z )  for a.a.  z Ω } .

By int C 0 1 ( Ω ¯ ) [ u , u ^ ] , we denote the interior of [ u , u ^ ] C 0 1 ( Ω ¯ ) in the C 0 1 ( Ω ¯ ) -norm. Also, if k : ( 0 , + ) C 0 1 ( Ω ¯ ) , then we say that k ( ) is strictly increasing if λ < λ implies that k ( λ ) - k ( λ ) int C + . Finally, if u W 1 , p ( Ω ) , then

[ u ) = { y W 0 1 , p ( Ω ) : u ( z ) y ( z )  for a.a.  z Ω } .

Now we can introduce the hypotheses on the perturbation term f ( z , x ) .

Hypothesis 2.6.

f : Ω × is a Caratheodory function such that f ( z , 0 ) = 0 for a.a. z Ω , which satisfies the following properties:

  1. For every ρ > 0 , there exists a ρ L ( Ω ) such that

    | f ( z , x ) | a ρ ( z ) for a.a.  z Ω  and all  | x | ρ .

  2. We have

    λ ^ 1 lim inf x ± f ( z , x ) | x | p - 2 x lim sup x ± f ( z , x ) | x | p - 2 x η ^ uniformly for a.a.  z Ω .

  3. If F ( z , x ) = 0 x f ( z , s ) 𝑑 s , then

    lim x ± f ( z , x ) x - p F ( z , x ) x 2 = - uniformly for a.a.  z Ω .

  4. There exist functions w ± W 1 , p ( Ω ) L ( Ω ) , with Δ p w ± , Δ w ± L p ( Ω ) , and θ ± > 0 such that, for a.a. z Ω ,

    w - ( z ) - c - < 0 < c + w + ( z ) ,
    - Δ p w - ( z ) - Δ w - ( z ) 0 - Δ p w + ( z ) - Δ w + ( z ) ,
    θ + w + ( z ) q - 1 + f ( z , w + ( z ) ) - c ^ + < 0 c ^ - θ - | w - ( z ) | q - 2 w - ( z ) + f ( z , w - ( z ) ) .

  5. If c 0 = min { c + , c - } > 0 , then we can find δ 0 ( 0 , c 0 ) such that for all K Ω compact and all 0 < m < δ 0 , we have

    0 < c K m f ( z , x ) x q F ( z , x ) for a.a.  z K  and all  m | x | δ 0 .

  6. For every ρ > 0 , there exists ξ ^ ρ > 0 such that for a.a. z Ω , the function

    x f ( z , x ) + ξ ^ ρ | x | p - 2 x

    is nondecreasing on [ - ρ , ρ ] .

Remark 2.7.

Hypotheses 2.6 (i), (ii) imply that

| f ( z , x ) | c 1 [ 1 + | x | p - 1 ] for a.a.  z Ω  and all  x ,  with  c 1 > 0 .

Hypothesis 2.6 (ii) says that at ± we can have resonance with respect to the principal eigenvalue λ ^ 1 > 0 . In the process of the proof we shall see that Hypothesis 2.6 (iii) implies that the resonance occurs from the right of λ ^ 1 > 0 in the sense that

λ ^ 1 | x | p - p F ( z , x ) - uniformly for a.a.  z Ω  as  x ± .

This makes the problem noncoercive and so the direct method of the calculus of variations is not directly applicable on (1.1). Hypothesis 2.6 (iv) is satisfied if we can find t - < 0 < t + such that

f ( z , t + ) - c ^ + < 0 < c ^ - f ( z , t - ) for a.a.  z Ω .

Therefore, Hypotheses 2.6 (iv), (v) dictate an oscillatory behavior for f ( z , ) near zero. Hypothesis 2.6 (vi) is satisfied if, for example, for a.a. z Ω , f ( z , ) is differentiable, and for every ρ > 0 , we can find ξ ^ ρ > 0 such that

f x ( z , x ) x 2 - ξ ^ ρ | x | p for a.a.  z Ω , | x | ρ .

Example 2.8.

The following function satisfies Hypothesis 2.6 (for the sake of simplicity, we drop the z-dependence):

f ( x ) = { | x | τ - 2 x - 2 | x | η - 2 x if  | x | 1 , β | x | p - 2 x + | x | μ - 2 x + ( β + 2 ) | x | γ - 2 x if  | x | > 1 ,

with 1 < τ < η < , β λ ^ 1 and 1 < γ 2 < μ < p .

3 Solutions of constant sign

In this section we produce solutions of constant sign (positive and negative solutions) of problem (1.1).

We start by considering the following auxiliary nonlinear parametric Dirichlet ( p , 2 ) -equation:

(3.1) - Δ p u ( z ) - Δ u ( z ) = λ | u ( z ) | q - 2 u ( z ) in  Ω , λ > 0 , u | Ω = 0 .

Proposition 3.1.

For every λ > 0 , problem (3.1) has a unique positive solution u ~ λ int C + , and since (3.1) is odd, v ~ λ = - u ~ λ - int C + is the unique negative solution of (3.1); moreover, λ u ~ λ is strictly increasing and

u ~ λ C 0 1 ( Ω ¯ ) 0 + as  λ 0 + .

Proof.

The existence of a positive solution for problem (3.1) can be established using the direct method of the calculus of variations. More precisely, let ψ λ : W 0 1 , p ( Ω ) be the C 1 -functional defined by

ψ λ ( u ) = 1 p D u p p + 1 2 D u 2 2 - λ q u + q q for all  u W 0 1 , p ( Ω ) .

Since 1 < q < 2 < p , we see that ψ λ ( ) is coercive. Also, using the Sobolev embedding theorem, we show that ψ λ ( ) is sequentially weakly lower semicontinuous. So, using the Weierstrass–Tonelli theorem, we can find u ~ λ W 0 1 , p ( Ω ) such that

(3.2) ψ λ ( u ~ λ ) = inf [ ψ λ ( u ) : u W 0 1 , p ( Ω ) ] .

Since q < 2 < p , for t ( 0 , 1 ) small, we have ψ λ ( t u ^ 1 ) < 0 , hence ψ λ ( u λ ) < 0 = ψ λ ( 0 ) (see (3.2)), and thus u λ 0 . From (3.2) we have ψ λ ( u ~ λ ) = 0 , that is,

(3.3) A p ( u ~ λ ) , h + A ( u ~ λ ) , h = λ Ω ( u ~ λ + ) q - 1 h 𝑑 z for all  h W 0 1 , p ( Ω ) .

In (3.3) we choose h = - u ~ λ - W 0 1 , p ( Ω ) . Then D u ~ λ - p p + D u ~ λ - 2 2 = 0 , and thus u ~ λ 0 and u ~ λ 0 . From (3.3) it follows that

- Δ p u ~ λ ( z ) - Δ u ~ λ ( z ) = λ u ~ λ ( z ) q - 1 for a.a.  z Ω , u ~ λ | Ω = 0 ,

therefore u ~ λ is a positive solution of (3.1).

From [18, Corollary 8.6, p. 208], we have u ~ λ L ( Ω ) . Then, invoking [15, Theorem 1], we infer that u ~ λ C + { 0 } . The nonlinear strong maximum principle of Pucci and Serrin [26, pp. 111, 120] implies that u ~ λ int C + .

Next we show that this positive solution is unique. To this end, we introduce the integral functional j : L 1 ( Ω ) ¯ = { + } defined by

j ( u ) = { 1 p D u 1 / 2 p p + 1 2 D u 1 / 2 2 2 if  u 0 , u 1 / 2 W 0 1 , p ( Ω ) , + otherwise .

From [7, Lemma 1], we know that j ( ) is convex.

Suppose that y ~ λ W 0 1 , p ( Ω ) is another positive solution of (3.1). Again we have y ~ λ int C + . Then for any h C 0 1 ( Ω ¯ ) and for | t | < 1 small, we have

u ~ λ + t h dom j and y ~ λ + t h dom j ,

where dom j = { u L 1 ( Ω ) : j ( u ) < + } (the effective domain of j ( ) ). We can easily see that j ( ) is Gateaux differentiable at u ~ λ 2 and at y ~ λ 2 in the direction of h. Moreover, the chain rule and Green’s identity (see [10, p. 211]) imply that

j λ ( u ~ λ 2 ) ( h ) = 1 2 Ω - Δ p u ~ λ - Δ u ~ λ u ~ λ h 𝑑 z , j λ ( y ~ λ 2 ) ( h ) = 1 2 Ω - Δ p y ~ λ - Δ y ~ λ y ~ λ h 𝑑 z for all  h C 0 1 ( Ω ¯ ) .

The convexity of j ( ) implies the monotonicity of j ( ) . Therefore,

0 Ω ( - Δ p u ~ λ - Δ u ~ λ u ~ λ - Δ p y ~ λ - Δ y ~ λ y ~ λ ) ( u ~ λ - y ~ λ ) 𝑑 z = Ω λ [ 1 u ~ λ 2 - q - 1 y ~ λ 2 - q ] ( u ~ λ - y ~ λ ) 𝑑 z ,

and hence u ~ λ = y ~ λ (since q < 2 ). This proves the uniqueness of the positive solution of problem (3.1).

Let 0 < η < λ and consider the Caratheodory function

(3.4) k η ( z , x ) = { η ( x + ) q - 1 if  x u ~ λ ( z ) , η u ~ λ ( z ) q - 1 if  u ~ λ ( z ) < x .

We set K η ( z , x ) = 0 x k η ( z , s ) 𝑑 s and consider the C 1 -functional ψ ^ η : W 0 1 , p ( Ω ) defined by

ψ ^ η ( u ) = 1 p D u p p + 1 2 D u 2 2 - Ω K η ( z , u ) 𝑑 z for all  u W 0 1 , p ( Ω ) .

Evidently ψ ^ η ( ) is coercive (see (3.4)) and sequentially weakly lower semicontinuous. So, we can find u ¯ η W 0 1 , p ( Ω ) such that

(3.5) ψ ^ η ( u ¯ η ) = inf [ ψ ^ η ( u ) : u W 0 1 , p ( Ω ) ] .

Since 1 < q < 2 < p , we see that ψ ^ η ( u ¯ η ) < 0 = ψ ^ η ( 0 ) , therefore u ¯ η 0 .

From (3.5) we have ψ ^ η ( u ¯ η ) = 0 , that is,

(3.6) A p ( u ¯ λ ) , h + A ( u ¯ λ ) , h = Ω k λ ( z , u ¯ η ) , d z for all  h W 0 1 , p ( Ω ) .

In (3.6), first we choose h = - u ¯ η - W 0 1 , p ( Ω ) . Then D u ¯ η - p p + D u ¯ η - 2 2 = 0 (see (3.4)), hence u ¯ η 0 , u ¯ η 0 . Also, in (3.6), we choose h = ( u ¯ η - u ~ λ ) + W 0 1 , p ( Ω ) . Then (from (3.4) and since η < λ )

A p ( u ¯ η ) , ( u ¯ η - u ~ λ ) + + A ( u ¯ η ) , ( u ¯ η - u ~ λ ) + = Ω η u ~ λ q - 1 ( u ¯ η - u ~ λ ) + 𝑑 z
Ω λ u ~ λ q - 1 ( u ¯ η - u ~ λ ) + 𝑑 z
= A p ( u ~ λ ) , ( u ¯ η - u ~ λ ) + + A ( u ~ λ ) , ( u ¯ η - u ~ λ ) + ,

hence u ¯ η u ~ λ (see Proposition 2.2).

So, we have proved that

(3.7) u ¯ η [ 0 , u ~ λ ] , u ¯ η 0 .

From (3.4), (3.6) and (3.7), we infer that u ¯ η = u ~ η int C + , thus

(3.8) u ~ η u ~ λ .

We have (see (3.8) and recall that η < λ , u ~ η int C + )

(3.9) - Δ p u ~ η - Δ u ~ η = η u ~ η q - 1 = λ u ~ η q - 1 - ( λ - η ) u ~ η q - 1 < λ u ~ λ q - 1 = - Δ p u ~ λ - Δ u ~ λ for a.a.  z Ω .

Let h 1 = η u ~ η q - 1 and h 2 = λ u ~ λ q - 1 . Evidently h 1 , h 2 C 0 1 ( Ω ¯ ) . For all K Ω compact, let c K = min K u ~ η > 0 (recall u ~ η int C + ). Then (see (3.8))

h 2 ( z ) - h 1 ( z ) = λ u ~ λ ( z ) q - 1 - η u ~ η ( z ) q - 1 ( λ - η ) u ~ η ( z ) q - 1 ( λ - η ) c K > 0 for a.a.  z K .

From (3.9) and Proposition 2.4 it follows that u ~ λ - u ~ η int C + , therefore λ u ~ λ is strictly increasing from ( 0 , + ) into C 0 1 ( Ω ¯ ) .

Finally, let λ > 0 and let u ~ λ int C + be the unique solution of (3.1). Then we have

A p ( u ~ λ ) , h + A ( u ~ λ ) , h = λ Ω u ~ λ q - 1 h 𝑑 z for all  h W 0 1 , p ( Ω ) .

Choosing h = u ~ λ W 0 1 , p ( Ω ) , we obtain D u ~ λ p p λ u ~ λ q q , hence u ~ λ p - q λ c 2 for some c 2 > 0 (recall that q < p ). Therefore, given μ > 0 , we see that

(3.10) { u ~ λ } λ > 0 W 0 1 , p ( Ω )  is bounded , u ~ λ 0 as  λ 0 + .

Invoking [18, Corollary 8.6, p. 208], we can find c 3 > 0 such that

u ~ λ c 3 for all  0 < λ μ .

Then [15, Theorem 1] implies that there exist α ( 0 , 1 ) and c 4 > 0 such that

(3.11) u ~ λ C 0 1 , α ( Ω ¯ ) , u ~ λ C 0 1 , α ( Ω ¯ ) c 4 for all  0 < λ μ .

From (3.10), (3.11) and the compact embedding of C 0 1 , α ( Ω ¯ ) into C 0 1 ( Ω ¯ ) , we conclude that

u ~ λ 0 in  C 0 1 ( Ω ¯ )  as  λ 0 + .

Since problem (3.1) is odd, v ~ λ = - u ~ λ - int C + is the unique negative solution of (3.1) for all λ > 0 . Also, λ v ~ λ is strictly decreasing from ( 0 , + ) into C 0 1 ( Ω ¯ ) and v ~ λ C 0 1 ( Ω ¯ ) 0 as λ 0 + . ∎

Let δ 0 > 0 be as postulated by Hypothesis 2.6 (v). On account of Proposition 2.5, we can find λ ± > 0 such that

u ~ λ ( z ) [ 0 , δ 0 ] for all  z Ω ¯ ,  0 < λ λ + ,
(3.12) v ~ λ ( z ) [ - δ 0 , 0 ] for all  z Ω ¯ ,  0 < λ λ - .

With θ ± > 0 as in Hypothesis 2.6 (iv), we set

λ + * = min { λ + , θ + } and λ - * = min { λ - , θ - } .

Proposition 3.2.

If Hypothesis 2.6 holds, then

  1. for all 0 < λ λ + * , problem ( 1.1 ) admits a positive solution u 0 int C 0 1 ( Ω ¯ ) [ u ~ λ , w + ] ,

  2. for all 0 < λ λ - * , problem ( 1.1 ) admits a negative solution v 0 int C 0 1 ( Ω ¯ ) [ w - , v ~ λ ] .

Proof.

(a) Recall that δ 0 < c 0 (see Hypothesis 2.6 (v)). This fact and (3.12) permit the definition of the Caratheodory function

(3.13) e λ + ( z , x ) = { λ u ~ λ ( z ) q - 1 + f ( z , u ~ λ ( z ) ) if  x < u ~ λ ( z ) , λ x q - 1 + f ( z , x ) if  u ~ λ ( z ) x w + ( z ) , λ w + ( z ) q - 1 + f ( z , w + ( z ) ) if  w + ( z ) < x .

We set E λ + ( z , x ) = 0 x e λ + ( z , x ) 𝑑 s and consider the C 1 -functional φ ^ λ + : W 0 1 , p ( Ω ) defined by

φ ^ λ + ( u ) = 1 p D u p p + 1 2 D u 2 2 - Ω E λ + ( z , u ) 𝑑 z for all  u W 0 1 , p ( Ω ) .

From (3.13) it is clear that φ ^ λ + ( ) is coercive. Also, it is sequentially weakly lower semicontinuous. So, we can find u 0 W 0 1 , p ( Ω ) such that φ ^ λ + ( u 0 ) = inf [ φ ^ λ + ( u ) : u W 0 1 , p ( Ω ) ] , hence ( φ ^ λ + ) ( u 0 ) = 0 , and therefore

(3.14) A p ( u 0 ) , h + A ( u 0 ) , h = Ω e λ + ( z , u 0 ) h 𝑑 z for all  h W 0 1 , p ( Ω ) .

In (3.14), first we choose h = ( u ~ λ - u 0 ) + W 0 1 , p ( Ω ) . Then (see (3.12), (3.13), Hypothesis 2.6 (v) and Proposition 3.1)

A p ( u 0 ) , ( u ~ λ - u 0 ) + + A ( u 0 ) , ( u ~ λ - u 0 ) + = Ω [ λ u ~ λ q - 1 + f ( z , u ~ λ ) ] ( u ~ λ - u 0 ) + 𝑑 z
Ω λ u ~ λ q - 1 ( u ~ λ - u 0 ) + 𝑑 z
= A p ( u ~ λ ) , ( u ~ λ - u 0 ) + + A ( u ~ λ ) , ( u ~ λ - u 0 ) + ,

hence u ~ λ u 0 (see Proposition 2.2).

Next in (3.14) we choose h = ( u 0 - w + ) + W 0 1 , p ( Ω ) . Then (see (3.13), Hypothesis 2.6 (iv) and recall that 0 < λ λ + * θ + )

A p ( u 0 ) , ( u 0 - w + ) + + A ( u 0 ) , ( u 0 - w + ) + = Ω [ λ w + q - 1 + f ( z , w + ) ] ( u 0 - w + ) + 𝑑 z
Ω [ θ + w + q - 1 + f ( z , w + ) ] ( u 0 - w + ) + 𝑑 x
A p ( w + ) , ( u 0 - w + ) + + A ( w + ) , ( u 0 - w + ) + ,

thus u 0 w + (see Proposition 2.2).

We have proved that

(3.15) u 0 [ u ~ λ , w + ] .

From (3.13), (3.14), (3.15), it follows that

(3.16) - Δ p u 0 ( z ) - Δ u 0 ( z ) = λ u 0 ( z ) q - 1 + f ( z , u 0 ( z ) ) for a.a.  z Ω , u 0 | Ω = 0 .

From (3.15), (3.16) and [15, Theorem 1], we infer that

(3.17) u 0 [ u ~ λ , w + ] int C + .

Now let ρ = u 0 and let ξ ^ ρ > 0 be as postulated by Hypothesis 2.6 (vi). Then we have (see (3.12), (3.16), (3.17), Hypotheses 2.6 (v)–(vi) and Proposition 3.1)

- Δ p u 0 ( z ) - Δ u 0 ( z ) + ξ ^ ρ u 0 ( z ) p - 1 = λ u 0 ( z ) q - 1 + f ( z , u 0 ( z ) ) + ξ ^ ρ u 0 ( z ) p - 1
λ u ~ λ ( z ) q - 1 + f ( z , u ~ λ ( z ) ) + ξ ^ ρ u ~ λ ( z ) p - 1
λ u ~ λ ( z ) q - 1 + ξ ^ ρ u ~ λ ( z ) p - 1
(3.18) = - Δ p u ~ λ ( z ) - Δ u ~ λ ( z ) + ξ ^ ρ u ~ λ ( z ) p - 1  for a.a.  z Ω .

Set

h 1 ( z ) = λ u ~ λ ( z ) q - 1 + ξ ^ ρ u ~ λ ( z ) p - 1 , h 2 ( z ) = λ u 0 ( z ) q - 1 + f ( z , u 0 ( z ) ) + ξ ^ ρ u 0 ( z ) p - 1 .

Evidently h 1 , h 2 L ( Ω ) . Also, for K Ω compact, we have (recall that u 0 int C + )

0 < m K u 0 ( z ) for all  z K .

Therefore (see (3.17) and Hypothesis 2.6 (v)),

h 2 ( z ) - h 1 ( z ) f ( z , u 0 ( z ) ) c K > 0 for a.a.  z Ω .

Then (3.18) and Proposition 2.4 imply that u 0 - u ~ λ int C + .

Also, we have (see (3.16), (3.17), Hypotheses 2.6 (iv), (vi), and recall that 0 < λ θ + )

- Δ p u 0 ( z ) - Δ u 0 ( z ) + ξ ^ ρ u 0 ( z ) p - 1 = λ u 0 ( z ) q - 1 + f ( z , u 0 ( z ) ) + ξ ^ ρ u 0 ( z ) p - 1
θ + w + ( z ) q - 1 + f ( z , w + ( z ) ) + ξ ^ ρ w + ( z ) p - 1
- c ^ 1 + ξ ^ ρ w + ( z ) p - 1
(3.19) - Δ p w + ( z ) - Δ w + ( z ) + ξ ^ ρ w + ( z ) p - 1 for a.a.  z Ω .

From (3.19) and Proposition 2.4, we have w + - u 0 int C + . We conclude that u 0 int C 0 1 ( Ω ¯ ) [ u ~ λ , w + ] .

(b) In the negative semiaxis, we consider the Caratheodory function

(3.20) e λ - ( z , x ) = { λ | w - ( z ) | q - 2 w - ( z ) + f ( z , w - ( z ) ) if  x < w - ( z ) , λ | x | q - 2 x + f ( z , x ) if  w - ( z ) x v ~ λ ( z ) , λ | v ~ λ | q - 2 v ~ λ ( z ) + f ( z , v ~ λ ( z ) ) if  v ~ λ ( z ) < x .

We set E λ - ( z , x ) = 0 x e λ - ( z , s ) 𝑑 s and consider the C 1 -functional φ ^ λ - : W 0 1 , p ( Ω ) defined by

φ ^ λ - ( u ) = 1 p D u p p + 1 2 D u 2 2 - Ω E λ - ( z , u ) 𝑑 z for all  u W 0 1 , p ( Ω ) .

Arguing as in part (a), using this time the functional φ ^ λ - and (3.20), we produce a negative solution v 0 for problem (1.1), with λ ( 0 , λ - * ] , such that v 0 int C 0 1 ( Ω ¯ ) [ w - , v ~ λ ] .

Next, using u 0 int C + , v 0 - int C + together with suitable variational, truncation and comparison arguments, we will generate a second pair of constant sign smooth solutions.

Proposition 3.3.

If Hypothesis 2.6 holds, then

  1. for all 0 < λ λ + * , problem ( 1.1 ) has a second positive solution u ^ int C + , with u ^ u 0 , u ^ - u ~ λ int C + ;

  2. for all 0 < λ λ - * , problem ( 1.1 ) has a second negative solution v ^ - int C + , with v ^ v 0 , v ~ λ - v ^ int C + .

Proof.

(a) Let u ~ λ int C + be the unique positive solution of (3.1) (see Proposition 3.1). We introduce the following truncation of the reaction in problem (1.1):

(3.21) r λ + ( z , x ) = { λ u ~ λ ( z ) q - 1 + f ( z , u ~ λ ( z ) ) if  x u ~ λ ( z ) , λ x q - 1 + f ( z , x ) if  u ~ λ ( z ) < x .

This is a Caratheodory function. Set R λ + ( z , x ) = 0 x r λ + ( z , s ) 𝑑 s and consider the C 1 -functional φ λ + : W 0 1 , p ( Ω ) defined by

φ λ + ( u ) = 1 p D u p p + 1 2 D u 2 2 - Ω R λ + ( z , u ) 𝑑 z for all  u W 0 1 , p ( Ω ) .

From (3.13) and (3.21), it is clear that

(3.22) φ λ + | [ u ~ λ , w + ] = φ ^ λ + | [ u ~ λ , w + ] .

Let u 0 int C + be the positive solution of problem (1.1) produced in Proposition 3.2. From the proof of that proposition, we know that u 0 int C + is a minimizer of φ ^ λ + , and u 0 int C 0 1 ( Ω ¯ ) [ u ~ λ , w + ] . Hence, from (3.22), it follows that u 0 int C + is a local C 0 1 ( Ω ¯ ) -minimizer of φ λ + , and thus (see Proposition 2.3)

(3.23) u 0 int C +  is a local  W 0 1 , p ( Ω ) -minimizer of  φ λ + .

Claim 1: K φ λ + [ u ~ λ ) int C + and u - u ~ λ int C + for all u K φ λ + .

Let u K φ λ + . Then

(3.24) A p ( u ) , h + A ( u ) , h = Ω r λ + ( z , u ) h 𝑑 z for all  h W 0 1 , p ( Ω ) .

In (3.24), we choose h = ( u ~ λ - u ) + W 0 1 , p ( Ω ) . Then (see (3.21), (3.12), Hypothesis 2.6 (v) and Proposition 3.1)

A p ( u ) , ( u ~ λ - u ) + + A ( u ) , ( u ~ λ - u ) + = Ω [ λ u ~ λ q - 1 + f ( z , u ~ λ ) ] ( u ~ λ - u ) + 𝑑 z
Ω λ u ~ λ q - 1 ( u ~ λ - u ) + 𝑑 z
= A p ( u ~ λ ) , ( u ~ λ - u ) + + A ( u ~ λ ) , ( u ~ λ - u ) + ,

thus u ~ λ u (see Proposition 2.2).

The nonlinear regularity theory of Lieberman [15] implies u int C + . In addition, as in the proof of Proposition 3.2, using Proposition 2.4, we show that u - u ~ λ int C + . This proves claim 1.

On account of claim 1, we may assume that

(3.25) K φ λ +  is finite .

Otherwise, we already have an infinity of smooth solutions of problem (1.1) satisfying u - u ~ λ int C + for all the solutions u (see (3.21) and claim 1). Hence, we are done.

From (3.23) and (3.25) it follows that we can find ρ ( 0 , 1 ) small such that

(3.26) φ λ + ( u 0 ) < inf [ φ λ + ( u ) : u - u 0 = ρ ] = m λ +

(see the proof of [1, Proposition 29]).

From Hypothesis 2.6 (iii), we see that given β > 0 , we can find M 1 = M 1 ( β ) > 0 such that

f ( z , x ) x - p F ( z , x ) - β x 2 for a.a.  z Ω , | x | M 1 .

Then we have

d d x [ F ( z , x ) | x | p ] = f ( z , x ) | x | p - p | x | p - 2 x F ( z , x ) | x | 2 p = f ( z , x ) x - p F ( z , x ) | x | p x { - β x p - 1 if  x M 1 , β | x | p - 1 if  x - M 1 ,

hence

(3.27) F ( z , x ) | x | p - F ( z , y ) | y | p β p - 2 [ 1 | x | p - 2 - 1 | y | p - 2 ] for a.a.  z Ω , | x | | y | M 1 .

From Hypothesis 2.6 (ii), we have

(3.28) λ ^ 1 p lim inf x ± F ( z , x ) | x | p lim sup x ± F ( z , x ) | x | p η ^ p uniformly for a.a.  z Ω .

So, if in (3.27) we let x ± and use (3.28), then

λ ^ 1 p - F ( z , y ) | y | p - β p - 2 1 | y | p - 2 ,

and thus

λ ^ 1 | y | p - p F ( z , y ) y 2 - β p - 2 - β for a.a.  z Ω , | y | M 1 .

Since β > 0 is arbitrary, we conclude that

(3.29) λ ^ 1 | x | p - p F ( z , x ) x 2 - uniformly for a.a.  z Ω  as  x ± .

Claim 2: φ λ + ( t u ^ 1 ) - as t + .

For t > 0 , we have

p φ λ + ( t u ^ 1 ) = t p λ ^ 1 + p t 2 2 D u ^ 1 2 2 - Ω R λ + ( z , t u ^ 1 ) 𝑑 z
t p λ ^ 1 + p t 2 2 D u ^ 1 2 2 - p λ t q q D u ^ 1 q q - Ω p F ( z , t u ^ 1 ) 𝑑 z + c 4
= Ω [ λ ^ 1 ( t u ^ 1 ) p - p F ( z , t u ^ 1 ) ] 𝑑 z + p t 2 2 D u ^ 1 2 2 - p q D u ^ 1 q q + c 4

for some c 4 > 0 (see (3.21)), therefore

p φ λ + ( t u ^ 1 ) t 2 = Ω [ λ ^ 1 ( t u ^ 1 ) p - p F ( z , t u ^ 1 ) ] ( t u ^ 1 ) 2 u ^ 1 2 𝑑 z + p 2 D ( t u ^ 1 ) 2 2 - p q 1 λ 2 - q D u ^ 1 q q + c 4 t 2 .

Passing to the limit as t + and using (3.29), Fatou’s lemma and the fact that q < 2 , we obtain

p φ λ + ( t u ^ 1 ) t 2 - as  t + ,

hence

φ λ + ( t u ^ 1 ) - as  t + .

This proves claim 2.

Claim 3: φ λ + satisfies the C-condition.

We consider a sequence { u n } n 1 W 0 1 , p ( Ω ) such that

(3.30) | φ λ + ( u n ) | M 2 for some  M 2 > 0  and all  n

and

(3.31) ( 1 + u n ) ( φ λ + ) ( u n ) 0 in  W - 1 , p ( Ω )  as  n .

From (3.31) we have

(3.32) | A p ( u n ) , h + A ( u n ) , h - Ω r λ + ( z , u n ) h 𝑑 z | ε n h 1 + u n

for all h W 0 1 , p ( Ω ) , with ε n 0 + .

In (3.32), we choose h = - v n - W 0 1 , p ( Ω ) . Using (3.21), we have

D u n - p p + D u n - 2 2 - Ω [ λ u ~ λ q - 1 + f ( z , u ~ λ ) ] ( - u n - ) 𝑑 z ε n for all  n ,

which implies u n - p c 5 u n - for some c 5 > 0 and all n (see Hypothesis 2.6 (i)), hence

(3.33) { u n - } n 1 W 0 1 , p ( Ω )  is bounded.

Next we show that { u n + } W 0 1 , p ( Ω ) is bounded. Arguing by contradiction, we assume that at least for a subsequence, we have u n + + as n . We let y n = u n + u n + , n . Then y n = 1 , y n 0 for all n . We may assume that

y n 𝑤 y in  W 0 1 , p ( Ω ) , y n y in  L p ( Ω ) , y 0 .

Multiplying (3.32) with 1 u n + p - 1 and using (3.33), we obtain

| A p ( y n ) , h + A ( y n ) , h u n + p - 2 - Ω r λ + ( z , u n + ) u n + p - 1 h 𝑑 z | ε n h

for all h W 0 1 , p ( Ω ) , with ε n 0 + . Using (3.21), we have

(3.34) | A p ( y n ) , h + A ( y n ) , h u n + p - 2 - Ω [ λ ( u n + ) q - 1 + f ( z , u n + ) ] u n + p - 1 h 𝑑 z | ε n ′′ h

for all h W 0 1 , p ( Ω ) , with ε n ′′ 0 + . On account of (3.2), we see that

{ λ ( u n + ) q - 1 + f ( z , u n + ) u n + p - 1 } n 1 L p ( Ω )

is bounded. Therefore, we may assume that

(3.35) λ ( u n + ) q - 1 + f ( z , u n + ) u n + p - 1 𝑤 η 0 ( z ) y p - 1 in  L p ( Ω ) ,

with λ ^ 1 η ( z ) η ^ for a.a. z Ω (see Hypothesis 2.6 (ii) and recall that q < 2 < p ).

In (3.34), we choose h = y n - y W 0 1 , p ( Ω ) . Passing to the limit as n , we obtain (recall that 2 < p )

lim n A p ( y n ) , y n - y = 0 ,

therefore (see Proposition 2.2)

(3.36) y n y in  W 0 1 , p ( Ω ) y = 1 , y 0 .

So, if in (3.34) we pass to the limit as n and use (3.35), (3.36), then

A p ( y ) , h = Ω η 0 ( z ) y p - 1 h 𝑑 z  for all  h W 0 1 , p ( Ω ) ,

and hence

(3.37) - Δ p y ( z ) = η 0 ( z ) y ( z ) p - 1 for a.a.  z Ω , y | Ω = 0 .

Suppose that η 0 λ ^ 1 (see (3.35)). We have λ ~ 1 ( p , η 0 ) < λ ~ 1 ( p , λ ^ 1 ) = 1 (see Proposition 2.5), and thus y must be nodal (see (3.37)), which contradicts (3.36).

Next assume that η 0 ( z ) = λ ^ 1 for a.a. z Ω . From (3.37) it follows that y = ξ u ^ 1 int C + ( ξ > 0 ), hence y ( z ) > 0 for all z Ω , and thus u n + ( z ) + for all z Ω . Therefore (see Hypothesis 2.6 (iii)),

f ( z , u n + ( z ) ) u n + ( z ) - p F ( z , u n + ( z ) ) u n + ( z ) 2 - for a.a.  z Ω ,

and thus (by Fatou’s lemma)

(3.38) Ω f ( z , u n + ) u n + - p F ( z , u n + ) u n + 2 𝑑 z - .

From (3.30) and (3.33), we have

(3.39) D u n + p p + p 2 D u n + 2 2 - Ω [ λ p q | u n + | q + p F ( z , u n + ) ] 𝑑 z - M 3

for some M 3 > 0 and all n (see (3.21)). Similarly, if we use (3.33) in (3.32) and also choose h = u n + W 0 1 , p ( Ω ) , then

(3.40) - D u n + p p - D u n + 2 2 + Ω [ λ ( u n + ) q + f ( z , u n + ) u n + ] 𝑑 z - M 4

for some M 4 > 0 and all n (see (3.21)). We add (3.39) and (3.40) and obtain

( p 2 - 1 ) D u n + 2 2 + Ω [ f ( z , u n + ) u n + - p F ( z , u n + ) ] 𝑑 z - M 5 + λ ( p q - 1 ) u n + q q ,

with M 5 = M 3 + M 4 > 0 , for all n , hence

( p 2 - 1 ) D y n 2 2 + Ω f ( z , u n + ) u n + - p F ( z , u n + ) u n + 2 𝑑 z - M 5 u n + 2 + λ ( p q - 1 ) y n q q u n + 2 - q .

Since q < 2 < p , passing to the limit as n , we have a contradiction (see (3.38)). Therefore, { u n + } n 1 W 0 1 , p ( Ω ) is bounded, and hence { u n } n 1 W 0 1 , p ( Ω ) is bounded (see (3.33)).

We may assume that

(3.41) u n 𝑤 u in  W 0 1 , p ( Ω ) , u n u in  L p ( Ω ) .

In (3.32) we choose h = u n - u W 0 1 , p ( Ω ) , pass to the limit as n and use (3.41). Then

lim n [ A p ( u n ) , u n - u + A ( u n ) , u n - u ] = 0 ,

which implies (recall that A ( ) is monotone)

lim sup n [ A p ( u n ) , u n - u + A ( u ) , u n - u ] 0 .

Hence (see (3.41)),

lim sup n A p ( u n ) , u n - u 0 ,

and thus (see Proposition 2.2)

u n u in  W 0 1 , p ( Ω ) .

Therefore, φ λ + satisfies the C-conditions and this proves claim 3.

Then (3.26) and claims 2 and 3, permit the use of Theorem 2.1 (the mountain pass theorem). So, we can find u ^ W 0 1 , p ( Ω ) such that u ^ K φ λ + [ u ~ λ ) int C + (see claim 1) and m λ + φ λ + ( u ^ ) (see (3.26)). From (3.21) and (3.26) we infer that u ^ is a positive solution of (1.1) and u ^ u 0 . As in the proof of Proposition 3.2, using Proposition 2.4, we can show that u ^ - u ~ λ int C + .

(b) Let v ~ λ = - u ~ λ - int C + be the unique negative solution of (3.1) (see Proposition 3.1). We consider the following truncation of the reaction in problem (3.1):

r λ - ( z , x ) = { λ | x | q - 2 x + f ( z , x ) if  x v ~ λ ( z ) , λ | v ~ λ ( z ) | q - 2 v ~ λ ( z ) + f ( z , v ~ λ ( z ) ) if  v ~ λ ( z ) < x .

This is a Caratheodory function. Set R λ - ( z , x ) = 0 x r λ - ( z , s ) 𝑑 s and consider the C 1 -function φ λ - : W 0 1 , p ( Ω ) defined by

φ λ - ( u ) = 1 p D u p p + 1 2 D u 2 2 - Ω R λ - ( z , u ) 𝑑 z for all  u W 0 1 , p ( Ω ) .

Working with φ λ - as in part (a), we see that for λ ( 0 , λ - * ] , we can find v ^ - int C + , a second negative solution of (1.1), such that v ^ v 0 and v ~ λ - v int C + . ∎

We will show that we have extremal constant sign solutions, that is, there is a smallest positive solution for problem (1.1), with λ ( 0 , λ + * ] , and a biggest negative solution for (1.1), with λ ( 0 , λ - * ] . These extremal constant sign solutions will be used in the next section to generate nodal solutions.

We introduce the following solution sets:

  1. S λ + is the set of positive solutions for problem (1.1),

  2. S λ - is the set of negative solutions for problem (1.1).

From Proposition 3.2 we know that

for all  0 < λ λ + * , S λ + int C + ,
for all  0 < λ λ - * , S λ - - int C + .

Proposition 3.4.

If Hypothesis 2.6 holds, then

  1. for all λ ( 0 , λ + * ] , problem ( 1.1 ) has a smallest positive solution u ¯ λ int C + , that is, u ¯ λ u for all u S λ + ,

  2. for all λ ( 0 , λ - * ] , problem ( 1.1 ) has a biggest negative solution v ¯ λ - int C + , that is, v v ¯ λ for all v S λ - .

Proof.

(a) First we show that

(3.42) u ~ λ u for all  u S λ + ( 0 < λ λ + * ) .

To this end, let u S λ + and consider the Caratheodory function

(3.43) β λ ( z , x ) = { λ ( x + ) q - 1 if  x u ( z ) , λ u ( z ) q - 1 if  u ( z ) < x .

We set B λ ( z , x ) = 0 x β λ ( z , s ) 𝑑 s and consider the C 1 -functional ψ ^ λ : W 0 1 , p ( Ω ) defined by

ψ ^ λ ( u ) = 1 p D u p p + 1 2 D u 2 2 - Ω B λ ( z , u ) 𝑑 z for all  u W 0 1 , p ( Ω ) .

From (3.43) it is clear that ψ ^ λ ( ) is coercive. Also, it is sequentially weakly lower semicontinuous. So, we can find u ~ λ * W 0 1 , p ( Ω ) such that

(3.44) ψ ^ λ ( u ~ λ * ) = inf [ ψ ^ λ ( u ) : u W 0 1 , p ( Ω ) ] .

Let y int C + . Since u S λ + int C + , using [17, Proposition 2.1], we can find t ( 0 , 1 ] small such that t y u . Then, from (3.43) and since q < 2 < p , we see that by choosing t ( 0 , 1 ] even smaller if necessary, we have ψ ^ λ ( t y ) < 0 , which implies ψ ^ λ ( u ~ λ * ) < 0 = ψ ^ λ ( 0 ) (see (3.44)), and thus u ~ λ * 0 .

From (3.44) we have ψ ^ λ ( u ~ λ * ) = 0 , hence

(3.45) A p ( u ~ λ * ) , h + A ( u ~ λ * ) , h = Ω β λ ( z , u ~ λ * ) h 𝑑 z for all  h W 0 1 , p ( Ω ) .

In (3.45), we choose h = - ( u ~ λ * ) - W 0 1 , p ( Ω ) . Then D ( u ~ λ * ) - p p + D ( u ~ λ * ) - 2 2 = 0 (see (3.43)), and therefore u ~ λ * 0 and u ~ λ * 0 .

Also, in (3.45), we choose h = ( u ~ λ * - u ) + W 0 1 , p ( Ω ) . Then (see (3.12), (3.43), Hypothesis 2.6 (v) and note that u S λ + )

A p ( u ~ λ * ) , ( u ~ λ * - u ) + + A ( u ~ λ * ) , ( u ~ λ * - u ) + = Ω λ u q - 1 ( u ~ λ * - u ) + 𝑑 z
Ω [ λ u q - 1 + f ( z , u ) ] ( u ~ λ * - u ) + 𝑑 z
= A p ( u ) , ( u ~ λ * - u ) + + A ( u ) , ( u ~ λ * - u ) + ,

hence u ~ λ * u .

We have proved that

(3.46) u ~ λ * [ 0 , u ] , u ~ λ * 0 .

From (3.46), (3.45), (3.43) and Proposition 3.1, we conclude that u ~ λ * = u ~ λ . Therefore, (3.42) is true.

From [8], we know that S λ + is downward directed (that is, if u 1 , u 2 S λ + , then we can find v S λ + such that u u 1 , u u 2 ). Invoking [13, Lemma 3.10, p. 178], we can find a decreasing sequence { u n } n 1 S λ + such that inf S λ + = inf n 1 u n . Evidently, { u n } n 1 W 0 1 , p ( Ω ) is bounded (see Hypothesis 2.6 (i) and recall that 0 u n u 1 for all n ). So, we may assume that

(3.47) u n 𝑤 u ¯ λ in  W 0 1 , p ( Ω ) , u n u ¯ λ in  L p ( Ω ) .

For every n , we have

(3.48) A p ( u n ) , h + A ( u n ) , h = Ω [ λ u n q - 1 + f ( z , u n ) ] h 𝑑 z for all  h W 0 1 , p ( Ω ) .

In (3.48), we choose h = u n - u ¯ λ W 0 1 , p ( Ω ) , pass to the limit as n , use (3.47), and reasoning as in the proof of Proposition 3.3, via Proposition 2.2, we conclude that

(3.49) u n u ¯ λ in  W 0 1 , p ( Ω ) .

So, in (3.48), we pass to the limit as n and use (3.49). Then

A p ( u ¯ λ ) , h + A ( u ¯ λ ) , h = Ω [ λ u ¯ λ q - 1 + f ( z , u ¯ λ ) ] h 𝑑 z for all  h W 0 1 , p ( Ω ) .

Also, from (3.42) and (3.49), we have u ~ λ u ¯ λ . Therefore,

u ¯ λ S λ + , u ¯ λ u for all  u S λ + .

(b) Similarly for the set S λ - , we have v v ~ λ for all v S λ - . Reasoning as in part (a), we produce a v ¯ λ S λ - such that v v ¯ λ for all v S λ - . ∎

Next we examine the maps λ u ¯ λ and λ v ¯ λ .

Proposition 3.5.

If Hypothesis 2.6 holds, then

  1. the map λ u ¯ λ from ( 0 , λ + * ] into C + is strictly increasing (that is, for 0 < θ < λ λ + * , u ¯ λ - u ¯ θ int C + ) and left continuous;

  2. the map λ v ¯ λ from ( 0 , λ - * ] into - C + is strictly decreasing (that is, for 0 < θ < λ λ - * , u ¯ θ - u ¯ λ int C + ) and left continuous.

Proof.

(a) First we show that λ u ¯ λ is increasing. We consider the Caratheodory function

(3.50) m θ ( z , x ) = { θ ( x + ) q - 1 + f ( z , x + ) if  x u ¯ λ ( z ) , θ u ¯ λ ( z ) q - 1 + f ( z , u ¯ λ ( z ) ) if  u ¯ λ ( z ) < x .

We set Γ θ ( z , x ) = 0 x m θ ( z , s ) 𝑑 s and consider the C 1 -functional σ θ : W 0 1 , p ( Ω ) defined by

σ θ ( u ) = 1 p D u p p + 1 2 D u 2 2 - Ω Γ θ ( z , u ) 𝑑 z for all  u W 0 1 , p ( Ω ) .

This is coercive (see (3.50)) and sequentially weakly lower semicontinuous. Hence, we can find y θ W 0 1 , p ( Ω ) such that

(3.51) σ θ ( y θ ) = inf [ σ θ ( u ) : u W 0 1 , p ( Ω ) ] .

Exploiting as before the fact that q < 2 < p , we have σ θ ( y θ ) < 0 = σ θ ( 0 ) . Hence, y θ 0 . From (3.51) we have σ θ ( y θ ) = 0 , therefore

(3.52) A p ( y θ ) , h + A ( y θ ) , h = Ω m θ ( z , y θ ) h 𝑑 z for all  h W 0 1 , p ( Ω ) .

In (3.52), first let h = - y θ - W 0 1 , p ( Ω ) . Then, using (3.50), we have y θ 0 , y θ 0 . Also, in (3.52), we choose h = ( y θ - u ¯ λ ) + W 0 1 , p ( Ω ) . Then (see (3.50) and note that θ < λ )

A p ( y θ ) , ( y θ - u ¯ λ ) + + A ( y θ ) , ( y θ - u ¯ λ ) + = Ω [ θ u ¯ λ q - 1 + f ( z , u ¯ λ ) ] ( y θ - u ¯ λ ) + 𝑑 z
Ω [ λ u ¯ λ q - 1 + f ( z , u ¯ λ ) ] ( y θ - u ¯ λ ) + 𝑑 z
= A p ( u ¯ λ ) , ( y θ - u ¯ λ ) + + A ( u ¯ λ ) , ( y θ - u ¯ λ ) + ,

hence y θ u ¯ λ .

We have proved that

(3.53) y θ [ 0 , u ¯ λ ] , y θ 0 ,

which implies y θ S θ + int C + (see (3.50), (3.52)), and thus u ¯ θ y θ u ¯ λ (see (3.53)). Hence, λ u ¯ λ is increasing. Now let ρ = u ¯ λ and let ξ ^ ρ > 0 be as postulated by Hypothesis 2.6 (vi). Then we have (see Hypothesis 2.6 (vi) and recall that θ < λ )

- Δ p u ¯ θ - Δ u ¯ θ + ξ ^ ρ u ¯ θ p - 1 = θ u ¯ θ q - 1 + f ( z , u ¯ θ ) + ξ ^ ρ u ¯ θ p - 1
= λ u ¯ θ q - 1 + f ( z , u ¯ θ ) + ξ ^ ρ u ¯ θ p - 1 - ( λ - θ ) u ¯ θ q - 1
λ u ¯ θ q - 1 + f ( z , u ¯ θ ) + ξ ^ ρ u ¯ θ p - 1
= - Δ p u ¯ λ - Δ u ¯ λ + ξ ^ ρ u ¯ λ p - 1 for a.a.  z Ω .

Let

h 1 ( z ) = θ u ¯ θ ( z ) q - 1 + f ( z , u ¯ θ ( z ) ) + ξ ^ ρ u ¯ θ ( z ) p - 1 ,
h 2 ( z ) = λ u ¯ λ ( z ) q - 1 + f ( z , u ¯ λ ( z ) ) + ξ ^ ρ u ¯ λ ( z ) p - 1 .

Evidently, h 1 , h 2 L ( Ω ) and ( λ - θ ) u ¯ θ ( z ) q - 1 h 2 ( z ) - h 1 ( z ) for a.a. z Ω . Since u ¯ θ int C + , it follows that h 1 h 2 and so, using Proposition 2.4, we conclude that u ¯ λ - u ¯ θ int C + . We have proved that λ u ¯ λ is strictly increasing.

Next we show that the map λ u ¯ λ is left continuous. To this end, let λ n λ - , λ λ + * . For each n , let u ¯ n = u ¯ λ n S λ n + int C + be the minimal positive solution of problem (1.1) (see Proposition 3.3). Then we have

(3.54) A p ( u ¯ n ) , h + A ( u ¯ n ) , h = Ω [ λ n u ¯ n q - 1 + f ( z , u ¯ n ) ] h 𝑑 z

for all h W 0 1 , p ( Ω ) , all n , and

(3.55) u ¯ n u ¯ λ for all  n .

In (3.54), we choose h = u ¯ n W 0 1 , p ( Ω ) . Then from Hypothesis 2.6 (i) and (3.55) we infer that { u ¯ n } n 1 W 0 1 , p ( Ω ) is bounded. Using [18, Corollary 8.6, p. 208], we can find c 6 > 0 such that

u ¯ n c 6 for all  n .

Invoking [15, Theorem 1], we can find α ( 0 , 1 ) and c 7 > 0 such that

u ¯ n C 0 1 , α ( Ω ¯ ) , u ¯ C 0 1 , α ( Ω ¯ ) c 7 for all  n .

Since C 0 1 , α ( Ω ¯ ) C 0 1 ( Ω ¯ ) compactly, we may assume that

(3.56) u ¯ n u ^ λ in  C 0 1 ( Ω ¯ ) .

Passing to the limit as n in (3.54) and using (3.56), we obtain u ^ λ S λ + . We claim that u ¯ λ = u ^ λ . If this is not true, then we can find z 0 Ω such that u ¯ λ ( z 0 ) < u ^ λ ( z 0 ) , hence (see (3.56))

u ¯ λ ( z 0 ) < u ¯ n ( z 0 ) for all  n n 0 .

This contradicts the fact that λ u ¯ λ is increasing. Hence, u ^ λ = u ¯ λ and so we conclude the left continuity of the map λ u ¯ λ . ∎

Summarizing the results obtained in this section, we can state the following theorem.

Theorem 3.6.

If Hypothesis 2.6 holds, then

  1. there exists λ + * > 0 such that for λ ( 0 , λ + * ] , we have

    1. problem ( 1.1 ) has at least two positive solutions

      u 0 , u ^ int C + , u 0 u ^ , u 0 - u ~ λ , u ^ - u ~ λ int C + ,

    2. problem ( 1.1 ) has a smallest positive solution u ¯ λ int C + and λ u ¯ λ is strictly increasing and left continuous;

  2. there exists λ - * > 0 such that for λ ( 0 , λ - * ] , we have

    1. problem ( 1.1 ) has at least two negative solutions

      v 0 , v ^ - int C + , v 0 v ^ , v ~ λ - v 0 , v ~ λ - v ^ int C + ,

    2. problem ( 1.1 ) has a biggest negative solution v ¯ λ - int C + and λ v ¯ λ is strictly decreasing and left continuous.

4 Nodal solutions

In this section we focus on nodal (sign changing) solutions for problem (1.1). So, let λ * = min { λ + * , λ - * } (see Theorem 3.6). Let λ ( 0 , λ * ] and consider the C 1 -functional τ ^ λ : H 0 1 ( Ω ) defined by

τ ^ λ ( u ) = 1 2 D u 2 2 - λ q u q q - Ω F ( z , u ) 𝑑 z for all  u H 0 1 ( Ω ) .

Hypothesis 2.6 (v) and Proposition 2.1 of [14] imply the following result.

Proposition 4.1.

If Hypothesis 2.6 holds and λ > 0 , then C k ( τ ^ λ , 0 ) = 0 for all k N 0 .

Let τ λ = τ ^ λ | W 0 1 , p ( Ω ) . Since W 0 1 , p ( Ω ) H 0 1 ( Ω ) densely, we can apply [19, Theorem 16] (see also [4, p. 14]) and have C k ( τ λ , 0 ) = C k ( τ ^ λ , 0 ) for all k 0 , thus (see [14])

(4.1) C k ( τ λ , 0 ) = 0 for all  k 0 .

Let φ λ : W 0 1 , p ( Ω ) be the energy functional for problem (1.1) defined by

φ λ ( u ) = 1 p D u p p + 1 2 D u 2 2 - λ q u q q - Ω F ( z , u ) 𝑑 z for all  u W 0 1 , p ( Ω ) .

Then we have

| φ λ ( u ) - τ λ ( u ) | = 1 p D u p p = 1 p u p ,
| φ λ ( u ) - τ λ ( u ) , h | = | A p ( u ) , h | D u p p - 1 h p for all  h W 0 1 , p ( Ω ) .

Therefore,

φ λ ( u ) - τ λ ( u ) * D u p p - 1 = u p - 1 .

Then the C 1 -continuity property of critical groups (see [11, Theorem 5.126, p. 836]) implies that

C k ( φ λ , 0 ) = C k ( τ λ , 0 ) for all  k 0 ,

hence (see (4.1))

(4.2) C k ( φ λ , 0 ) = 0 for all  k 0 .

Let λ ( 0 , λ * ] and let u ¯ λ int C + , v ¯ λ - int C + be two extremal constant sign solutions for problem (1.1) (see Theorem 3.6). We introduce the following trunction of the reaction in problem (1.1):

(4.3) g λ ( z , x ) = { λ | v ¯ λ ( z ) | q - 2 v ¯ λ ( z ) + f ( z , v ¯ λ ( z ) ) if  x v ¯ λ ( z ) , λ | x | q - 1 x + f ( z , x ) if  v ¯ λ ( z ) x u ¯ λ ( z ) λ u ¯ λ ( z ) q - 1 + f ( z , u ¯ λ ( z ) ) if  u ¯ λ ( z ) < x .

This is a Caratheodory function. Set G λ ( z , x ) = 0 x g λ ( z , s ) 𝑑 s and consider the C 1 -functional φ ~ λ : W 0 1 , p ( Ω ) defined by

φ ~ λ ( u ) = 1 p D u p p + 1 2 D u 2 2 - Ω G λ ( z , u ) 𝑑 z for all  u W 0 1 , p ( Ω ) .

We know that φ ~ λ C 1 ( W 0 1 , p ( Ω ) ) . Using (4.3), we can easily show that

(4.4) K φ ~ λ [ v ¯ λ , u ¯ λ ] C 0 1 ( Ω ¯ ) .

Therefore, the nontrivial critical points of φ ~ λ , distinct from v ¯ λ and u ¯ λ , are nodal solutions of (1.1). So, we assume that K φ ~ λ is finite. Otherwise, we already have an infinity of nodal solutions for problem (1.1).

We also consider the positive and negative truncations of φ ~ λ , namely, we consider the C 1 -functionals φ ~ λ ± : W 0 1 , p ( Ω ) defined by

φ ~ λ ± ( u ) = 1 p D u p p + 1 2 D u 2 2 - Ω G λ ( z , ± u ± ) 𝑑 z for all  u W 0 1 , p ( Ω ) .

We can easily show that

K φ ~ λ + [ 0 , u ¯ λ ] C 0 1 ( Ω ¯ ) , K φ ~ λ - [ v ¯ λ , 0 ] C 0 1 ( Ω ¯ ) .

The extremality of u ¯ λ and v ¯ λ implies that

(4.5) K φ ~ λ + = { 0 , u ¯ λ } , K φ ~ λ - = { 0 , v ¯ λ } .

We compute the critical groups of φ ~ λ .

Proposition 4.2.

If Hypothesis 2.6 holds and λ ( 0 , λ * ] , then C k ( φ λ , 0 ) = C k ( φ ~ λ , 0 ) for all k N 0 .

Proof.

We consider the homotopy h ^ ( t , u ) defined by

h ^ ( t , u ) = ( 1 - t ) φ λ ( u ) + t φ ~ λ ( u ) for all  t [ 0 , 1 ] , u W 0 1 , p ( Ω ) .

Suppose we can find { t n } n 1 [ 0 , 1 ] and { u n } n 1 W 0 1 , p ( Ω ) such that

(4.6) t n t , u n 0 in  W 0 1 , p ( Ω ) , h u ( t n , u n ) = 0 for all  n .

Then we have

A p ( u n ) , h + A ( u n ) , h = ( 1 - t n ) Ω [ λ | u n | q - 2 u n + f ( z , u n ) ] h 𝑑 z + t n Ω g λ ( z , u n ) h 𝑑 z

for all h W 0 1 , p ( Ω ) and all n , which implies

(4.7) { - Δ p u n ( z ) - Δ u n ( z ) = ( 1 - t n ) [ λ | u n ( z ) | q - 2 u n ( z ) + f ( z , u n ( z ) ) ] + t n g λ ( z , u n ( z ) ) for a.a.  z Ω , u n | Ω = 0 .

Evidently, { u n } n 1 W 0 1 , p ( Ω ) is bounded (see (4.6), (4.7)). Then as before using (4.7) and [15, Theorem 1], we can find α ( 0 , 1 ) and c 8 > 0 such that

(4.8) u n C 0 1 , α ( Ω ¯ ) , u n C 0 1 , α ( Ω ¯ ) c 8 for all  n .

From (4.6), (4.8) and the compact embedding of C 0 1 , α ( Ω ¯ ) into C 0 1 ( Ω ¯ ) , we have

u n 0 in  C 0 1 ( Ω ¯ ) ,

hence

u n [ v ¯ λ , u ¯ λ ] for all  n n 0 ,

and therefore { u n } n n 0 K φ ~ λ (see (4.3)), a contradiction to our assumption that K φ ~ λ is finite. Therefore, (4.6) can not occur, and from homotopy invariance of critical groups (see [11, Theorem 5.125, p. 836]), we have C k ( φ λ , 0 ) = C k ( φ ~ λ , 0 ) for all k 0 . ∎

From Proposition 4.2 and (4.2), we infer that

(4.9) C k ( φ ~ λ , 0 ) = 0 for all  k 0 .

Proposition 4.3.

If Hypothesis 2.6 holds and 0 < λ λ * , then problem (1.1) admits two nodal solutions

y 0 , y ^ C 0 1 ( Ω ¯ ) , y 0 inf C 0 1 ( Ω ¯ ) [ v ¯ λ , u ¯ λ ] .

Proof.

From (4.4) and (4.5) we know that

(4.10) K φ ~ λ [ v ¯ λ , u ¯ λ ] C 0 1 ( Ω ¯ ) , K φ ~ λ + = { 0 , u ¯ λ } , K φ ~ λ - = { 0 , v ¯ λ } .

Claim: u ¯ λ int C + and v ¯ λ - int C + are local minimizers of φ ~ λ .

From (4.3) it is clear that φ ~ λ + is coercive. Also, it is sequentially weakly lower semicontinuous. So, we can find u ¯ λ * W 0 1 , p ( Ω ) such that

(4.11) φ ~ λ + ( u ¯ λ * ) = inf [ φ ~ λ + ( u ) : u W 0 1 , p ( Ω ) ] .

As before, since q < 2 < p , we have φ ~ λ + ( u ¯ λ * ) < 0 = φ ~ λ + ( 0 ) , hence u ¯ λ * 0 and u ¯ λ * K φ ~ λ + (see (4.11)), and thus u ¯ λ * = u ¯ λ (see (4.10)). It is clear from (4.3) that φ ~ λ | C + = φ ~ λ + | C + . Since u ¯ λ int C + , it follows that u ¯ λ is a local C 0 1 ( Ω ¯ ) -minimizer of φ ~ λ , hence u ¯ λ is a local W 0 1 , p ( Ω ) -minimizer of φ ~ λ (see Proposition 2.3). We can argue in a similar manner for v ¯ λ - int C + , using this time the functional φ ~ λ - . This proves the claim.

Without loss of generality, we may assume that

(4.12) φ ~ λ ( v ¯ λ ) φ ~ λ ( u ¯ λ ) .

Recall that K φ ~ λ is finite. So, on account of the claim and (4.12), we can find ρ ( 0 , 1 ) small such that

(4.13) φ ~ λ ( v ¯ λ ) φ ~ λ ( u ¯ λ ) < inf [ φ ~ λ ( u ) : u - u ¯ λ = ρ ] = m ~ λ , v ¯ λ - u ¯ λ > ρ .

We know that φ ~ λ is coercive (see (4.3)). Therefore, we have that

(4.14) φ ~ λ  satisfies the  C -condition

(see [17, Proposition 2.2]). From (4.13), (4.14) we see that we can use Theorem 2.1 (the mountain pass theorem). Therefore, we can find y 0 W 0 1 , p ( Ω ) such that y 0 K φ ~ λ [ v ¯ λ , u ¯ λ ] C 0 1 ( Ω ¯ ) (see (4.10)) and m ~ λ φ ~ λ ( y 0 ) (see (4.13)), thus y 0 { v ¯ λ , u ¯ λ } (see (4.13)). Since y 0 is a critical point of mountain pass type for φ ~ λ , we have

(4.15) C 1 ( φ ~ λ , y 0 ) 0

(see [18, Proposition 6.100, p. 176]). From (4.9) and (4.15), we infer that y 0 0 , hence y 0 C 0 1 ( Ω ¯ ) is a nodal solution of (1.1).

Let a ( y ) = | y | p - 2 y + y for all y N . Evidently, a C ( N , N ) (recall that 2 < p ) and

div a ( D u ) = Δ p u + Δ u for all  u W 0 1 , p ( Ω ) .

We have

a ( y ) = | y | p - 2 [ id + y y | y | 2 ] + id for all  y N ,

hence

(4.16) ( a ( y ) ξ , ξ ) N | ξ | 2 for all  y , ξ N .

We know that

v ¯ λ y 0 u ¯ λ , with  y 0 v ¯ λ , y 0 u ¯ λ .

Using (4.16) and the tangency principle of Pucci and Serrin [26, p. 35], we have

(4.17) v ¯ λ ( z ) < y 0 ( z ) < u ¯ λ ( z ) for all  z Ω .

Let ρ = max { u ¯ λ , v ¯ λ } and let ξ ^ ρ > 0 be as postulated by Hypothesis 2.6 (vi). Then (see (4.17) and Hypothesis 2.6 (vi))

- Δ p y 0 - Δ y 0 + ξ ^ ρ | y 0 | p - 2 y 0 = λ | y 0 | p - 2 y 0 + f ( z , y 0 ) + ξ ^ ρ | y 0 | p - 2 y 0
λ u ¯ λ q - 1 + f ( z , u ¯ λ ) + ξ ^ ρ u ¯ λ p - 1
(4.18) = - Δ p u ¯ λ - Δ u ¯ λ + ξ ^ ρ u ¯ λ p - 1 for a.a.  z Ω .

Let

h 1 ( z ) = λ | y 0 ( z ) | q - 2 y 0 ( z ) + f ( z , y 0 ( z ) ) + ξ ^ ρ | y 0 ( z ) | p - 2 y 0 ( z ) ,
h 2 ( z ) = λ u ¯ λ ( z ) q - 1 + f ( z , u ¯ λ ( z ) ) + ξ ^ ρ u ¯ λ ( z ) p - 1 .

Evidently, h 1 , h 2 L ( Ω ) and

h 2 ( z ) - h 1 ( z ) λ [ u λ ( z ) q - 1 - | y 0 ( z ) | q - 2 y 0 ( z ) ] ,

which implies h 1 h 2 (see Hypothesis 2.6 (vi), (4.17) and recall u ¯ λ int C + , y 0 C 0 1 ( Ω ¯ ) ).

Then from (4.18) and Proposition 2.4, we infer that

u ¯ λ - y 0 int C + .

Similarly, we show that

y 0 - v ¯ λ int C + .

Therefore,

(4.19) y 0 int C 0 1 ( Ω ¯ ) [ v ¯ λ , u ¯ λ ] .

On the other hand, from [12, Proposition 17] (see also [20]), we have a nodal solution of (1.1) such that

(4.20) y ^ int C 0 1 ( Ω ¯ ) [ v ¯ λ , u ¯ λ ] .

From (4.19) and (4.20) it follows that y 0 y ^ . ∎

We can state the following multiplicity result for problem (1.1).

Theorem 4.4.

If Hypothesis 2.6 holds, then there exists λ * > 0 such that for all λ ( 0 , λ * ] , problem (1.1) has at least six nontrivial smooth solutions

u 0 , u ^ int C + , u 0 u ^ ,
v 0 , v ^ - int C + , v 0 v ^ ,
y 0 , y ^ C 0 1 ( Ω ¯ ) both nodal .

Award Identifier / Grant number: 11671111

Award Identifier / Grant number: LBHQ16082

Funding statement: This work was supported by the NSFC (No. 11671111) and Heilongjiang Province Postdoctoral Startup Foundation (Grant No. LBHQ16082).

References

[1] S. Aizicovici, N. S. Papageorgiou and V. Staicu, Degree theory for operators of monotone type and nonlinear elliptic equations with inequality constraints, Mem. Amer. Math. Soc. 196 (2008), no. 915, 1–70. 10.1090/memo/0915Suche in Google Scholar

[2] S. Aizicovici, N. S. Papageorgiou and V. Staicu, Nodal solutions for ( p , 2 ) -equations, Trans. Amer. Math. Soc. 367 (2015), no. 10, 7343–7372. 10.1090/S0002-9947-2014-06324-1Suche in Google Scholar

[3] V. Benci, D. Fortunato and L. Pisani, Soliton like solutions of a Lorentz invariant equation in dimension 3, Rev. Math. Phys. 10 (1998), no. 3, 315–344. 10.1142/S0129055X98000100Suche in Google Scholar

[4] K.-C. Chang, Infinite-dimensional Morse Theory and Multiple Solution Problems, Progr. Nonlinear Differential Equations Appl. 6, Birkhäuser, Boston, 1993. 10.1007/978-1-4612-0385-8Suche in Google Scholar

[5] L. Cherfils and Y. Il’yasov, On the stationary solutions of generalized reaction diffusion equations with p & q -Laplacian, Commun. Pure Appl. Anal. 4 (2005), no. 1, 9–22. 10.3934/cpaa.2005.4.9Suche in Google Scholar

[6] S. Cingolani and M. Degiovanni, Nontrivial solutions for p-Laplace equations with right-hand side having p-linear growth at infinity, Comm. Partial Differential Equations 30 (2005), no. 7–9, 1191–1203. 10.1080/03605300500257594Suche in Google Scholar

[7] J. I. Díaz and J. E. Saá, Existence et unicité de solutions positives pour certaines équations elliptiques quasilinéaires, C. R. Acad. Sci. Paris Sér. I Math. 305 (1987), no. 12, 521–524. Suche in Google Scholar

[8] M. E. Filippakis and N. S. Papageorgiou, Multiple constant sign and nodal solutions for nonlinear elliptic equations with the p-Laplacian, J. Differential Equations 245 (2008), no. 7, 1883–1922. 10.1016/j.jde.2008.07.004Suche in Google Scholar

[9] G. Fragnelli, D. Mugnai and N. S. Papageorgiou, Positive and nodal solutions for parametric nonlinear Robin problems with indefinite potential, Discrete Contin. Dyn. Syst. 36 (2016), no. 11, 6133–6166. 10.3934/dcds.2016068Suche in Google Scholar

[10] L. Gasiński and N. S. Papageorgiou, Nonlinear Analysis, Ser. Math. Anal. Appl. 9, Chapman & Hall/CRC, Boca Raton, 2006. Suche in Google Scholar

[11] L. Gasiński and N. S. Papageorgiou, Exercises in Analysis. Part 2. Nonlinear Analysis, Problem Books in Math., Springer, Cham, 2016. 10.1007/978-3-319-27817-9Suche in Google Scholar

[12] T. He, P. Guo, Y. Huang and Y. Lei, Multiple nodal solutions for nonlinear nonhomogeneous elliptic problems with a superlinear reaction, Nonlinear Anal. Real World Appl. 42 (2018), 207–219. 10.1016/j.nonrwa.2017.12.010Suche in Google Scholar

[13] S. Hu and N. S. Papageorgiou, Handbook of Multivalued Analysis. Vol. I: Theory, Math. Appl. 419, Kluwer Academic, Dordrecht, 1997. 10.1007/978-1-4615-6359-4Suche in Google Scholar

[14] Q. Jiu and J. Su, Existence and multiplicity results for Dirichlet problems with p-Laplacian, J. Math. Anal. Appl. 281 (2003), no. 2, 587–601. 10.1016/S0022-247X(03)00165-3Suche in Google Scholar

[15] G. M. Lieberman, Boundary regularity for solutions of degenerate elliptic equations, Nonlinear Anal. 12 (1988), no. 11, 1203–1219. 10.1016/0362-546X(88)90053-3Suche in Google Scholar

[16] S. A. Marano and S. J. N. Mosconi, Some recent results on the Dirichlet problem for ( p , q ) -Laplace equations, Discrete Contin. Dyn. Syst. Ser. S 11 (2018), no. 2, 279–291. 10.3934/dcdss.2018015Suche in Google Scholar

[17] S. A. Marano and N. S. Papageorgiou, Positive solutions to a Dirichlet problem with p-Laplacian and concave-convex nonlinearity depending on a parameter, Commun. Pure Appl. Anal. 12 (2013), no. 2, 815–829. 10.3934/cpaa.2013.12.815Suche in Google Scholar

[18] D. Motreanu, V. V. Motreanu and N. Papageorgiou, Topological and Variational Methods with Applications to Nonlinear Boundary Value Problems, Springer, New York, 2014. 10.1007/978-1-4614-9323-5Suche in Google Scholar

[19] R. S. Palais, Homotopy theory of infinite dimensional manifolds, Topology 5 (1966), 1–16. 10.1016/0040-9383(66)90002-4Suche in Google Scholar

[20] N. S. Papageorgiou and F. Papalini, Seven solutions with sign information for sublinear equations with unbounded and indefinite potential and no symmetries, Israel J. Math. 201 (2014), no. 2, 761–796. 10.1007/s11856-014-1050-ySuche in Google Scholar

[21] N. S. Papageorgiou and V. D. Rădulescu, Qualitative phenomena for some classes of quasilinear elliptic equations with multiple resonance, Appl. Math. Optim. 69 (2014), no. 3, 393–430. 10.1007/s00245-013-9227-zSuche in Google Scholar

[22] N. S. Papageorgiou and V. D. Rădulescu, Noncoercive resonant ( p , 2 ) -equations, Appl. Math. Optim. 76 (2017), no. 3, 621–639. 10.1007/s00245-016-9363-3Suche in Google Scholar

[23] N. S. Papageorgiou, V. D. Rădulescu and D. D. Repovš, On a class of parametric ( p , 2 ) -equations, Appl. Math. Optim. 75 (2017), no. 2, 193–228. 10.1007/s00245-016-9330-zSuche in Google Scholar

[24] N. S. Papageorgiou, V. D. Rădulescu and D. D. Repovš, ( p , 2 ) -equations asymmetric at both zero and infinity, Adv. Nonlinear Anal. 7 (2018), no. 3, 327–351. 10.1515/anona-2017-0195Suche in Google Scholar

[25] N. S. Papageorgiou and P. Winkert, Resonant ( p , 2 ) -equations with concave terms, Appl. Anal. 94 (2015), no. 2, 342–360. 10.1080/00036811.2014.895332Suche in Google Scholar

[26] P. Pucci and J. Serrin, The Maximum Principle, Progr. Nonlinear Differential Equations Appl. 73, Birkhäuser, Basel, 2007. 10.1007/978-3-7643-8145-5Suche in Google Scholar

[27] M. Sun, Multiplicity of solutions for a class of the quasilinear elliptic equations at resonance, J. Math. Anal. Appl. 386 (2012), no. 2, 661–668. 10.1016/j.jmaa.2011.08.030Suche in Google Scholar

[28] M. Sun, M. Zhang and J. Su, Critical groups at zero and multiple solutions for a quasilinear elliptic equation, J. Math. Anal. Appl. 428 (2015), no. 1, 696–712. 10.1016/j.jmaa.2015.03.033Suche in Google Scholar

[29] H. Wilhelmsson, Explosive instabilities of reaction-diffusion equations, Phys. Rev. A (3) 36 (1987), no. 2, 965–966. 10.1103/PhysRevA.36.965Suche in Google Scholar PubMed

Received: 2018-08-04
Accepted: 2018-08-16
Published Online: 2018-10-11

© 2020 Walter de Gruyter GmbH, Berlin/Boston

This work is licensed under the Creative Commons Attribution 4.0 Public License.

Artikel in diesem Heft

  1. Frontmatter
  2. On the moving plane method for boundary blow-up solutions to semilinear elliptic equations
  3. Regularity of solutions of the parabolic normalized p-Laplace equation
  4. Cahn–Hilliard equation on the boundary with bulk condition of Allen–Cahn type
  5. Blow-up solutions for fully nonlinear equations: Existence, asymptotic estimates and uniqueness
  6. Radon measure-valued solutions of first order scalar conservation laws
  7. Ground state solutions for a semilinear elliptic problem with critical-subcritical growth
  8. Generalized solutions of variational problems and applications
  9. Existence and non-existence results for Kirchhoff-type problems with convolution nonlinearity
  10. Nonlinear Sherman-type inequalities
  11. Global regularity for systems with p-structure depending on the symmetric gradient
  12. Homogenization of a net of periodic critically scaled boundary obstacles related to reverse osmosis “nano-composite” membranes
  13. Noncoercive resonant (p,2)-equations with concave terms
  14. Evolutionary quasi-variational and variational inequalities with constraints on the derivatives
  15. Sharp estimates on the first Dirichlet eigenvalue of nonlinear elliptic operators via maximum principle
  16. Localization and multiplicity in the homogenization of nonlinear problems
  17. Remarks on a nonlinear nonlocal operator in Orlicz spaces
  18. A Picone identity for variable exponent operators and applications
  19. On the weakly degenerate Allen-Cahn equation
  20. Continuity results for parametric nonlinear singular Dirichlet problems
  21. Construction of type I blowup solutions for a higher order semilinear parabolic equation
  22. Singularly perturbed Choquard equations with nonlinearity satisfying Berestycki-Lions assumptions
  23. Comparison results for nonlinear divergence structure elliptic PDE’s
  24. Constant sign and nodal solutions for parametric (p, 2)-equations
  25. Monotonicity formulas for coupled elliptic gradient systems with applications
  26. Berestycki-Lions conditions on ground state solutions for a Nonlinear Schrödinger equation with variable potentials
  27. A class of semipositone p-Laplacian problems with a critical growth reaction term
  28. The role of superlinear damping in the construction of solutions to drift-diffusion problems with initial data in L1
  29. Reconstruction of Tesla micro-valve using topological sensitivity analysis
  30. Lewy-Stampacchia’s inequality for a pseudomonotone parabolic problem
  31. Global well-posedness of nonlinear wave equation with weak and strong damping terms and logarithmic source term
  32. Regularity Criteria for Navier-Stokes Equations with Slip Boundary Conditions on Non-flat Boundaries via Two Velocity Components
  33. Homoclinics for singular strong force Lagrangian systems
  34. A constructive method for convex solutions of a class of nonlinear Black-Scholes equations
  35. On a class of nonlocal nonlinear Schrödinger equations with potential well
  36. Superlinear Schrödinger–Kirchhoff type problems involving the fractional p–Laplacian and critical exponent
  37. Regularity for minimizers for functionals of double phase with variable exponents
  38. Boundary blow-up solutions to the Monge-Ampère equation: Sharp conditions and asymptotic behavior
  39. Homogenisation with error estimates of attractors for damped semi-linear anisotropic wave equations
  40. A-priori bounds for quasilinear problems in critical dimension
  41. Critical growth elliptic problems involving Hardy-Littlewood-Sobolev critical exponent in non-contractible domains
  42. On the Sobolev space of functions with derivative of logarithmic order
  43. On a logarithmic Hartree equation
  44. Critical elliptic systems involving multiple strongly–coupled Hardy–type terms
  45. Sharp conditions of global existence for nonlinear Schrödinger equation with a harmonic potential
  46. Existence for (p, q) critical systems in the Heisenberg group
  47. Periodic traveling fronts for partially degenerate reaction-diffusion systems with bistable and time-periodic nonlinearity
  48. Some hemivariational inequalities in the Euclidean space
  49. Existence of standing waves for quasi-linear Schrödinger equations on Tn
  50. Periodic solutions for second order differential equations with indefinite singularities
  51. On the Hölder continuity for a class of vectorial problems
  52. Bifurcations of nontrivial solutions of a cubic Helmholtz system
  53. On the exact multiplicity of stable ground states of non-Lipschitz semilinear elliptic equations for some classes of starshaped sets
  54. Sign-changing multi-bump solutions for the Chern-Simons-Schrödinger equations in ℝ2
  55. Positive solutions for diffusive Logistic equation with refuge
  56. Null controllability for a degenerate population model in divergence form via Carleman estimates
  57. Eigenvalues for a class of singular problems involving p(x)-Biharmonic operator and q(x)-Hardy potential
  58. On the convergence analysis of a time dependent elliptic equation with discontinuous coefficients
  59. Multiplicity and concentration results for magnetic relativistic Schrödinger equations
  60. Solvability of an infinite system of nonlinear integral equations of Volterra-Hammerstein type
  61. The superposition operator in the space of functions continuous and converging at infinity on the real half-axis
  62. Estimates by gap potentials of free homotopy decompositions of critical Sobolev maps
  63. Pseudo almost periodic solutions for a class of differential equation with delays depending on state
  64. Normalized multi-bump solutions for saturable Schrödinger equations
  65. Some inequalities and superposition operator in the space of regulated functions
  66. Area Integral Characterization of Hardy space H1L related to Degenerate Schrödinger Operators
  67. Bifurcation of time-periodic solutions for the incompressible flow of nematic liquid crystals in three dimension
  68. Morrey estimates for a class of elliptic equations with drift term
  69. A singularity as a break point for the multiplicity of solutions to quasilinear elliptic problems
  70. Global and non global solutions for a class of coupled parabolic systems
  71. On the analysis of a geometrically selective turbulence model
  72. Multiplicity of positive solutions for quasilinear elliptic equations involving critical nonlinearity
  73. Lack of smoothing for bounded solutions of a semilinear parabolic equation
  74. Gradient estimates for the fundamental solution of Lévy type operator
  75. π/4-tangentiality of solutions for one-dimensional Minkowski-curvature problems
  76. On the existence and multiplicity of solutions to fractional Lane-Emden elliptic systems involving measures
  77. Anisotropic problems with unbalanced growth
  78. On a fractional thin film equation
  79. Minimum action solutions of nonhomogeneous Schrödinger equations
  80. Global existence and blow-up of weak solutions for a class of fractional p-Laplacian evolution equations
  81. Optimal rearrangement problem and normalized obstacle problem in the fractional setting
  82. A few problems connected with invariant measures of Markov maps - verification of some claims and opinions that circulate in the literature
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