Boundary Value Problems

, 2019:171 | Cite as

Existence of solutions for a nonhomogeneous Dirichlet problem involving \(p(x)\)-Laplacian operator and indefinite weight

  • Aboubacar MarcosEmail author
  • Aboubacar Abdou
Open Access


We obtain multiplicity and uniqueness results in the weak sense for the following nonhomogeneous quasilinear equation involving the \(p(x)\)-Laplacian operator with Dirichlet boundary condition:
$$ -\Delta _{p(x)}u+V(x) \vert u \vert ^{q(x)-2}u =f(x,u)\quad \text{in }\varOmega , u=0 \text{ on }\partial \varOmega , $$
where Ω is a smooth bounded domain in \(\mathbb{R}^{N}\), V is a given function with an indefinite sign in a suitable variable exponent Lebesgue space, \(f(x,t)\) is a Carathéodory function satisfying some growth conditions. Depending on the assumptions, the solutions set may consist of a bounded infinite sequence of solutions or a unique one. Our technique is based on a symmetric version of the mountain pass theorem.


Generalized Lebesgue–Sobolev spaces \(p(x)\)-Laplacian operator Symmetric mountain pass lemma 


35B38 35J20 35J60 35J66 

1 Introduction

In this work, we study the existence of solutions for the following nonlinear Dirichlet problem involving the \(p(x)\)-Laplacian operator:
$$ \textstyle\begin{cases} - \Delta _{p(x)}u+V(x) \vert u \vert ^{q(x)-2}u=f(x,u)\quad \text{in } \varOmega , \\ u=0\quad \text{on } \partial \varOmega , \end{cases} $$
where \(\varOmega \subset \mathbb{R}^{N}\) is a smooth bounded domain, \(p, q, s:\overline{\varOmega }\rightarrow \mathbb{R}^{+}\) are continuous functions, \(V \in L^{s(x)}(\varOmega )\) has an indefinite sign, and \(f(x,t)\) is a Carathéodory function. Let us recall that the \(p(x)\)-Laplacian operator \(\Delta _{p(x)}\) is defined by
$$ \Delta _{p(x)}u = \operatorname{div}\bigl(\lvert \nabla u \rvert ^{p(x)-2} \nabla u\bigr) $$
and is an extension of the classical p-Laplacian operator obtained in the case where the function \(p(x)\) is just a positive constant p. A growing interest in the study of the \(p(x)\)-Laplacian operator has arisen during the last two decades, in regard to its involvement in the modelings of a large number of phenomena. One can name for instance electrorheological fluids [30, 32, 36], elastic mechanics, flows in porous media and image processing [11], curl systems emanating from electromagnetism [4, 7]. For more details, we refer to Acerbi and Mingione [2] and Růžička [32] about electrorheological fluids, and to Antontsev and Shmarev [5] about nonlinear Darcy’s law in porous media. Because of its nonhomogeneous feature, the \(p(x)\)-Laplacian operator is reasonably expected to be appropriate for modeling nonhomogeneous materials. Recently, several works devoted to existence problems for p-Laplacian operator have been naturally generalized to the case of \(p(x)\)-Laplacian operator. In relevance with the present work are the papers in [1, 8, 9, 16, 17, 20, 21, 25, 27, 29] and the references therein. To the best of our knowledge, the earliest paper that deals with the variable exponent case with \(V\equiv 0\) in (1.1) is the article by Fan and Zhang (cf. [22]). Indeed, those authors studying the problem
$$ \textstyle\begin{cases} - \Delta _{p(x)}u=f(x,u)\quad \text{in } \varOmega , \\ u=0\quad \text{on } \partial \varOmega , \end{cases} $$
proved by the mountain pass theorem and by the fountain theorem that problem (1.2) admits respectively at least one nontrivial weak solution and infinitely many pairs of weak solutions. In an earlier work, Dinca et al. (cf. [17]), using variational and topological methods, proved the existence and multiplicity of weak solutions for problem (1.2) for the case \(p(x)\equiv p \) (p is a constant). Their main tool was the mountain pass theorem of Ambrosetti and Rabinowitz (cf. [3]). The approach for dealing with the case where \(V > 0 \) is technically similar to that of \(V = 0 \). In [8, 9], the authors considered problem (1.1) with V bounded and \(p(\cdot ) = q(\cdot )\) and proved the existence of nonnegative solutions using a mountain pass theorem. Most of the results are derived from a \(\beta (x)\)-growth assumption on the nonlinearity \(f(x,t) \) with additional assumptions preventing the range of \(p(\cdot )\) to interfere with that of \(\beta (\cdot )\). Motivated by the works in [8, 9, 17, 20, 22, 25, 27], we study here the nonlinear Dirichlet problem (1.1) when the function \(V(\cdot )\) has an indefinite sign and belongs to the generalized Lebesgue space \(L^{s(x)}(\varOmega )\). We assume furthermore that \(p(\cdot ) \neq q(\cdot ) \) and the nonlinearity \(f(x,t)\) satisfies a \(\beta (x)\)-growth assumption. The interest in this work is twofold: the weight V is not bounded and may change sign, and besides the range of \(p(\cdot )\) may interfere with that of \(q(\cdot )\) or \(\beta (\cdot ) \) as well. To the best of our knowledge, our setting is more general than those of [8, 9, 22] and our method contrasts with other treatments of (1.1). The functional framework is the generalized Lebesgue and Sobolev spaces with variable exponents and our technique is based on a \(\mathbb{Z}_{2}\) symmetric version of the mountain pass theorem.

The remainder of this paper is organized as follows. In Section 2, we introduce some technical results and formulate the required hypotheses on (1.1). Sections 3 and 4 are devoted to the statement of our main results along with some auxiliary results and to their proofs. In the appendix we present the appropriate version of the mountain pass theorem related to our problem.

2 Preliminaries and hypotheses

In order to study problem (1.1), some of the properties on variable exponent Lebesgue spaces and Sobolev spaces, \(L^{p(x)}(\varOmega )\) and \(W^{1,p(x)} (\varOmega )\) respectively, are required and listed below. We refer to [15, 23, 26] for exhaustive details on properties of those spaces.

Suppose that Ω is a bounded domain of \(\mathbb{R}^{N}\) with a smooth boundary ∂Ω. Let us denote
$$\begin{aligned}& L_{+}^{\infty }(\varOmega )=\Bigl\{ p\in L^{\infty }(\varOmega ): \operatorname{ess}\inf_{ \varOmega } p(x)\geq 1 \Bigr\} , \\& C_{+}(\overline{\varOmega })=\bigl\{ p\in C(\overline{\varOmega }): p(x)>1 \text{ for every } x\in \overline{\varOmega } \bigr\} , \\& p^{-}=\min_{x\in \overline{\varOmega }}p(x), \qquad p^{+}= \max _{x\in \overline{\varOmega } }p(x) \quad \text{for }p\in C_{+}(\overline{ \varOmega }), \\& M=\{ u:\varOmega \to \mathbb{R} : u \text{ is a measurable real-valued function} \}. \end{aligned}$$

Definition 2.1

For any \(p \in L_{+}^{\infty }(\varOmega )\), the variable exponent Lebesgue space \(L^{p(x)}(\varOmega )\) is defined by
$$ L^{p(x)}(\varOmega )= \biggl\{ u \in M: \int _{\varOmega } \vert u \vert ^{p(x)}\,dx < + \infty \biggr\} . $$
For any \(u \in L^{p(x)}(\varOmega ) \), we define the so-called Luxemburg norm on \(L^{p(x)}(\varOmega )\) by
$$ \vert u \vert _{p(\cdot)}=\inf \biggl\{ \lambda >0: \int _{\varOmega } \biggl\vert \frac{u}{\lambda } \biggr\vert ^{p(x)}\,dx \leq 1 \biggr\} . $$

Throughout this paper, \(L^{p(x)}(\varOmega ) \) will be endowed with this norm.

The modular, which is the mapping \(\varrho _{p(\cdot)}:L^{p(x)}(\varOmega ) \to \mathbb{R}\) defined by
$$ \varrho _{p(\cdot)} (u)= \int _{\varOmega } \vert u \vert ^{p(x)}\,dx, $$
is at many aspects an important tool in studying generalized Lebesgue–Sobolev spaces.

Remark 2.2

Variable exponent Lebesgue spaces have many properties similar to those of classical Lebesgue spaces, namely they are separable Banach spaces and the Hölder inequality holds. The inclusions between Lebesgue spaces are also naturally generalized, that is, if \(0< \operatorname{mes}(\varOmega )<\infty \) and p, q are variable exponents such that \(p(x)< q(x)\) a.e. in Ω, then there exists a continuous embedding \(L^{q(x)}(\varOmega ) \hookrightarrow L^{p(x)}(\varOmega )\).

We recall below some statements whose details can be found in [18, 20, 23].

Let us denote by \(L^{p'(x)}(\varOmega )\) the conjugate space of \(L^{p(x)}(\varOmega )\), with \(\frac{1}{p(x)}+\frac{1}{p'(x)}=1\). There is a counterpart of the Hölder inequality for variable exponent Lebesgue spaces when \(p \in L_{+}^{\infty }(\varOmega )\) in the literature (cf. [18]). We give below a version relevant to the need of this work.

Proposition 2.3

(Hölder inequality)

$$ \int _{\varOmega } \vert uv \vert \,dx \leq \biggl(\frac{1}{p^{-}}+ \frac{1}{p^{\prime -}} \biggr) \vert u \vert _{p(\cdot)} \vert v \vert _{p'(\cdot)} \leq 2 \vert u \vert _{p(\cdot)} \vert v \vert _{p'(\cdot)} $$
for all\(u\in L^{p(x)}(\varOmega ) \), \(v\in L^{p'(x)}(\varOmega ) \), and\(p , q \in C_{+}(\overline{\varOmega }) \).

Proposition 2.4

For\(p\in C_{+}(\overline{\varOmega }) \), we have the following:
$$ \min \bigl( \vert u \vert _{p(\cdot)}^{p^{-}}, \vert u \vert _{p(\cdot)}^{p^{+}}\bigr)\leq \varrho _{p(\cdot)} (u) \leq \max \bigl( \vert u \vert _{p(\cdot)}^{p^{-}}, \vert u \vert _{p(\cdot)}^{p^{+}}\bigr). $$

It is worth noticing that this relation between the norm and the modular shows an equivalence between the topology defined by the norm and that defined by the modular.

Proposition 2.5

Letpandqbe measurable functions such that\(p\in C_{+}(\overline{ \varOmega })\)and\(p q \in L_{+}^{\infty }(\varOmega )\). Let\(u\in L^{q(x)}( \varOmega )\), \(u\neq 0\). Then
$$\begin{aligned}& \vert u \vert _{p(\cdot)q(\cdot)}\leq 1\quad \Rightarrow\quad \vert u \vert _{p(\cdot)q(\cdot)}^{p^{+}} \leq \bigl\vert | u|^{p(\cdot)} \bigr\vert _{q(\cdot)} \leq \vert u \vert _{p(\cdot)q(\cdot)}^{p^{-}}, \\& \vert u \vert _{p(\cdot)q(\cdot)}\geq 1 \quad \Rightarrow\quad \vert u \vert _{p(\cdot)q(\cdot)}^{p^{-}}\leq \bigl\vert | u|^{p(\cdot)} \bigr\vert _{q(\cdot)}\leq \vert u \vert _{p(\cdot)q(\cdot)}^{p^{+}}. \end{aligned}$$
In particular, if\(p(x)=p\)is a constant, then
$$ \bigl\vert | u|^{p} \bigr\vert _{q(\cdot)}= \vert u \vert _{pq(\cdot)}^{p}. $$

Definition 2.6

The variable exponent Sobolev space is defined by
$$ W^{1,p(x)}(\varOmega )= \bigl\{ u\in L^{p(x)}(\varOmega ): \vert \nabla u \vert \in L^{p(x)}(\varOmega ) \bigr\} , $$
with the norm
$$\begin{aligned}& \Vert u \Vert _{1,p(\cdot)}=\|\nabla u\|_{p(\cdot)}+ \vert u \vert _{p(\cdot)}, \end{aligned}$$
where \(|\nabla u|=\sqrt{\sum_{i=1}^{N} (\frac{\partial u}{ \partial x_{i}} )^{2}}\).

Proposition 2.7

\(L^{p(x)}(\varOmega )\)and\(W^{1,p(x)}(\varOmega )\)are separable Banach spaces when\(p \in L_{+}^{\infty }(\varOmega )\), reflexive and uniformly convex for\(p \in L^{\infty }(\varOmega )\)and\(\operatorname{ess}\inf_{\varOmega } p(x) > 1 \).

Definition 2.8

For \(p\in C_{+}(\overline{\varOmega })\), let us define the so-called critical Sobolev exponent of p by
$$ p^{*}(x)= \textstyle\begin{cases} \frac{Np(x)}{N-p(x)} & \text{if } p(x)< N, \\ +\infty & \text{if } p(x)\geq N, \end{cases} $$
for every \(x\in \overline{\varOmega }\).

We also define the space \(W_{0}^{1,p(x)}(\varOmega )\) as the closure of the space \(C_{0}^{\infty }(\varOmega )\) (\(C^{\infty }\)-functions with compact support in Ω) in the space \(W^{1,p(x)}(\varOmega )\) with respect to the norm \(\| u \|_{1,p(x)}\).

The dual space of \(W_{0}^{1,p(x)}(\varOmega )\) is denoted by \(W^{-1,p'(x)}( \varOmega )\), where \(\frac{1}{p(x)}+\frac{1}{p'(x)}=1\), for every \(x\in \overline{\varOmega }\).

With respect to those spaces, we recall from [15, 23] the following.

Proposition 2.9

  1. (i)

    \(W_{0}^{1,p(x)}(\varOmega )\)is a separable Banach space when\(p \in L_{+}^{\infty }(\varOmega )\), reflexive and uniformly convex when\(p \in L^{\infty }(\varOmega )\)and\(\operatorname{ess}\inf_{\varOmega } p(x) > 1 \).

    Assume that\(p,q\in C_{+}(\overline{\varOmega })\). Then

  2. (ii)
    if\(p(x) < N \)and\(q(x)< p^{*}(x) = \frac{Np(x)}{N-p(x)}\)for every\(x\in \overline{\varOmega }\), then there is a compact and continuous embedding
    $$ W^{1,p(x)}(\varOmega )\hookrightarrow L^{q(x)}(\varOmega ); $$
  3. (iii)
    if\(p(x) > N\)for every\(x\in \overline{\varOmega }\), then
    $$ W^{1,p(x)(\varOmega )\hookrightarrow L^{\infty }(\varOmega )}; $$
  4. (iv)
    there is a constant \(C>0\) such that
    $$ \vert u \vert _{p()}\leq C \Vert \nabla u \Vert _{p()},\quad \textit{for all }u\in W_{0}^{1,p(x)}( \varOmega ). $$

Remark 2.10

Using estimate (iv) of Proposition 2.9, we derive that the norm \(\| u \|_{1,p(\cdot)}=\|\nabla u\|_{p(\cdot)}+| u |_{p(\cdot)}\) is equivalent to the norm \(\| u \|=\|\nabla u\|_{p(\cdot)}\) in \(W_{0}^{1,p(x)}(\varOmega )\). Here and henceforth, we will consider the space \(W_{0}^{1,p(x)}( \varOmega )\) equipped with the norm \(\| u \|=\|\nabla u\|_{p(\cdot)}\).

Moreover, one can prove (cf. [15]) that the norm \(\| u \|\) is weakly sequentially lower semicontinuous.

Remark 2.11

If \(q\in C_{+}(\overline{\varOmega })\) and \(q(x)< p^{*}(x)\) for every \(x\in \overline{\varOmega }\), then the embedding of \(W_{0}^{1,p(x)}( \varOmega )\) into \(L^{q(x)}(\varOmega )\) is compact.

As in the case \(p(x)\equiv p\) (a constant), we consider the \(p(x)\)-Laplacian operator
$$ -\Delta _{p(x)}:W_{0}^{1,p(x)}(\varOmega )\to W^{-1,p'(x)}(\varOmega ) $$
defined by
$$ \langle -\Delta _{p(x)}u,v \rangle = \int _{\varOmega } \vert \nabla u \vert ^{p(x)-2} \nabla u \nabla v\,dx\quad \text{for all } u,v\in W_{0}^{1,p(x)}(\varOmega ), $$
where \(\langle \,, \,\rangle \) denotes the duality pairing between \(W_{0}^{1,p(x)}(\varOmega )\) and \(W^{-1,p'(x)}(\varOmega )\).

We have the following properties (cf. [10, 22]).

Proposition 2.12

  1. (i)

    \(-\Delta _{p(x)}:W_{0}^{1,p(x)}(\varOmega )\to W^{-1,p'(x)}( \varOmega )\)is a homeomorphism.

  2. (ii)
    \(-\Delta _{p(x)}:W_{0}^{1,p(x)}(\varOmega )\to W^{-1,p'(x)}( \varOmega )\)is a strictly monotone operator, that is,
    $$ -\langle \Delta _{p(x)}u-\Delta _{p(x)}v,u-v \rangle >0\quad \textit{for all } u\neq v \in W_{0}^{1,p(x)}(\varOmega ). $$
  3. (iii)
    \(-\Delta _{p(x)}:W_{0}^{1,p(x)}(\varOmega )\to W^{-1,p'(x)}( \varOmega )\)is a mapping of type\((S_{+})\), that is,
    $$ \textit{if } u_{n} \rightharpoonup u\quad \textit{in }W_{0}^{1,p(x)}( \varOmega )\quad \textit{and}\quad \limsup_{n\to \infty }\langle -\Delta _{p(x)}u_{n},u _{n}-u \rangle \leq 0, $$
    then\(u_{n}\to u\)in\(W_{0}^{1,p(x)}(\varOmega )\).

Proposition 2.13

The functional\(\varPsi :W_{0}^{1,p(x)}(\varOmega )\to \mathbb{R}\)defined by
$$ \varPsi (u)= \int _{\varOmega }\frac{1}{p(x)} \vert \nabla u \vert ^{p(x)}\,dx $$
is continuously Fréchet differentiable, sequentially weakly lower semicontinuous, and\(\varPsi '(u)=-\Delta _{p(x)}u\)for all\(u\in W_{0} ^{1,p(x)}(\varOmega )\).

Note that, if \(f:\varOmega \times \mathbb{R}\to \mathbb{R}\) is a Carathéodory function and \(u\in M\), then the function \(N_{f}u: \varOmega \to \mathbb{R}\) defined by \((N_{f}u)(x)=f(x,u(x))\) for \(x\in \varOmega \) is measurable in Ω. Thus, the Carathéodory function \(f:\varOmega \times \mathbb{R}\to \mathbb{R}\) generates an operator \(N_{f}:M\to M\), which is called the Nemytskii operator. The properties of \(N_{f}\) are recalled through the propositions below (see [35] for details).

Proposition 2.14

Suppose that\(f:\varOmega \times \mathbb{R}\to \mathbb{R}\)is a Carathéodory function satisfying the following growth condition:
$$ \bigl\vert f(x,t) \bigr\vert \leq c \vert t \vert ^{\frac{\alpha (x)}{\beta (x)}}+h(x) \quad \textit{for every }x\in \varOmega , t\in \mathbb{R}, $$
where\(\alpha ,\beta \in C_{+}(\overline{\varOmega })\), \(c\geq 0\)is constant, and\(h\in L^{\beta (x)}(\varOmega )\). Then\(N_{f}(L^{\alpha (x)}( \varOmega ))\subseteq L^{\beta (x)} (\varOmega )\). Moreover, \(N_{f}\)is continuous from\(L^{\alpha (x)}(\varOmega )\)into\(L^{\beta (x)}(\varOmega )\)and maps a bounded set into a bounded set.

Proposition 2.15

Suppose that\(f:\varOmega \times \mathbb{R}\to \mathbb{R}\)is a Carathéodory function satisfying the following growth condition:
$$ \bigl\vert f(x,t) \bigr\vert \leq c \vert t \vert ^{\alpha (x)-1}+h(x) \quad \textit{for every }x\in \varOmega , t\in \mathbb{R}, $$
where\(c\geq 0\)is constant, \(\alpha \in C_{+}(\overline{\varOmega })\), \(h\in L^{\alpha '(x)} (\varOmega )\)with\(\alpha '\)the conjugate exponent ofα, i.e., \(\alpha '(x)=\frac{\alpha (x)}{\alpha (x)-1}\). Let\(F:\varOmega \times \mathbb{R}\to \mathbb{R}\)defined by
$$ F(x,t)= \int _{0}^{t}f(x,s)\,ds . $$
  1. (i)
    F is a Carathéodory function and there exist a constant \(c_{1}\geq 0\) and \(\sigma \in L^{1}(\varOmega )\) such that
    $$ \bigl\vert F(x,t) \bigr\vert \leq c_{1} \vert t \vert ^{\alpha (x)}+\sigma (x)\quad \textit{for all }x \in \varOmega , t\in \mathbb{R}. $$
  2. (ii)

    The functional\(\varPhi :L^{\alpha (x)}(\varOmega )\to \mathbb{R}\)defined by\(\varPhi (u)=\int _{\varOmega }F(x,u(x))\,dx\)is continuously Fréchet differentiable and\(\varPhi '(u)=N_{f}(u) \)for all\(u\in L^{\alpha (x)}(\varOmega )\).


Remark 2.16

Since the embedding \(W_{0}^{1,p(x)}(\varOmega )\hookrightarrow L^{\alpha (x)}(\varOmega )\) is compact for \(\alpha \in C_{+}(\overline{\varOmega })\) with \(\alpha (x)< p^{*}(x)\) for every \(x\in \overline{\varOmega }\), we derive from Proposition 2.15 the following diagram:
$$ W_{0}^{1,p(x)}(\varOmega )\stackrel{I}{\hookrightarrow } L^{\alpha (x)}( \varOmega ) \stackrel{N_{f}}{\to }L^{\alpha '(x)}( \varOmega )\stackrel{I ^{*}}{\hookrightarrow } W^{-1,p'(x)}( \varOmega ), $$
which shows that \(N_{f}:W_{0}^{1,p(x)}(\varOmega )\to W^{-1,p'(x)}( \varOmega )\) is strongly continuous on \(W_{0}^{1,p(x)}(\varOmega )\). Using the same argument, the functional \(\varPhi :W_{0}^{1,p(x)}(\varOmega )\to \mathbb{R}\) defined by \(\varPhi (u)= [4]\int _{\varOmega }F(x,u(x))\,dx\) is also strongly continuous on \(W_{0}^{1,p(x)}(\varOmega )\).
Throughout this work, \(f:\varOmega \times \mathbb{R}\to \mathbb{R}\) is a Carathéodory function, \(V\in L^{s(x)}(\varOmega )\). Let us denote
$$ \varOmega _{-} = \bigl\{ x \in \varOmega , V(x) < 0\bigr\} . $$
We require the following assumptions on problem (1.1):
\(p, q, s \text{ and } \beta \in C_{+}(\overline{\varOmega })\) such that, for all \(x\in {\overline{\varOmega }}\),
$$ q(x) < p(x) \leq N ,\qquad \beta (x) < p(x) \leq N, \qquad N < s(x)\quad \text{for } V\in L^{s(x)}(\varOmega ). $$
Moreover, we assume that
$$ q^{+}_{\varOmega _{-}}= \sup_{\varOmega _{-}}q(x) < p^{-}\quad \text{and} \quad \beta ^{+} < p^{-} . $$
$$ \bigl\vert f(x,t) \bigr\vert \leq c \vert t \vert ^{\beta (x)-1}+h(x)\quad \text{for all }x\in \varOmega , t\in \mathbb{R}, $$
where \(c\geq 0\) is constant, \(\beta \in C_{+}(\overline{\varOmega })\) with \(\beta (x)< p^{*}(x)\) for every \(x\in \overline{\varOmega }\), and \(h\in L^{\infty }(\varOmega ) \).
There exist \(\gamma \in L^{\infty }(\varOmega )\), \(\gamma > 0 \), and θ satisfying \(\theta < p^{-}\) such that
$$ F(x,t)\geq \gamma (x) \vert t \vert ^{\theta }\quad \text{for all }x\in \varOmega \text{ and any } t \in [0,1[, $$
where \(F(x,s)=\int _{0}^{s}f(x,t)\,dt\).

\(f(x,-t)=-f(x,t)\) for \(x\in \varOmega \), \(t\in \mathbb{R}\).

3 Main results

In this section, we give some auxiliary results prior to the establishment of our main results. Here and henceforth, we denote by X the generalized Sobolev space \(W_{0}^{1,p(x)}(\varOmega )\) equipped with the norm \(\| \cdotp \|\), \(X^{*}\) its dual space, and we define the continuous function α by
$$ \alpha (x)=\frac{s(x)q(x)}{s(x)-q(x)}. $$

From assumptions (A1) on the functions p, q, s and from (3.1), a straightforward computation gives \(q(x)< p^{*}(x)\), \(\beta (x) < p^{*}(x)\), \(s'(x)q(x) < p^{*}(x)\), \(\alpha (x) < p^{*}(x) \) for every \(x \in \overline{\varOmega }\). Hence, we have the following remark.

Remark 3.1

From (ii) of Proposition 2.9, the embeddings \(X\hookrightarrow L^{q(x)}(\varOmega )\), \(X\hookrightarrow L^{\alpha (x)}(\varOmega )\), \(X \hookrightarrow L^{s'(x)q(x)}(\varOmega )\), and \(X\hookrightarrow L ^{\beta (x)}(\varOmega )\) are compact and continuous. Therefore, there exists a positive constant C such that
$$ \vert u \vert _{q(\cdot)}\leq C \Vert u \Vert ,\qquad \vert u \vert _{\alpha (\cdot)}\leq C \Vert u \Vert ,\qquad \vert u \vert _{s'(\cdot)q(\cdot)} \leq C \Vert u \Vert , \qquad \vert u \vert _{\beta (\cdot)}\leq C \Vert u \Vert $$
for all \(u\in X\). Without any loss of generality, we can suppose that \(C>1\).
We are interested in the investigation of weak solution of (1.1), say a function \(u\in X\) satisfying
$$ \int _{\varOmega } \vert \nabla u \vert ^{p(x)-2}\nabla u \nabla v\,dx + \int _{\varOmega }V(x) \vert u \vert ^{q(x)-2}uv\,dx = \int _{\varOmega }f(x,u)v\,dx,\quad \forall v\in X. $$
Let us consider the Euler–Lagrange functional or the energy functional \(H: X\to \mathbb{R}\) associated with problem (1.1) defined by
$$ H(u)= \int _{\varOmega }\frac{1}{p(x)} \vert \nabla u \vert ^{p(x)}\,dx + \int _{\varOmega }\frac{V(x)}{q(x)} \vert u \vert ^{q(x)}\,dx- \int _{\varOmega }F(x,u)\,dx . $$
Let us denote \(J(u)=\int _{\varOmega }\frac{V(x)}{q(x)}|u|^{q(x)}\,dx \).
Then the energy functional H can be written as
$$ H(u)=\varPsi (u)+J(u)-\varPhi (u), $$
where we recall that
$$ \varPsi (u)= \int _{\varOmega }\frac{1}{p(x)} \vert \nabla u \vert ^{p(x)}\,dx \quad \text{and}\quad \varPhi (u)= \int _{\varOmega }F(x,u)\,dx. $$

The functional H is obviously well defined and satisfies the following.

Proposition 3.2

The functionalHis continuously Fréchet differentiable and is weakly lower semicontinuous. Moreover, \(u \in X\)is a critical point ofHif and only ifuis a weak solution of (1.1).


By Proposition 2.13 and Proposition 2.15, we have that the functional \(H \in C^{1}(X,\mathbb{R})\) and its derivative function is given by
$$ \bigl\langle dH(u),v \bigr\rangle = \int _{\varOmega } \vert \nabla u \vert ^{p(x)-2}\nabla u \nabla v\,dx+ \int _{\varOmega }V(x) \vert u \vert ^{q(x)-2}uv\,dx - \int _{\varOmega }f(x,u)v\,dx $$
for all \(u,v\in X\). Now, let u be a critical point of H, then we have \(dH(u)=0_{X^{*}}\), which implies that
$$ \bigl\langle dH(u),v \bigr\rangle =0\quad \text{for all }v\in X. $$
$$ \int _{\varOmega } \vert \nabla u \vert ^{p(x)-2}\nabla u \nabla v\,dx+ \int _{\varOmega }V(x) \vert u \vert ^{q(x)-2}uv\,dx= \int _{\varOmega }f(x,u)v\,dx,\quad \forall v\in X. $$
It follows that u is a weak solution of (1.1). On the other hand, if u is a weak solution of (1.1), by definition, we have
$$ \int _{\varOmega } \vert \nabla u \vert ^{p(x)-2}\nabla u \nabla v\,dx+ \int _{\varOmega }V(x) \vert u \vert ^{q(x)-2}uv\,dx= \int _{\varOmega }f(x,u)v\,dx,\quad \forall v\in X, $$
which implies that
$$ \bigl\langle dH(u),v \bigr\rangle =0\quad \text{for all }v\in X. $$
So, \(dH(u)=0_{X^{*}}\) and hence u is a critical point of H.

Let us show that H is sequentially weakly lower semicontinuous.

Consider a weakly convergent sequence \((u_{n})\) to u in X, we have \(\varPsi (u) \leq [4]\liminf_{n \rightarrow +\infty }\varPsi (u_{n})\). By using the Hölder inequality, Proposition 2.5, and inequality (3.2), for any \(n\in \mathbb{N}\), we deduce that
$$ \begin{aligned}[b] \int _{\varOmega }\frac{1}{q(x)} \bigl\vert V(x) \bigr\vert \vert u_{n} \vert ^{q(x)}\,dx & \leq \frac{2}{q ^{-}} \vert V \vert _{s(x)} \max \bigl\{ \vert u_{n} \vert _{s'(x)q(x)}^{q^{-}}, \vert u_{n} \vert _{s'(x)q(x)} ^{q^{+}}\bigr\} \\ & \leq \frac{2}{q^{-}} \vert V \vert _{s(x)}\max \bigl\{ C^{q^{-}} \Vert u _{n} \Vert ^{q^{-}},C^{q^{+}} \Vert u_{n} \Vert ^{q^{+}}\bigr\} . \end{aligned} $$
Recalling assumption (A2) and integrating the growth condition with respect to t, we obtain
$$ \biggl\vert \int _{\varOmega }F(x,u)\,dx \biggr\vert \leq a\varrho _{\beta (\cdot)}(u) + \vert h \vert _{ \infty } \vert u \vert , $$
where \(a\geq 0\) is a constant depending on the real numbers c, \(\beta ^{-}\) provided by (A2). Recalling the compact embeddings \(X \hookrightarrow L^{s'(x)q(x)}(\varOmega )\) and \(X\hookrightarrow L ^{\beta (x)}(\varOmega )\), we deduce that \(J(u_{n}) \rightarrow J(u) \) and \(\varPhi (u) \leq \liminf_{n \rightarrow +\infty }\varPhi (u_{n})\); and hence, H is sequentially weakly lower semicontinuous. The proof is complete. □

Next, we have the following.

Proposition 3.3

Under assumptions (A1), (A1′), (A2), the energy functional (H) is coercive.


Suppose that \(\| u\| >1 \), then
$$ \begin{aligned}[b] H(u) & = \int _{\varOmega }\frac{1}{p(x)} \vert \nabla u \vert ^{p(x)}\,dx + \int _{ \varOmega }\frac{V(x)}{q(x)} \vert u \vert ^{q(x)}\,dx- \int _{\varOmega }F(x,u)\,dx \\ & \geq \frac{1}{p^{+}} \int _{\varOmega } \vert \nabla u \vert ^{p(x)}\,dx - \frac{1}{q ^{-}} \int _{\varOmega _{-}}V(x) \vert u \vert ^{q(x)}\,dx -aC^{\beta ^{+}} \Vert u \Vert ^{\beta ^{+}} - b \Vert u \Vert \\ &\geq \frac{1}{p^{+}} \Vert u \Vert ^{p^{-}} - \frac{1}{q ^{-}}K^{q^{+}_{\varOmega _{-}}} \Vert u \Vert ^{q^{+}_{\varOmega _{-}}} -aC^{\beta ^{+}} \Vert u \Vert ^{\beta ^{+}} - b \Vert u \Vert , \end{aligned} $$
where K, C, and b are positive constants depending on the Sobolev embeddings V and F. Since \(q^{+}_{\varOmega _{-}} < p^{-} \) and \(\beta ^{+} < p^{-} \), we get that H is coercive. □

We are now ready to state the first existence result of this work.

Theorem 3.4

Under assumptions (A1), (A1′), and (A2), problem (1.1) has a weak solution.


Since H is differentiable, coercive, weakly lower semicontinuous, H has a critical minimum point u in X which is a weak solution of (1.1) and then the proof is complete. □

Remark 3.5

Notice that when V is positive, the first condition in (A1′) is not necessary and the proof of the coercivity of H is of course trivial.

Corollary 3.6

Suppose that hypotheses of Theorem 3.4are satisfied. If besides\(V\in L^{s(x)}(\varOmega )\)is a nonnegative function and\(f(x,\cdot ): \mathbb{R}\to \mathbb{R}\)defined by\(f(x,\cdot )(t)=f(x,t)\)is a decreasing function for a.e. \(x\in \varOmega \), then problem (1.1) has a unique weak solution inX.


Let \(u_{1},u_{2}\in E\) such that
$$ \bigl\langle dH(u_{1}),v \bigr\rangle =\bigl\langle dH(u_{2}),v \bigr\rangle = 0\quad \text{for all }v\in X, $$
then we have
$$ \bigl\langle dH(u_{1})-dH(u_{2}), u_{1}- u_{2} \bigr\rangle = 0. $$
From the strict monotonicity of \(-\Delta _{p(x)}\), we have
$$ \begin{aligned}[b] 0 &\leq -\langle \Delta _{p(x)}u_{1}-\Delta _{p(x)}u_{2},u_{1} - u_{2} \rangle \\ &= \int _{\varOmega } \bigl(f\bigl(x,u_{1}(x)\bigr)-f \bigl(x,u_{2}(x)\bigr) \bigr) \bigl(u_{1}(x)-u_{2}(x) \bigr)\,dx \\ &\quad {}- \int _{\varOmega }V(x) \bigl( \bigl\vert u_{1}(x) \bigr\vert ^{q(x)-2}u _{1}(x)- \bigl\vert u_{2}(x) \bigr\vert ^{q(x)-2}u_{2}(x) \bigr) \bigl(u_{1}(x)-u_{2}(x) \bigr)\,dx. \end{aligned} $$
Since the potential V is nonnegative and the function \(f(x,\cdotp )\) is decreasing for a.e. \(x\in \varOmega \), the right-hand side of (3.8) is nonpositive, and then we get
$$ -\langle \Delta _{p(x)}u_{1}-\Delta _{p(x)}u_{2},u_{1} - u_{2}\rangle = 0. $$
Recalling again the strict monotonicity of \(-\Delta _{p(x)}\), we get \(u_{1}=u_{2}\). □

Remark 3.7

In addition to the assumptions in Corollary 3.6, if the Carathéodory function \(f:\varOmega \times \mathbb{R}\to \mathbb{R}\) satisfies (A4), then \(u\equiv 0\) is a trivial solution, and since the solution is unique when it exists, then \(u\equiv 0\) is the only one solution in X for Dirichlet problem (1.1).

In the next step, we will suppose that condition (A1′) is no longer satisfied and of course the coercivity of the functional H fails since V is a sign-changing function. In this case, we prove that H satisfies the Palais–Smale (PS) condition and show multiplicity results for our problem.

Definition 3.8

The \(C^{1}\)-functional H is said to satisfy the Palais–Smale condition, in short the (PS) condition, if any sequence \((u_{n})_{n\in \mathbb{N}}\subseteq X\), for which \((H(u_{n}))_{n \in \mathbb{N}}\subseteq \mathbb{R}\) is bounded and \(dH(u_{n})\to 0\) as \(n\to \infty \), has a convergent subsequence.

Proposition 3.9

Under assumptions (A1) and (A2), the functionalHsatisfies the (PS) condition.

The following lemma plays a key role in the proof of Proposition (3.9). The constant exponent version (\(p(x) \equiv p\)) of the lemma can be found in [14].

Lemma 3.10

Letωbe positive onΩand satisfy\(\omega \in L ^{r(x)} (\varOmega )\)with\(r(x)> \frac{N}{q(x)}\)if\(1< q (x)\leq N \)or\(r(x)= 1 \)if\(q (x) >N\), \(x \in \overline{\varOmega }\), and let\(T > 0 \)be a constant. Then there are three positive constants\(C_{1}\), \(C_{2}\), \(C_{3}\)depending only onωandTsuch that
$$ \varrho _{p(\cdot)}(\nabla u) \leq C_{1} H(u) + C_{2}\min \bigl( \vert u \vert ^{q^{+}}_{ \omega , q(\cdot)}, \vert u \vert ^{q^{-}}_{\omega , q(\cdot)}\bigr) + C_{3}\min \bigl( \vert u \vert ^{\beta ^{+}}_{ \beta (\cdot)}, \vert u \vert ^{\beta ^{-}}_{ \beta (\cdot)}\bigr) $$
for every\(V \in L^{s(x)} (\varOmega )\)such that\(|V|_{s(\cdot)} \leq T\), \(r \in L_{+}^{\infty }(\varOmega ) \), \(p, q, \beta \in C_{+}(\overline{ \varOmega }) \), and for\(u\in X\), where\(|u|_{\omega , q(\cdot)} = \inf \{ \lambda >0: \int _{\varOmega } \omega | \frac{u}{\lambda }|^{q(x)}\,dx \leq 1 \}\).

Remark 3.11

Clearly, in Lemma (3.10), \(\int _{\varOmega }\omega |u|^{q(x)} \,dx < \infty \) for any \(u \in X\). The case \(r(x)= 1 \) with \(q (x) >N \) is obvious since \(p (x) >N \) and then \(X\hookrightarrow L^{\infty }(\varOmega )\).

For the case \(r(x)> \frac{N}{q(x)}\) and \(1< q (x)\leq N \), Propositions 2.3 and 2.5 yield
$$ \int _{\varOmega }\omega \vert u \vert ^{q(x)} \,dx \leq 2 \vert \omega \vert _{r(\cdot)} \bigl\vert |u|^{q(\cdot)} \bigr\vert _{r'(\cdot)}\leq \max \bigl( \vert \omega \vert _{r(\cdot)} \vert u \vert _{q(\cdot)r'(\cdot)}^{q^{+}}, \vert \omega \vert _{r(\cdot)} \vert u \vert _{q(\cdot)r'(\cdot)}^{q^{-}}\bigr). $$
And since \(q(x)r'(x) < p^{*}(x)\) for any \(x\in \overline{ \varOmega }\), \(\int _{\varOmega }\omega |u|^{q(x)} \,dx < \infty\) for any \(u \in X \). Accordingly, \(|u|_{\omega , q(\cdot)} = \inf \{ \lambda >0: \int _{\varOmega }\omega | \frac{u}{\lambda }|^{q(x)}\,dx \leq 1 \}\) stands for the Luxembourg norm on the Orlicz–Musielak space \(L_{M} \) where \(M(x, t ) = \omega (x) t^{q(x)}\). Endowed with this norm, \(L_{M}\) is separable and reflexive (see [19, 24]).

Proof of Lemma 3.10

Recalling (3.4) and (3.5), we have
$$\begin{aligned}& \biggl\vert \int _{\varOmega }\frac{V}{q(x)} \vert u \vert ^{q(x)} \,dx \biggr\vert \leq \frac{2}{q^{-}} \vert V \vert _{s(\cdot)} \max \bigl( \vert u \vert _{q(\cdot)s'(\cdot)}^{q^{+}}, \vert u \vert _{q(\cdot)s'(\cdot)}^{q^{-}}\bigr), \end{aligned}$$
$$\begin{aligned}& \biggl\vert \int _{\varOmega }F(x,u)\,dx \biggr\vert \leq a\varrho _{\beta (\cdot)}(u) + \vert h \vert _{ \infty } \vert u \vert \leq M \max \bigl( \vert u \vert ^{\beta ^{+}}_{ \beta (\cdot)}, \vert u \vert ^{\beta ^{-}} _{ \beta (\cdot)}\bigr), \end{aligned}$$
where \(M\geq 0\) is a constant depending on the real numbers \(|h|_{\infty }\), c, \(\beta ^{-}\) provided by (A2), and on Ω.
Let us claim that, for any \(\epsilon > 0 \), there are \(M_{\epsilon }\) and \(M'_{\epsilon }\) such that
$$\begin{aligned}& \max \bigl( \vert u \vert _{q(\cdot)s'(\cdot)}^{q^{+}}, \vert u \vert _{q(\cdot)s'(\cdot)}^{q^{-}}\bigr) \leq \epsilon \varrho _{p(\cdot)}(\nabla u) + M_{\epsilon }\min \bigl( \vert u \vert _{\omega , q(\cdot)}^{q ^{+}}, \vert u \vert _{\omega , q(\cdot)}^{q^{-}} \bigr), \end{aligned}$$
$$\begin{aligned}& \max \bigl( \vert u \vert ^{\beta ^{+}}_{ \beta (\cdot)}, \vert u \vert ^{\beta ^{-}}_{ \beta (\cdot)}\bigr) \leq \epsilon \varrho _{p(\cdot)}(\nabla u) + M'_{\epsilon }\min \bigl( \vert u \vert ^{ \beta ^{+}}_{ \beta (\cdot)}, \vert u \vert ^{\beta ^{-}}_{ \beta (\cdot)} \bigr). \end{aligned}$$
Let us deal first with (3.12) by assuming to the contrary that there exist \(\epsilon _{0} > 0 \) and a sequence \(u_{n} \text{ in } X \) such that \(|u_{n}|_{q(\cdot)s'(\cdot)} = 1 \) and
$$ \epsilon _{0}\varrho _{p(\cdot)}(\nabla u_{n}) + n\min \bigl( \vert u_{n} \vert _{\omega , q(\cdot)} ^{q^{+}}, \vert u_{n} \vert _{\omega , q(\cdot)}^{q^{-}}\bigr)< 1. $$
Then \((u_{n})\) is a bounded sequence in X and up to a subsequence, \((u_{n})\) converges weakly to some \(u_{0} \in X \) and strongly in \(L^{q(\cdot)s'(\cdot)} (\varOmega ) \). Consequently, \(|u_{0}|_{q(\cdot)s'(\cdot)} = 1 \), and then \(\min (|u_{0}|_{\omega , q(\cdot)}^{q^{+}},|u_{0}|_{\omega , q(\cdot)} ^{q^{-}})< 0\). Contradiction.

A similar approach enables us to get (3.13).

Let \(T > 0 \) be such that \(\max (\frac{2}{q^{-}}|V_{s}(\cdot)|, M) < T\), and let ϵ satisfy \(0 < \epsilon < T^{-1}\). By combining (3.10), (3.11), (3.12), we get
$$ (1-\epsilon T) \varrho _{p(\cdot)}(\nabla u) \leq H(u) +TM_{\epsilon } \min \bigl( \vert u \vert _{\omega , q(\cdot)}^{q^{+}}, \vert u \vert _{\omega , q(\cdot)}^{q^{-}}\bigr) +TM'_{ \epsilon } \min \bigl( \vert u \vert ^{\beta ^{+}}_{ \beta (\cdot)}, \vert u \vert ^{\beta ^{-}}_{ \beta (\cdot)}\bigr), $$
and the proof of the lemma is complete. □

Proof of Proposition 3.9

Let \((u_{n})_{n\in \mathbb{N}}\subset X\) be a (PS) sequence for the functional H, i.e., there exists a positive constant \(k>0\) such that
$$ \bigl\vert H(u_{n}) \bigr\vert \leq k \quad \text{for all }n\in \mathbb{N}, \quad \text{and}\quad \bigl\vert \bigl\langle dH(u _{n}), v\bigr\rangle \bigr\vert \leq \epsilon _{n} \Vert v \Vert \quad \text{for } v \in X, $$
and \(\epsilon _{n} \to 0 \) as \(n\to \infty \).
Let us show that the sequence \((u_{n})_{n\in \mathbb{N}}\) is bounded in X. By contradiction, assume that \(\| u_{n}\| \to \infty\) as \(n \to \infty \). Up to a subsequence, we have \(\| u_{n}\|>1\) for any \(n\in \mathbb{N}\), and by choosing \(\omega = 1 \) in Lemma 3.10 and replacing u by \(u_{n}\) in (3.9), we obtain
$$ \varrho _{p(\cdot)}(\nabla u_{n} ) \leq C_{1} H(u_{n} ) + C_{2} \min \bigl( \vert u _{n} \vert ^{q^{-}}_{ q()}, \vert u_{n} \vert ^{q^{+}}_{ q()}\bigr) + C_{3}\min \bigl( \vert u_{n} \vert ^{ \beta ^{-}}_{ \beta ()}, \vert u_{n} \vert ^{\beta ^{+}}_{ \beta ()}\bigr), $$
and consequently
$$ \Vert u_{n} \Vert ^{p^{-}} \leq C_{1} k + C_{2}' \Vert u_{n} \Vert ^{q^{-}} + C_{3}' \Vert u_{n} \Vert ^{\beta ^{-}}, $$
where \(C_{2}'\), \(C_{3}'\) are constants depending on \(C_{2}\), \(C_{3}\) and the Sobolev embedding constants.
Dividing (3.16) by \(\|u_{n}\|^{p^{-}}\), we have
$$ 1 \leq \frac{C_{1} k }{ \Vert u_{n} \Vert ^{p^{-}}} + C_{2}' \Vert u_{n} \Vert ^{q^{-} - p^{-}} + C_{3}' \Vert u_{n} \Vert ^{\beta ^{-} - p^{-}}, $$
and passing to the limit leads to a contradiction since \(p^{-} > q ^{-} \) and \(p^{-} > \beta ^{-} \). So \((u_{n})\) is bounded in X and up to a subsequence converges weakly in X and strongly in \(L^{r(x)} \) with \(1< r(x) < p^{*}(x) \) to \(u_{0}\).
Let us show now that \((u_{n})_{n\in \mathbb{N}}\) converges strongly to \(u_{0}\) in X. Indeed, since the functional H satisfies the Palais–Smale condition, we have
$$ \begin{aligned} &\langle -\Delta _{p(x)}u_{n},u_{n}-u_{0} \rangle \\ &\quad =\bigl\langle dH(u_{n}),u_{n}-u_{0} \bigr\rangle - \int _{\varOmega }V(x) \vert u_{n} \vert ^{q(x)-2}u_{n}(u_{n}-u_{0})\,dx + \int _{\varOmega }f(x,u_{n}) (u_{n}-u_{0})\,dx \\ &\quad \leq \varepsilon _{n} \Vert u _{n}-u_{0} \Vert +M_{0} \vert V \vert _{s(\cdot)} \vert u_{n} \vert ^{k}_{q(\cdot)} \vert u_{n}-u_{0} \vert _{\alpha (\cdot)} +M_{1}\bigl| \vert u_{n} \vert ^{\beta (\cdotp )-1}\bigr|_{ \frac{\beta (\cdot)}{\beta (\cdot)-1}}|u_{n}-u_{0}|_{\beta (\cdot)} \\ &\qquad {}+M_{2}|h|_{ \infty }|u_{n}-u_{0}|_{\beta (\cdot)},& \end{aligned} $$
where \(\varepsilon _{n}\to 0\) and \(M_{0}\), \(M_{1}\), \(M_{2}\), and \(k\in \{q ^{-}-1,q^{+}-1 \}\) are positive constants. Next, using the Sobolev compact embeddings, we obtain
$$ \limsup_{n\to \infty }\langle -\Delta _{p(x)}u_{n},u_{n}-u_{0} \rangle \leq 0 $$
and then, by the \((S_{+})\) property of \(-\Delta _{p(x)}\), we get that \(u_{n}\to u_{0}\) strongly in X, and the proof is complete. □

We are now ready to state the following existence result for multiple solutions.

Theorem 3.12

Under assumptions (A1), (A2), (A3), and (A4), problem (1.1) has a bounded sequence of weak solutions\((u_{n})_{n\in \mathbb{N}}\subseteq X\)such that\(H(u_{n}) = c_{n} < 0 \). Moreover, the sequence of the critical values\((c_{n})_{n}\)tends to\(c= \inf_{X}H\).

Theorem 3.12 is derived by searching solutions as critical values of the functional H by means of a symmetric version of the mountain pass theorem.

Many versions of the mountain pass theorem stated according to the geometry of the problem under consideration exist in the literature (see [3, 31, 33, 34]). We state below an appropriate version to our situation.

Theorem 3.13

(A symmetric mountain pass theorem )

Suppose thatXis an infinite dimensional real Banach space. Let\(H \in C^{1}(X,\mathbb{R})\)be even and satisfy the (PS) condition and\(H(0)=0\). Assume that
  1. (i)

    there are some constants\(\rho ,\alpha >0\)such that\(H(u)\geq \alpha \)for all\(u\in X\)with\(\| u\|\geq \rho \);

  2. (ii)

    for each finite dimensional subspaceFofX, there is a constant\(R > 0\)such that\(H(u)< 0\)for all\(u\in F\)with\(\| u\| \leq R\).

ThenHhas a bounded sequence of negative critical values\((c_{n})\)tending to\(\inf_{X}H\).

The proof of Theorem 3.13 can be adapted from some works in the literature. We give it as an appendix to this work for the sake of completeness.

4 Proof of Theorem 3.12 and auxiliary results

The proof will consist in showing that H satisfies the geometry required to apply the symmetric mountain pass theorem.

Lemma 4.1

Under assumptions (A1), (A2), and (A3), there exist some constants\(\rho ,\alpha > 0\)such that\(H|_{ \{u\in X: \lVert u\rVert \geq \rho \} }\geq \alpha \).


Suppose on the contrary that, for any \(n \in \mathbb{N}^{*}\), there exists a sequence \((u_{n} ) \in X \) such that \(H(u_{n}) \leq \frac{1}{n}\) for \(\|u_{n} \| \geq n \). Recalling (3.9), we have
$$ 1 \leq C_{1} \frac{1}{n} + C_{2} ' \Vert u_{n} \Vert ^{q^{-} - p^{-} } + C_{3}' \Vert u_{n} \Vert ^{\beta ^{-} - p^{-}} $$
with \(q^{-} - p^{-} < 0 \), \(\beta ^{-} - p^{-} < 0 \) and passing to the limit yields a contradiction. Then Lemma 4.1 holds. □

Lemma 4.2

Suppose that assumptions (A1) and (A3) are satisfied. Then, for each finite dimensional subspaceF, there is a constant\(R>0\)such that\(H(u)\leq 0\)for all\(u\in F\)with\(\| u\| \leq R\).


By assumption (A3),
$$ F(x,t)\geq \gamma (x) \vert t \vert ^{\theta }\quad \text{for all }x\in \varOmega \text{ and for any } t \in [0, 1[. $$
On the other hand, it is easy to see that the functional \(\| \cdotp \|_{\theta }:X\to \mathbb{R}\) defined by
$$ \Vert u \Vert _{\theta }= \biggl( \int _{\varOmega }\gamma (x) \bigl\vert u(x) \bigr\vert ^{\theta }\,dx \biggr)^{\frac{1}{\theta }} $$
is a norm on the space X. Then on the finite dimensional subspace F the norms \(\| \cdotp \|\) and \(\| \cdotp \|_{\theta }\) are equivalent, so there exists a constant \(\nu >0\) such that
$$ \nu \Vert u \Vert \leq \Vert u \Vert _{\theta }\quad \text{for all }u\in F. $$
Now, let \(v\in F\) and write \(v= su \) with \(\|u\| = 1\), and \(0< s < 1 \),
$$ \begin{aligned}[b] H(v)& = \int _{\varOmega }\frac{1}{p(x)} s^{p(x)} \vert \nabla u \vert ^{p(x)}\,dx+ \int _{\varOmega }\frac{V(x)}{q(x)}s^{q(x)} \vert u \vert ^{q(x)}\,dx - \int _{\varOmega } F(x,v)\,dx \\ &\leq \frac{s^{p^{-}}}{p^{-}} \Vert u \Vert ^{p^{+}}+\frac{2C^{q ^{+}}}{q^{-}} \vert V \vert _{s(x)}s^{q^{-}} \Vert u \Vert ^{q^{+}} - s^{\theta } \Vert u \Vert _{\theta }^{\theta }, \end{aligned} $$
where C is a constant deriving from the Sobolev embedding.
Using the fact that \(\|u\| = 1\) and (4.3), we obtain
$$ \begin{aligned}[b] H(v) &\leq \frac{s^{p^{-}}}{p^{-}}+ \frac{2C^{q^{+}}}{q^{-}} \vert V \vert _{s(x)}s ^{q^{-}} -\nu ^{\theta } s^{\theta } \\ &\leq s^{p^{-}} \biggl[\frac{1}{p ^{-}}+\frac{1}{s^{p^{-} - q^{-}}}\biggl( \frac{2C^{q^{+}}}{q^{-}} \vert V \vert _{s(x)} - \nu ^{\theta } \frac{1}{s^{p^{-} -\theta }}\biggr) \biggr]. \end{aligned} $$
Since \(p^{-} > q^{-}\) and \(p^{-} > \theta \), we have for \(s < 1\) small enough that H is nonpositive, that is, there is η such that
$$ H(v) < 0\quad \text{for } 0 < s \leq \eta < 1. $$
Take \(R = \eta \) and then Lemma 4.2 is proved. □

To conclude with the proof of Theorem 3.12, we notice from (A4) that \(H(0) = 0 \) and H is even, and from Lemmas 4.1, 4.2, H satisfies the conditions required in Theorem 3.12, and then the result is achieved.

An auxiliary result in terms of multiple solutions in the same spirit of the works in [1, 6], and [22] can be obtained in our context by means of the fountain theorem. However, the sequence of solutions obtained either by the mountain pass theorem or the fountain theorem are quite different. Of course, in our context, the sequence of critical values obtained via the mountain pass theorem converges to a nonzero limit, while the use of the fountain theorem gives rise to critical values sequence converging to 0 as stated in Theorem 4.3 below.

Theorem 4.3

(Dual fountain theorem)

Suppose thatXis an infinite dimensional reflexive separable Banach space. Let\(H \in C^{1}(X,\mathbb{R})\)be even and satisfy the (PS) condition and\(H(0)=0\).

Write\(X = \overline{\operatorname{span}\{e_{n}, e_{n} \in X \mbox{ } \forall n \geq 1 \}}\), \(X^{*} = \overline{\operatorname{span}\{e^{*} _{n}, e^{*}_{n} \in X^{*} \mbox{ } \forall n \geq 1\}}\)with\(e_{n}^{*}(e _{m}) = 1\)if\(m =n \)and\(e_{n}^{*}(e_{m}) = 0\)if\(m \neq n \), and define the subspaces
$$ X_{k}= \operatorname{span}\{e_{k}, k\geq 1 \}, \qquad Y_{k} = \bigoplus_{j=1}^{k}X_{j}, \qquad Z_{k} =\overline{ \bigoplus_{j=k}^{\infty }X_{j}}. $$
Assume that there are some constants\(\rho _{k} > r_{k} >0\)such that
  1. (i)

    \(H(u)\geq 0\)for all\(u\in Z_{k}\)with\(\| u\|=\rho _{k}\),

  2. (ii)

    \(H(u)< 0\)for all\(u\in Y_{k} \)with\(\| u\| = r_{k}\),

  3. (iii)

    \(d_{k} = \inf_{\{u\in Z_{k}, \|u\| \leq \rho _{k}\}}H(u) \to 0 \)as\(k \to +\infty \).

ThenHhas a sequence of negative critical values\((c_{n})\)tending to 0.

Sketch of the proof

H satisfies of course (PS), and by means of Lemmas 4.1 and 4.2, (i), (ii) are also satisfied. Thus, to prove Theorem 4.3, we need only to show that (iii) is satisfied. Accordingly, we need the following lemma whose proof can be pointed out similarly as in [1, 6], and [22].

Lemma 4.4

$$\beta _{k}= \sup_{\{u\in Z_{k} , \Vert u \Vert = 1\}} \int _{\varOmega } \frac{ \vert V \vert }{q(x)} \vert u_{k} \vert ^{p} \,dx \to 0 $$
$$\beta _{k}'= \sup_{\{u\in Z_{k} , \Vert u \Vert = 1\}} \int _{\varOmega } \bigl\vert F(x, u _{k}) \bigr\vert \,dx \to 0 \quad \textit{as } k \to +\infty . $$
To conclude the proof of Theorem 4.3, we notice that
$$\begin{aligned} H(u)&\geq \int _{\varOmega }\frac{1}{p(x)} \vert \nabla u \vert ^{p(x)}\,dx - \int _{ \varOmega }\frac{ \vert V(x) \vert }{q(x)} \vert u \vert ^{q(x)}\,dx- \int _{\varOmega } \bigl\vert F(x,u) \bigr\vert \,dx . \\ &\geq - \int _{\varOmega }\frac{ \vert V(x) \vert }{q(x)} \vert u \vert ^{q(x)}\,dx- \int _{\varOmega } \bigl\vert F(x,u) \bigr\vert \,dx. \end{aligned}$$
Choosing \(u = tv \) with \(0< t \leq \rho _{k} \) and \(\|v\| = 1\), we have
$$ 0 = H(0) \geq d_{k} = \inf_{\{u\in Z_{k}, \|u\| \leq \rho _{k}\}}H(u) \geq - \beta _{k} - \beta _{k}' \quad \text{for $k$ large enough}. $$
$$ \inf_{\{u\in Z_{k}, \|u\| \leq \rho _{k}\}}H(u)\to 0 \quad \text{as } k \to +\infty . $$



The authors wish to thank the referees for their interesting remarks and suggestions which contributed to the quality of the paper.

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Authors and Affiliations

  1. 1.Institut de Mathématiques et de Sciences PhysiquesUACDangboBénin

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