1 Introduction

In this paper, we are interested in the existence, energy estimates and the convergence property of the least energy sign-changing solution for the following fractional Kirchhoff problems with logarithmic and critical nonlinearity:

$$\begin{aligned} \left\{ \begin{array}{l@{\quad }l} \left( a+b[u]_{s,p}^p\right) (-\Delta )^s_pu = \lambda |u|^{q-2}u\ln |u|^2 + |u|^{ p_s^{*}-2 }u &{}\text {in } \Omega , \\ u=0 &{} \text {in } {\mathbb {R}}^N{\setminus } \Omega , \end{array}\right. \end{aligned}$$
(1.1)

where \(N >sp\) with \(s \in (0, 1)\), \(p>1\), and

$$\begin{aligned}{}[u]_{s,p}^p =\iint _{{\mathbb {R}}^{2N}}\frac{|u(x)-u(y)|^p}{|x-y|^{N+ps}}dxdy, \end{aligned}$$

\(p_s^*=Np/(N-ps)\) is the fractional critical Sobolev exponent, \(\Omega \subset {\mathbb {R}}^N\) \((N\ge 3)\) is a bounded domain with Lipshcitz boundary and \(\lambda \) is a positive parameter. We denote by \((-\Delta )^s_p\) the fractional p-Laplace operator which, up to a normalization constant, is defined as

$$\begin{aligned} (-\Delta )_p^s\varphi (x)=2 \lim _{\varepsilon \rightarrow 0^+}\int _{{\mathbb {R}}^N{\setminus } B_\varepsilon (x)}\frac{|\varphi (x)-\varphi (y)|^{p-2}(\varphi (x)-\varphi (y))}{|x-y|^{N+ps}}\,dy,\quad x\in {\mathbb {R}}^{N}, \end{aligned}$$

for all \(\varphi \in C_0^\infty ({\mathbb {R}}^N)\). Henceforward, \(B_\varepsilon (x)\) denotes the open ball of \({\mathbb {R}}^N\) centered at \(x\in {\mathbb {R}}^N\) and radius \(\varepsilon >0\).

One of the classical topics in the qualitative analysis of PDEs is the study of existence and multiplicity properties of solutions for both the Kirchhoff problems and the fractional Kirchhoff problems under various hypotheses on the nonlinearity. In the recent past there is a vast literature concerning the existence and multiplicity of solutions for the following Dirichlet problem of Kirchhoff type

$$\begin{aligned} \left\{ \begin{array}{l}- \left( a+b\displaystyle \int _{\Omega }|\nabla u|^2dx\right) \Delta u = f(x, u), \quad x\in \Omega ,\\ u|_{\partial \Omega } = 0.\end{array}\right. \end{aligned}$$
(1.2)

Problem (1.2) is a generalization of a model introduced by Kirchhoff [24]. More precisely, Kirchhoff proposed a model given by the equation

$$\begin{aligned} \rho \frac{\partial ^2u}{\partial t^2}-\left( \frac{\rho _0}{h} + \frac{E}{2L}\int _0^L\left| \frac{\partial u}{\partial x}\right| ^2dx\right) \frac{\partial ^2u}{\partial x^2} = 0, \end{aligned}$$
(1.3)

where \(\rho \), \(\rho _0\), hEL are constants. This nonlocal model extends the classical D’Alembert’s wave equation, by considering the effects of the changes in the length of the strings during the vibrations. Since Lions [29] introduced an abstract framework to Kirchhoff-type equations, the solvability of these nonlocal problems has been well studied in the general dimension by various authors. We refer to D’Ancona and Shibata [13] and D’Ancona and Spagnolo [14] for the global solvability of various classes of Kirchhoff-type problems. We also refer to Carrier [9, 10] who used a more rigorous method to model transverse vibration via the coupled governing equation of planar vibration in order to recover the nonlinear integro partial-differential equation, in which a more general Kirchhoff function was considered. In addition, the nonlocal Kirchhoff problems of parabolic type can model several biological systems, such as population density, see for example Ghergu and Rădulescu [21]. For more details on mathematical theories and its applications of Kirchhoff-type problems, we refer the readers to [4, 14, 23, 26, 27, 41].

Problem (1.2) is a nonlocal problem because the term \(b\int _{\Omega }|\nabla u|^2dx\Delta u\) appears in the left-hand side of the equation, which results that (1.2) is not a pointwise identity. Moreover, the energy functional associated to (1.2) has different properties with respect to the local case corresponding to \(b = 0\), hence several mathematical difficulties are brought naturally out in the study of the nonlocal problems \((b \ne 0)\) by means of variational methods. Recently, Fiscella and Valdinoci [19] proposed a steady-state Kirchhoff model involving the fractional Laplacian by taking into account the nonlocal aspect of the tension arising from nonlocal measurements of the fractional length of the string, see [19, Appendix A] for more details. Fractional Kirchhoff-type Laplacian problems have been studied by many authors, refer [2, 3, 20, 28, 34, 36, 37, 48, 49, 51]. Here we can refer the recent monograph about nonlocal fractional problems [35]. We note that the results dealing with the problem (1.2) with critical nonlinearity are relatively scarce. The main difficulty in the study of these problems is due to the lack of compactness caused by the presence of the critical Sobolev exponent.

Recently, most of the literature deals with fractional Laplacian problems with power type nonlinearities, there are a few papers that deal with the existence and multiplicity of solutions for fractional problems involving logarithmic nonlinearity. In [15], d’Avenia et al. considered the following fractional logarithmic Schorödinger equation

$$\begin{aligned} (-\Delta )^su+\omega u=u\log \vert u\vert ^2,\quad x\in {\mathbb {R}}^N, \end{aligned}$$

where \(\omega >0\). By employing the fractional logarithmic Sobolev inequality, [15] obtained the existence of infinitely many solutions. Moreover, the regularity of solutions was also discussed in [15]. In [42], Truong studied the following problem fractional p-Laplacian equations with logarithmic nonlinearity

$$\begin{aligned} (-\Delta )^s_pu+V(x)\vert u\vert ^{p-2}u=\lambda a(x)\vert u\vert ^{p-2}u\ln \vert u\vert ,\quad x\in {\mathbb {R}}^N, \end{aligned}$$

where a is a sign-changing function. Under some assumptions on V, a and \(\lambda \), [42] obtained two nontrivial solutions by using Nehari manifold approach. Very recently, Xiang, Hu and Yang in [47] considered the following Kirchhoff problems with combined nonlinearity of logarithmic and power type

$$\begin{aligned} \left\{ \begin{array}{ll}M([u]^p_{s,p})(-\Delta )^s_pu=h(x)\vert u\vert ^{\theta p-2}u\ln \vert u\vert +\lambda \vert u\vert ^{q-2}u &{}\quad x\in \Omega ,\\ u=0 &{}\quad x\in {\mathbb {R}}^N{\setminus }\Omega ,\end{array}\right. \end{aligned}$$

where \(s\in (0,1)\), \(1<p<N/s\), \(\Omega \) is a bounded domain in \({\mathbb {R}}^N\) with Lipschitz boundary, \(M([u]^p_{s,p})=[u]^{(\theta -1)p}_{s,p}\) with \(\theta \ge 1\), and h is a sign-changing function. When \(\lambda \) is sufficiently small, [47] obtained two nonnegative local least energy solutions by using Nehari manifold approach.

On the other hand, the existence of sign-changing solution of nonlinear elliptic PDEs with power nonlinearities has been studied extensively for the p-Laplacian operator as well as the fractional p-Laplacian operator. We refer the reader to see [5, 6, 11, 12, 30, 44] and the references therein. Consider the nonlocal problem

$$\begin{aligned} \left\{ \begin{array}{l}(-\Delta )^s_pu = f(x, u),\quad x\in \Omega ,\\ u = 0, \quad x\in {\mathbb {R}}^N{\setminus }\Omega . \end{array}\right. \end{aligned}$$
(1.4)

For \(p = 2\), the authors in [11], have studied the problem (1.4), where the fractional Laplacian operator is defined through spectral decomposition to obtain the sign-changing solution. The method of harmonic extension was introduced by Caffarelli and Silvestre [8] to transform the nonlocal problem in \(\Omega \) to a local problem in the half cylinder \(\Omega \times (0, \infty )\), by using an equivalent definition of the fraction Laplacian operator [7]. For \(p \in (1, \infty )\), the problem studied by Chang et al. [12], where the authors have guaranteed the existence of a sign-changing solutions by using Nehari manifold method.

Recently, many authors pay their attention to find sign-changing solutions to problem (1.2) or similar Kirchhoff-type equations, and indeed, some interesting results were obtained. For example, Zhang and Perera [50] and Mao and Zhang [32] used the method of invariant sets of descent flow to obtain the existence of a sign-changing solution of problem (1.2). In [17], Figueiredo and Nascimento considered the following Kirchhoff equation of the type:

$$\begin{aligned} \left\{ \begin{array}{l}- M\left( \int _{\Omega }|\nabla u|^2dx\right) \Delta u = g(u),\quad x\in \Omega ,\\ u|_{\partial \Omega } = 0,\end{array}\right. \end{aligned}$$
(1.5)

where \(\Omega \) is a bounded domain in \({\mathbb {R}}^3\), M is a general \(C^1\) class function, and g is a superlinear \(C^1\) class function with subcritical growth. By using the minimization argument and a quantitative deformation lemma, the existence of a sign-changing solution for this Kirchhoff equation was obtained. In unbounded domains, Figueiredo and Santos Júnior [18] studied a class of nonlocal Schrödinger–Kirchhof problems involving only continuous functions. Using a minimization argument and a quantitative deformation lemma, they obtained a least energy sign-changing solution to Schrödinger–Kirchhof problems. Moreover, when the problem presents symmetry, the authors showed that it has infinitely many nontrivial solutions.

It is noted that combining constraint variational methods and quantitative deformation lemma, Shuai [38] proved that problem (1.2) has one least energy sign-changing solution \(u_b\) and the energy of \(u_b\) strictly larger than the ground state energy. Moreover, the author investigated the asymptotic behavior of \(u_b\) as the parameter \(b\searrow 0\). Later, under some more weak assumptions on g (especially, Nehari type monotonicity condition been removed), with the aid of some new analytical skills and Non-Nehari manifold method, Tang and Cheng [40] improved and generalized some results obtained in [38].

In [16], Deng, Peng, and Shuai studied the following Kirchhoff problem:

$$\begin{aligned} - \left( a+b\displaystyle \int _{{\mathbb {R}}^3}|\nabla u|^2dx\right) \Delta u = f(x, u),\quad x\in {\mathbb {R}}^3. \end{aligned}$$
(1.6)

The authors obtained the existence of radial sign-changing solutions with prescribed numbers of nodal domains for Kirchhoff problem (1.6) in \(H^1_r ({\mathbb {R}}^3)\), the subspace of radial functions of \(H^1({\mathbb {R}}^3)\) by using a Nehari manifold and gluing solution pieces together, when \(V(x)= V(|x|)\), \(f(x, u)= f(|x|, u)\) and satisfies some conditions. Precisely, they proved the existence of a sign-changing solution, which changes signs exactly k times for any \(k \in {\mathbb {N}}\). Moreover, they investigated the energy property and the asymptotic behavior of the sign-changing solution. By using a combination of the invariant set method and the Ljusternik–Schnirelman type minimax method, Sun et al. [39] obtained infinitely many sign-changing solutions for Kirchhoff problem (1.6) when \(f(x, u) = f(u)\) and f is odd in u. It is worth noticing that, in [39], the nonlinear term may not be 4-superlinear at infinity; in particular, it includes the power-type nonlinearity \(|u|^{p-2}u\) with \(p \in (2, 4]\). In [43], the authors obtained the existence of least energy sign-changing solutions of Kirchhoff-type equation with critical growth by using the constraint variational method and the quantitative deformation lemma. For more results on sign-changing solutions for Kirchhoff-type equations, we refer the reader to [16, 25, 31] and the references therein.

2 Abstract setting and main results

To the best of our knowledge, there are no results concerning the existence of sign-changing solutions for fractional Kirchhoff problems with logarithmic and critical nonlinearity. Hence, a natural question is whether or not there exist nodal solutions of problem (1.1)? The goal of the present paper is to develop a thorough qualitative analysis in this direction.

We first recall some preliminary results on the fractional Sobolev space \(W^{s,p}_0(\Omega )\) with respect to the norm \( \Vert u\Vert = [u]_{s,p}\). We then have that \(W^{s,p}_0(\Omega )\) is continuously and compactly embedded into the Lebesgue space \(L^r(\Omega )\) endowed the norm \(|u|_r=\left( \int _\Omega |u|^r\,dx\right) ^\frac{1}{r}\), \(p<r<p_s^{*}\). Denote by \(S_r\) the best constant for this embedding, that is,

$$\begin{aligned} S_r|u|_r\le \Vert u\Vert , \quad \forall u\in W^{s,p}_0(\Omega ). \end{aligned}$$
(2.1)

In particular, if S is the the best constant for the embedding \(W^{s,p}_0(\Omega ) \hookrightarrow L^{p_s^{*}} (\Omega )\), then it is defined by

$$\begin{aligned} S=\inf _{u\in W^{s,p}_0(\Omega )\backslash \{0\}}\frac{\iint _{{\mathbb {R}}^{2N}}\frac{|u(x)-u(y)|^p}{|x-y|^{N+ps}}dxdy}{\left( \int _\Omega |u|^{p_s^{*}}\,dx\right) ^\frac{p}{p_s^{*}}}. \end{aligned}$$
(2.2)

For the weak solution, we mean the one satisfying the given definition.

Definition 2.1

We say that \(u\in W^{s,p}_0(\Omega )\) is a (weak) solution of problem (1.1) if

$$\begin{aligned} \left( a + b[u]_{s,p}^p\right) L(u,v) = \lambda \int _\Omega |u|^{q-2}uv\ln |u|^2dx + \int _\Omega |u|^{p_s^{*}-2}uv\,dx, \end{aligned}$$
(2.3)

where

$$\begin{aligned} L(u,v) := L(u,v) \iint _{\mathbb R^{2N}}\frac{|u(x)-u(y)|^{p-2}(u(x)-u(y))}{|x-y|^{N+sp}}(v(x)-v(y))dxdy \end{aligned}$$
(2.4)

for any \(v\in W^{s,p}_0(\Omega )\).

The corresponding energy functional \(I_b^{\lambda } : W^{s,p}_0(\Omega ) \rightarrow {\mathbb {R}}\) to problem (1.1) is defined by

$$\begin{aligned} I_b^{\lambda }(u) = \frac{a}{p}[u]_{s, p}^p + \frac{b}{2p}[u]_{s, p}^{2p} + \frac{2\lambda }{q^2}\int _\Omega |u|^{q}dx - \frac{\lambda }{q}\int _\Omega |u|^{q}\ln |u|^2dx -\frac{1}{p_s^{*}}\int _\Omega |u|^{p_s^{*}}\,dx. \end{aligned}$$
(2.5)

It is easy to see that \(I_b^{\lambda }\) belongs to \(C^1(W^{s,p}_0(\Omega ), {\mathbb {R}})\) and the critical points of \(I_b^{\lambda }\) are the solutions of (1.1). Furthermore, if we write

$$\begin{aligned} u^+(x) = \max \{u(x), 0\} \quad \text{ and }\quad u^-(x) = \min \{u(x), 0\} \end{aligned}$$

for \(u \in W^{s,p}_0(\Omega )\), then every solution \(u \in W^{s,p}_0(\Omega )\) of problem (1.1) with the property that \(u^\pm \ne 0\) is a sign-changing solution of problem (1.1).

It is noticed that if \(u^\pm \not \equiv 0\)

$$\begin{aligned}&I_b^{\lambda }(u) = I_b^{\lambda }(u^+) + I_b^{\lambda }(u^-) + \frac{b}{p}\Vert u^+\Vert ^p\Vert u^-\Vert ^p,\\&\langle (I_b^{\lambda })'(u), u^+\rangle = \langle (I_b^{\lambda })'(u^+), u^+\rangle + b\Vert u^+\Vert ^p\Vert u^-\Vert ^p,\\&\langle (I_b^{\lambda })'(u), u^-\rangle = \langle (I_b^{\lambda })'(u^-), u^-\rangle + b\Vert u^+\Vert ^p\Vert u^-\Vert ^p. \end{aligned}$$

Our goal in this paper is then to seek the least energy sign-changing solutions of problem (1.1). As we know, there are some very interesting studies, which studied the existence and multiplicity of sign-changing solutions for the following problem:

$$\begin{aligned} -\Delta u+V(x)u=f(x,u), \quad x\in \Omega , \end{aligned}$$
(2.6)

where \(\Omega \) is an open subset of \({\mathbb {R}}^N\). However, these methods of seeking sign-changing solutions heavily rely on the following decompositions:

$$\begin{aligned} J(u)= & {} J(u^+)+ J(u^-), \end{aligned}$$
(2.7)
$$\begin{aligned} \langle J'(u),u^+\rangle= & {} \langle J'(u^+),u^+\rangle , \langle J'(u),u^-\rangle =\langle J'(u^-),u^-\rangle , \end{aligned}$$
(2.8)

where J is the energy functional of (2.4) given by

$$\begin{aligned} J(u) = \frac{1}{2}\int _{\Omega }(|\nabla u|^2+V(x)u^2)dx-\int _{\Omega }F(x,u)dx. \end{aligned}$$

However, if \(b>0\), the energy functional \(I_b^{\lambda }\) does not possess the same decompositions as (2.7) and (2.8). In fact, a straightforward computation yields that

$$\begin{aligned}&I_b^{\lambda } (u)> I_b^{\lambda }(u^{+}) + I_b^{\lambda }(u^{-}),\\&\langle (I_b^{\lambda })'(u), u^{+}\rangle> \langle (I_b^{\lambda })'(u^+), u^+\rangle \quad \text{ and }\quad \langle (I_b^{\lambda })'(u), u^-\rangle > \langle (I_b^{\lambda })'(u^-), u^-\rangle \end{aligned}$$

for \(u^\pm \ne 0\). Therefore, the method to obtain sign-changing solutions for the local problem (2.6) do not seem applicable to problem (1.1). In this paper, we follow the approach in [5] by defining the following constrained set

$$\begin{aligned} {\mathcal {M}}_b^{\lambda } =\{u\in W^{s,p}_0(\Omega ), u^\pm \ne 0\,\, \text{ and }\,\, \langle (I_b^{\lambda })'(u), u^+\rangle = \langle (I_b^{\lambda })'(u),u^-\rangle =0\} \end{aligned}$$
(2.9)

and considering a minimization problem of \(I_b^{\lambda }\) on \({\mathcal {M}}_b^{\lambda }\). Indeed, by using the parametric method and implicit theorem, Shuai [38] proved \({\mathcal {M}}_b^{\lambda } \ne \emptyset \) in the absence of the nonlocal term. However, the nonlocal term in problem (1.1), consisting of the biharmonic operator and the nonlocal term will cause some difficulties. Roughly speaking, compared to the general Kirchhoff type problem (1.2), decompositions (2.7) and (2.8) corresponding to \(I_b^{\lambda }\) are much more complicated. This results in some technical difficulties during the proof of the nonempty of \({\mathcal {M}}_b^{\lambda }\). Moreover, we find that the parametric method and implicit theorem are not applicable for problem (1.1) due to the complexity of the nonlocal term there. Therefore, our proof takes a different route which is inspired by [1], namely, we make use of a modified Miranda’s theorem (see [33]). We are also able to prove that the minimizer of the constrained problem is also a sign-changing solution via the quantitative deformation lemma and degree theory. We can now present our first main result.

Theorem 2.1

There exists \(\lambda ^{*} > 0\) such that for all \(\lambda \ge \lambda ^{*}\), problem (1.1) has a least energy sign-changing solution \(u_b \in {\mathcal {M}}_b^{\lambda }\) with precisely two nodal domains such that \(I_b^{\lambda }(u_b) = \inf _{u\in {\mathcal {M}}_b^{\lambda }} I_b^{\lambda }(u)\).

Another goal of this paper is to establish the so-called energy doubling property (cf. [45]), i.e., the energy of any sign-changing solution of problem (1.1) is strictly larger than twice the ground state energy. For the semilinear equation problem (2.6), the conclusion is trivial. Indeed, if we denote the Nehari manifold associated to problem (2.6) by

$$\begin{aligned} {\mathcal {N}} = \left\{ u\in W^{s,p}_0(\Omega ){\setminus }\{0\}\ |\ \ \langle J'(u), u\rangle = 0\right\} \end{aligned}$$

and define

$$\begin{aligned} c = \inf _{u\in {\mathcal {N}}} J(u) \end{aligned}$$
(2.10)

then it is easy to verify that \(u^\pm \in {\mathcal {N}}\) for any sign-changing solution \(u \in W^{s,p}_0(\Omega )\) to problem (2.6). We can deduce that

$$\begin{aligned} J(w) = J(w^+) + J(w^-) \ge 2c. \end{aligned}$$
(2.11)

We may point out that the minimizer of (2.10) is indeed a ground state solution of problem (2.6) and \(c > 0\) is the least energy of all weak solutions of problem (2.6). Therefore, by (2.11), it follows that the energy of any sign-changing solution of problem (2.6) is larger than twice the least energy. When \(b > 0\), a similar result was obtained by Shuai [38] in a bounded domain \(\Omega \). We are also interested in that whether property (2.11) is still true for problem (1.1). To answer this question, we have the following result:

Theorem 2.2

There exists \(\lambda ^{**} > 0\) such that for all \(\lambda \ge \lambda ^{**}\), the \(c^*:= \inf _{u \in {\mathcal {N}}_b^{\lambda }}I_b^{\lambda }(u) > 0\) is achieved and \(I_b^{\lambda }(u) > 2c^*\), where \({\mathcal {N}}_b^{\lambda } = \left\{ u \in W^{s,p}_0(\Omega ){\setminus }\{0\}\ |\ \langle (I_b^{\lambda })'(u), u \rangle = 0 \right\} \) and u is the least energy sign-changing solution obtained in Theorem 2.1. In particular, \(c^*> 0\) is achieved either by a positive or a negative function.

It is obvious that the energy of the sign-changing solution \(u_b\) obtained in Theorem 2.1 depends on b. In the following, we give a convergence property of \(u_b\) as \(b \rightarrow 0\), which reflects some relationship between \(b > 0\) and \(b = 0\) for problem (1.1).

Theorem 2.3

For any sequence \(\{b_n\}\) with \(b_n \rightarrow 0\) as \(n \rightarrow \infty \), there exists a subsequence, still denoted by \(\{b_n\}\), such that \(\{u_n\}\) converges to \(u_0\) strongly in \(W^{s,p}_0(\Omega )\) as \(n \rightarrow \infty \), where \(u_0\) is a least energy sign-changing solution to the following problem

$$\begin{aligned} \left\{ \begin{array}{l@{\quad }l} a(-\Delta )^s_pu = \lambda |u|^{q-2}u\ln |u|^2 + |u|^{ p_s^{*}-2 }u &{} \text{ in }\ \ \Omega ,\\ u=0 &{} \text{ in }\ \ {\mathbb {R}}^N{\setminus } \Omega . \end{array}\right. \end{aligned}$$
(2.12)

The plan of this paper is as follows: Sect. 2 covers the proof of the achievement of least energy for the constraint problem (1.1), Sect. 3 is devoted to the proofs of our main theorems.

Throughout this paper, we use standard notations. For simplicity, we use “\(\rightarrow \)” and “\(\rightharpoonup \)” to denote the strong and weak convergence in the related function space respectively. Various positive constants are denoted by C and \( C_{i}.\) We use “:=” to denote definitions and \(B_r (x) := \{y \in {\mathbb {R}}^N \ |\ |x -y| < r\}.\) We denote a subsequence of a sequence \(\{u_n \}_n\) as \(\{u_n \}_n\) to simplify the notation unless specified.

3 Some technical lemmas

Now, fixed \(u \in W^{s,p}_0(\Omega )\) with \(u^{\pm } \ne 0\), we define function \(\psi _u : [0, \infty ) \times [0, \infty ) \rightarrow {\mathbb {R}}\) and mapping \(T_u : [0, \infty ) \times [0, \infty ) \rightarrow {\mathbb {R}}^2\) by

$$\begin{aligned} \psi _u(\alpha , \beta ) = I_b^{\lambda }(\alpha u^+ + \beta u^-) \end{aligned}$$
(3.1)

and

$$\begin{aligned} T_u(\alpha , \beta ) = \left( \langle (I_b^{\lambda })'(\alpha u^+ + \beta u^-), \alpha u^+\rangle , \langle (I_b^{\lambda })'(\alpha u^+ + \beta u^-), \beta u^- \rangle \right) . \end{aligned}$$
(3.2)

Lemma 3.1

For any \(u \in W^{s,p}_0(\Omega )\) with \(u^{\pm } \ne 0\), then there is the unique maximum point pair \((\alpha _u, \beta _u)\) of the function \(\psi _u\) such that \(\alpha _uu^+ + \beta _uu^- \in \mathcal {M}_b^{\lambda }\).

Proof

Our proof will be divided into three steps.

Step 1 For any \(u \in W^{s,p}_0(\Omega )\) with \(u^{\pm } \ne 0\), in the following, we will prove the existence of \(\alpha _u\) and \(\beta _u\).

From assumptions, we have that

$$\begin{aligned} \lim _{t\rightarrow 0}\frac{|t|^{q-1}\ln |t|^2}{|t|^{p-1}} = 0 \quad \text{ and }\quad \lim _{t\rightarrow \infty }\frac{|t|^{q-1}\ln |t|^2}{|t|^{r-1}} = 0 \end{aligned}$$
(3.3)

for all \(r \in (q, p_s^*)\). Then for any \(\varepsilon > 0\), there exists \(C_\varepsilon > 0\) such that

$$\begin{aligned} |t|^{q-1}\ln |t|^2 \le \varepsilon |t|^{p-1} + C_\varepsilon |t|^{r-1}. \end{aligned}$$
(3.4)

Since \(4 \le 2p< q < p_s^*\), it follows from (3.4) and the Sobolev embedding theorem that

$$\begin{aligned} \langle (I_b^{\lambda })'(\alpha u^+ + \beta u^-), \alpha u^+\rangle&= a\alpha ^p\Vert u^+\Vert ^p + b\alpha ^{2p}\Vert u^+\Vert ^{2p} + b\alpha ^p\beta ^p\Vert u^+\Vert ^p\Vert u^-\Vert ^p\\&\quad - \lambda \int _{\Omega }|\alpha u^+|^q\ln |\alpha u^+|^2dx -\alpha ^{p_s^{*}}\int _\Omega |u^+|^{p_s^{*}}dx\\&\ge a\alpha ^p\Vert u^+\Vert ^p + b\alpha ^{2p}\Vert u^+\Vert ^{2p} + b\alpha ^p\beta ^p\Vert u^+\Vert ^p\Vert u^-\Vert ^p\\&\quad -\lambda \alpha ^p\varepsilon \int _\Omega |u^+|^{p}dx -\lambda C_\varepsilon \alpha ^{r}\int _\Omega |u^+|^{r}dx- \alpha ^{p_s^{*}}\int _\Omega |u|^{p_s^{*}}dx\\&\ge a\alpha ^p\Vert u^+\Vert ^{p} + b\alpha ^{2p}\Vert u^+\Vert ^{2p} -\lambda \alpha ^p\varepsilon C_1\Vert u^+\Vert ^p \\&\quad - \lambda C_\varepsilon \alpha ^{r}C_2\Vert u^+\Vert ^r - C_3\alpha ^{p_s^{*}}\Vert u^+\Vert ^{p_s^{*}} \\&= \left( a - \lambda \varepsilon C_1\right) \alpha ^p\Vert u^+\Vert ^p + b\alpha ^{2p}\Vert u^+\Vert ^{2p} - \lambda C_\varepsilon \alpha ^{r}C_2\Vert u^+\Vert ^r \\&\quad - C_3\alpha ^{p_s^{*}}\Vert u^+\Vert ^{p_s^{*}}. \end{aligned}$$

Choose \(\varepsilon > 0\) such that \((a - \lambda \varepsilon C_1) > 0\). Since \(p_s^{*}, r > 2p\), we have that \(\langle (I_b^{\lambda })'(\alpha u^+ + \beta u^-), \alpha u^+\rangle > 0\) for \(\alpha \) small enough and all \(\beta \ge 0\).

Similarly, we obtain that \(\langle (I_b^{\lambda })'(\alpha u^+ + \beta u^-), \beta u^-\rangle > 0\) for \(\beta \) small enough and all \(\alpha \ge 0\).

Therefore, there exists \(\delta _1 > 0\) such that

$$\begin{aligned} \langle (I_b^{\lambda })'(\delta _1 u^+ + \beta u^-), \delta _1 u^+\rangle> 0, \quad \langle (I_b^{\lambda })'(\alpha u^+ + \delta _1 u^-), \delta _1 u^-\rangle > 0 \end{aligned}$$
(3.5)

for all \(\alpha , \beta \ge 0\).

On the other hand, we can choose \(\alpha = \delta _2^*> \delta _1\), if \(\beta \in [\delta _1, \delta _2^*]\) and \(\delta _2^*\) is large enough, it follows that

$$\begin{aligned} \langle (I_b^{\lambda })'(\delta _2^*u^+ + \beta u^-), \delta _2^*u^+\rangle&\le a(\delta _2^*)^p\Vert u^+\Vert ^p + b(\delta _2^*)^{2p}\Vert u^+\Vert ^{2p} + b(\delta _2^*)^{2p}\Vert u^+\Vert ^p\Vert u^-\Vert ^p \\&\quad - (\delta _2^*)^{p_s^{*}}\int _\Omega |u^+|^{p_s^{*}}dx \le 0. \end{aligned}$$

Similarly, we have that

$$\begin{aligned} \langle (I_b^{\lambda })'(\alpha u^+ + \delta _2^*u^-), \delta _2^*u^-\rangle&\le (\delta _2^*)^p\Vert u^-\Vert ^p + b(\delta _2^*)^{2p}\Vert u^+\Vert ^{2p} + b(\delta _2^*)^{2p}\Vert u^+\Vert ^p\Vert u^-\Vert ^p \\&\quad - (\delta _2^*)^{p_s^{*}}\int _\Omega |u^-|^{p_s^{*}}dx \le 0. \end{aligned}$$

Let \(\delta _2 > \delta _2^*\) be large enough, we obtain that

$$\begin{aligned} \langle (I_b^{\lambda })'(\delta _2^*u^+ + \beta u^-), \delta _2^*u^+\rangle< 0 \quad \text{ and }\quad \langle (I_b^{\lambda })'(\alpha u^+ + \delta _2^*u^-), \delta _2^*u^-\rangle < 0 \end{aligned}$$
(3.6)

for all \(\alpha , \beta \in [\delta _1, \delta _2]\).

Combining (3.5) and (3.6) with Miranda’s theorem [33], there exists \((\alpha _u, \beta _u) \in (0, +\infty ) \times (0, +\infty )\) such that \(T_u(\alpha , \beta )= (0, 0)\), i.e., \(\alpha u^+ + \beta u^- \in \mathcal {M}_b^{\lambda }\).

Step 2 In this step, we prove the uniqueness of the pair \((\alpha _u, \beta _u)\).

\(\bullet \) Case \(u \in \mathcal {M}_b^{\lambda }\).

If \(u \in \mathcal {M}_b^{\lambda }\), we have that

$$\begin{aligned} \Vert u^+\Vert ^p + b\Vert u^+\Vert ^{2p} + b\Vert u^+\Vert ^p\Vert u^-\Vert ^p = \lambda \int _{\Omega }|u^+|^q\ln |u^+|^2dx +\int _\Omega |u^+|^{p_s^{*}}dx \end{aligned}$$
(3.7)

and

$$\begin{aligned} \Vert u^-\Vert ^2 + b\Vert u^-\Vert ^{2p} + b\Vert u^+\Vert ^p\Vert u^-\Vert ^p = \lambda \int _{\Omega }|u^-|^q\ln |u^-|^2dx +\int _\Omega |u^-|^{p_s^{*}}dx. \end{aligned}$$
(3.8)

We show that \((\alpha _u, \beta _u) = (1, 1)\) is the unique pair of numbers such that \(\alpha _u u^+ + \beta _u u^- \in \mathcal {M}_b^{\lambda }\).

Let \((\alpha _0, \beta _0)\) be a pair of numbers such that \(\alpha _0 u^+ + \beta _0 u^- \in \mathcal {M}_b^{\lambda }\) with \(0 < \alpha _0 \le \beta _0\). Hence, one has that

$$\begin{aligned} \alpha _0^p\Vert u^+\Vert ^p + b\alpha _0^{2p}\Vert u^+\Vert ^{2p} + b\alpha _0^p\beta _0^p\Vert u^+\Vert ^p\Vert u^-\Vert ^p&= \lambda \int _{\Omega }|\alpha _0u^+|^q\ln |\alpha _0u^+|^2dx\nonumber \\&\quad +\, \alpha _0^{p_s^{*}}\int _\Omega |u^+|^{p_s^{*}}dx \end{aligned}$$
(3.9)

and

$$\begin{aligned} \beta _0^p\Vert u^-\Vert ^p + b\beta _0^{2p}\Vert u^-\Vert ^{2p} + b\alpha _0^p\beta _0^p\Vert u^+\Vert ^p\Vert u^-\Vert ^p&= \lambda \int _{\Omega }|\beta _0u^-|^q\ln |\beta _0u^-|^2dx \nonumber \\&\quad +\, \beta _0^{p_s^{*}}\int _\Omega |u^-|^{p_s^{*}}dx. \end{aligned}$$
(3.10)

According to \(0 < \alpha _0 \le \beta _0\) and (3.10), we have that

$$\begin{aligned} \frac{\Vert u^-\Vert ^p}{\beta _0^p} + b\Vert u^-\Vert ^{2p} + b\Vert u^+\Vert ^p\Vert u^-\Vert ^p \ge \lambda \int _{\Omega }\frac{|\beta _0u^-|^q\ln |\beta _0u^-|^2}{\beta _0^{2p}}dx + \beta _0^{p_s^{*}-2p}\int _\Omega |u^-|^{p_s^{*}}dx. \end{aligned}$$
(3.11)

If \(\beta _0 > 1\), by (3.8) and (3.11), one has that

$$\begin{aligned} 0> \left( \frac{1}{\beta _0^p}-1\right) \Vert u^-\Vert ^p&\ge \lambda \int _{\Omega }\left[ \frac{|\beta _0u^-|^q\ln |\beta _0u^-|^2}{\beta _0^{2p}}-|u^-|^q\ln |u^-|^2\right] dx\\&\quad +\, (\beta _0^{p_s^{*}-2p}-1)\int _\Omega |u^-|^{p_s^{*}}dx\\&\ge (\beta _0^{p_s^{*}-2p}-1)\int _\Omega |u^-|^{p_s^{*}}dx > 0. \end{aligned}$$

This is a contradiction. Therefore, we conclude that \(0 < \alpha _0 \le \beta _0 \le 1\).

Similarly, by (3.9) and \(0 < \alpha _0 \le \beta _0\), we have that

$$\begin{aligned} \left( \frac{1}{\alpha _0^p}-1\right) \Vert u^+\Vert ^p&\le \lambda \int _{\Omega }\left[ \frac{|\alpha _0u^+|^q\ln |\alpha _0u^+|^2}{\alpha _0^{2p}}-|u^+|^q\ln |u^+|^2\right] dx\\&\quad + (\beta _0^{p_s^{*}-2p}-1)\int _\Omega |u^+|^{p_s^{*}}dx. \end{aligned}$$

This fact implies that \(\alpha _0 \ge 1\). Consequently, \(\alpha _0 = \beta _0 = 1\).

\(\bullet \) Case \(u \not \in \mathcal {M}_b^{\lambda }\).

Suppose that there exist \((\alpha _1, \beta _1)\), \((\alpha _2, \beta _2)\) such that

$$\begin{aligned} \omega _1 = \alpha _1u^+ + \beta _1u^- \in \mathcal {M}_b^{\lambda } \quad \text{ and }\quad \omega _2 = \alpha _2u^+ + \beta _2u^- \in \mathcal {M}_b^{\lambda }. \end{aligned}$$

Hence

$$\begin{aligned} \omega _2 = \left( \frac{\alpha _2}{\alpha _1}\right) \alpha _1u^+ + \left( \frac{\beta _2}{\beta _1}\right) \beta _1u^- = \left( \frac{\alpha _2}{\alpha _1}\right) \omega _1^+ + \left( \frac{\beta _2}{\beta _1}\right) \omega _1^- \in \mathcal {M}_b^{\lambda }. \end{aligned}$$

By \(\omega _1 \in \mathcal {M}_b^{\lambda }\), one has that

$$\begin{aligned} \frac{\alpha _2}{\alpha _1} = \frac{\beta _2}{\beta _1} = 1. \end{aligned}$$

Hence, \(\alpha _1 = \alpha _2\), \(\beta _1 = \beta _2\).

Step 3 In this step, we will prove that \((\alpha _u, \beta _u)\) is the unique maximum point of \(\psi _u\) on \([0, \infty ) \times [0, \infty )\).

First, it is easy to see that

$$\begin{aligned} 2\tau ^q -q\tau ^q\ln |\tau |^2 \le 2 \quad \text{ for } \text{ all }\ \tau \in (0, \infty ). \end{aligned}$$
(3.12)

Let \(\Omega ^+ = \{x \in \Omega : u(x) > 0\}\) and \(\Omega ^- = \{x \in \Omega : u(x) < 0\}\), \(u \in W^{s,p}_0(\Omega )\) with \(u^{\pm } \ne 0\), we have that

$$\begin{aligned} \int _\Omega |\alpha u^+ + \beta u^-|^{q}\ln |\alpha u^+ + \beta u^-|^2dx&=\nonumber \int _{\Omega ^+} |\alpha u^+ + \beta u^-|^{q}\ln |\alpha u^+ + \beta u^-|^2dx \\&\quad \nonumber + \int _{\Omega ^-} |\alpha u^+ + \beta u^-|^{q}\ln |\alpha u^+ + \beta u^-|^2dx\\&=\nonumber \int _{\Omega ^+} |\alpha u^+|^{q}\ln |\alpha u^+|^2dx + \int _{\Omega ^-} |\beta u^-|^{q}\ln |\beta u^-|^2dx\\&= \int _{\Omega }\left[ |\alpha u^+|^{q}\ln |\alpha u^+|^2 + |\beta u^-|^{q}\ln |\beta u^-|^2\right] dx. \end{aligned}$$
(3.13)

Combining (3.12) and (3.13), we have that

$$\begin{aligned} \psi _u(\alpha , \beta )&= I_b^{\lambda }(\alpha u^+ + \beta u^-)\\&= \frac{1}{p}\Vert \alpha u^+ + \beta u^-\Vert ^p + \frac{b}{2p}\Vert \alpha u^+ + \beta u^-\Vert ^{2p} + \frac{2\lambda }{q^2}\int _\Omega |\alpha u^+ + \beta u^-|^{q}dx\\&\quad - \frac{\lambda }{q}\int _\Omega |\alpha u^+ + \beta u^-|^{q}\ln |\alpha u^+ + \beta u^-|^2dx - \frac{1}{p_s^{*}}\int _{\Omega }|\alpha u^+ + \beta u^-|^{p_s^{*}}dx\\&= \frac{\alpha ^p}{p}\Vert u^+\Vert ^p + \frac{\beta ^p}{p}\Vert u^-\Vert ^p + \frac{b\alpha ^{2p}}{2p}\Vert u^+\Vert ^{2p} + \frac{b\beta ^{2p}}{2p}\Vert u^-\Vert ^{2p} + \frac{b\alpha ^p\beta ^p}{p}\Vert u^+\Vert ^p\Vert u^-\Vert ^p \\&\quad + \frac{\lambda }{q^2}\int _\Omega \left( 2|\alpha u^+|^{q} - q|\alpha u^+|^{q}\ln |\alpha u^+|^2\right) dx \\&\quad + \frac{\lambda }{q^2}\int _\Omega \left( 2|\beta u^-|^{q} - q|\beta u^-|^{q}\ln |\beta u^-|^2\right) dx\\&\quad - \frac{\alpha ^{p_s^{*}}}{p_s^{*}}\int _{\Omega }|u^+|^{p_s^{*}}dx - \frac{\beta ^{p_s^{*}}}{p_s^{*}}\int _{\Omega }|u^-|^{p_s^{*}}dx\\&\le \frac{\alpha ^p}{p}\Vert u^+\Vert ^p + \frac{\beta ^p}{p}\Vert u^-\Vert ^p + \frac{b\alpha ^{2p}}{2p}\Vert u^+\Vert ^{2p} + \frac{b\beta ^{2p}}{2p}\Vert u^-\Vert ^{2p} + \frac{b\alpha ^p\beta ^p}{p}\Vert u^+\Vert ^p\Vert u^-\Vert ^p \\&\quad + \frac{4}{q^2}\lambda |\Omega | - \frac{\alpha ^{p_s^{*}}}{p_s^{*}}\int _{\Omega }|u^+|^{p_s^{*}}dx - \frac{\beta ^{p_s^{*}}}{p_s^{*}}\int _{\Omega }|u^-|^{p_s^{*}}dx \end{aligned}$$

which implies that \(\lim _{|(\alpha ,\beta )|\rightarrow \infty }\psi _u(\alpha ,\beta ) = -\infty \) since \(p_s^{*} > 2p\). Hence, \((\alpha _u, \beta _u)\) is the unique critical point of \(\psi _u\) in \([0, \infty ) \times [0, \infty )\). So it is sufficient to check that a maximum point cannot be achieved on the boundary of \([0, \infty ) \times [0, \infty )\). By contradiction, we suppose that \((0, \beta _0)\) is a maximum point of \(\psi _u\) with \(\beta _0 \ge 0\). Then, we have that

$$\begin{aligned} \psi _u(\alpha , \beta _0)&= \frac{\alpha ^p}{p}\Vert u^+\Vert ^p + \frac{b\alpha ^{2p}}{2p}\Vert u^+\Vert ^{2p} + \frac{2\alpha ^q\lambda }{q^2}\int _\Omega |u^+|^{q}dx - \frac{\alpha ^q\lambda }{q}\int _\Omega |u^+|^{q}\ln |\alpha u^+|^2dx \\&- \frac{\alpha ^{p_s^{*}}}{p_s^{*}}\int _{\Omega }|u^+|^{p_s^{*}}dx +\frac{\beta _0^p}{p}\Vert u^-\Vert ^p + \frac{b\beta _0^{2p}}{2p}\Vert u^-\Vert ^{2p} + \frac{2\beta ^q\lambda }{q^2}\int _\Omega |u^-|^{q}dx \\&\quad - \frac{\beta ^q\lambda }{q}\int _\Omega |u^-|^{q}\ln |\beta u^-|^2dx - \frac{\beta ^{p_s^{*}}}{p_s^{*}}\int _{\Omega }|u^-|^{p_s^{*}}dx\\&\quad + \frac{b\alpha ^p\beta _0^p}{p}\Vert u^+\Vert ^p\Vert u^-\Vert ^p. \end{aligned}$$

Therefore, it is obvious that

$$\begin{aligned} (\psi _u)_\alpha '(\alpha , \beta _0)&= \alpha ^{p-1}\Vert u^+\Vert ^p + b\alpha ^{2p-1}\Vert u^+\Vert ^{2p} + b\alpha ^{p-1}\beta _0^p\Vert u^+\Vert ^p\Vert u^-\Vert ^p \\&\quad + \frac{2\alpha ^{q-1}\lambda }{q}\int _\Omega |u^+|^{q}dx - \alpha ^{q-1}\int _\Omega |u^+|^{q}\ln |\alpha u^+|^2dx - \alpha ^{p_s^{*}-1}\int _{\Omega }|u^+|^{p_s^{*}}dx\\&> 0, \end{aligned}$$

if \(\alpha \) is small enough. That is, \(\psi _u\) is an increasing function with respect to \(\alpha \) if \(\alpha \) is small enough. This yields the contradiction. Similarly, \(\psi _u\) can not achieve its global maximum on \((\alpha , 0)\) with \(\alpha \ge 0\). \(\square \)

Lemma 3.2

For any \(u \in W^{s,p}_0(\Omega )\) with \(u^{\pm } \ne 0\) such that \(\langle (I_b^{\lambda })'(u), u^{\pm }\rangle \le 0\). Then, the unique maximum point of \(\psi _u\) on \([0, \infty ) \times [0, \infty )\) satisfies \(0 < \alpha _u, \beta _u \le 1\).

Proof

Without loss of generality, let \(\alpha _u \ge \beta _u > 0\).

On the one hand, by \(\alpha _uu^+ + \beta _uu^- \in \mathcal {M}_b^{\lambda }\), we have

$$\begin{aligned} a\alpha _u^p\Vert u^+\Vert ^p + b\alpha _u^{2p}\Vert u^+\Vert ^{2p} + b\alpha _u^{2p}\Vert u^+\Vert ^p\Vert u^-\Vert ^p&= \lambda \alpha _u^q\int _\Omega |u^+|^{q}\ln |\alpha _uu^+|^2dx \nonumber \\&\quad - \alpha _u^{p_s^{*}}\int _{\Omega }|u^+|^{p_s^{*}}dx. \end{aligned}$$
(3.14)

On the other hand, by \(\langle (I_b^{\lambda })'(u), u^+ \rangle \le 0\), we have

$$\begin{aligned} a\Vert u^+\Vert ^p + b\Vert u^+\Vert ^{2p} + b\Vert u^+\Vert ^p\Vert u^-\Vert ^p \le \lambda \int _\Omega |u^+|^{q}\ln |u^+|^2dx + \int _{\Omega }|u^+|^{p_s^{*}}dx. \end{aligned}$$
(3.15)

So, according to (3.14) and (3.15), we have that

$$\begin{aligned} \left( \frac{1}{\alpha _u^p}-1\right) a\Vert u^+\Vert ^p&\ge \lambda \int _{\Omega }\left[ \alpha _u^{q-2p}|u^+|^{q}\ln |\alpha _uu^+|^2-|u^+|^{q}\ln |u^+|^2\right] dx \nonumber \\&\quad + (\alpha _u^{p_s^{*}-2p}-1)\int _{\Omega }|u^+|^{p_s^{*}}dx. \end{aligned}$$
(3.16)

If \(\alpha _u \ge 1\), one has

$$\begin{aligned} \int _{\Omega }\left[ \alpha _u^{q-2p}|u^+|^{q}\ln |\alpha _uu^+|^2-|u^+|^{q}\ln |u^+|^2\right] dx \ge 0. \end{aligned}$$

This fact together with (3.16), we have

$$\begin{aligned} 0> \left( \frac{1}{\alpha _u^p}-1\right) a\Vert u^+\Vert ^p&\ge (\alpha _u^{p_s^{*}-2p}-1)\int _{\Omega }|u^+|^{p_s^{*}}dx > 0. \end{aligned}$$

This is a contradiction. Thus, we conclude that \(\alpha _u \le 1\). Thus, we have that \(0 < \alpha _u, \beta _u \le 1\). \(\square \)

Lemma 3.3

Let \(c_b^{\lambda } = \inf _{u \in \mathcal {M}_b^{\lambda }}I_b^{\lambda }(u)\), then we have that \(\lim _{\lambda \rightarrow \infty }c_b^{\lambda } = 0\).

Proof

For any \(u \in \mathcal {M}_b^{\lambda }\), we have

$$\begin{aligned} a\Vert u^\pm \Vert ^p + b\Vert u^\pm \Vert ^{2p} + b\Vert u^+\Vert ^p\Vert u^-\Vert ^p = \lambda \int _\Omega |u^\pm |^{q}\ln |u^\pm |^2dx + \int _{\Omega } |u^\pm |^{p_s^{*}}dx. \end{aligned}$$

Then, by (3.4) and the Sobolev theorem, we have that

$$\begin{aligned} a\Vert u^\pm \Vert ^p&\le \lambda \int _\Omega |u^\pm |^{q}\ln |u^\pm |^2dx + \int _{\Omega } |u^\pm |^{p_s^{*}}dx \\&\le \lambda \varepsilon C_1 \Vert u^\pm \Vert ^p + \lambda C_\varepsilon C_2\Vert u^\pm \Vert ^r + C_3\Vert u^\pm \Vert ^{p_s^{*}}. \end{aligned}$$

Thus, we get

$$\begin{aligned} (1 - \lambda \varepsilon C_1)a\Vert u^\pm \Vert ^p \le \lambda C_\varepsilon C_2\Vert u^\pm \Vert ^r + C_3\Vert u^\pm \Vert ^{p_s^{*}}. \end{aligned}$$

Choosing \(\varepsilon \) small enough such that \(1 - \lambda \varepsilon C_1 > 0\), since \(r, p_s^{*} > p\), there exists \(\rho > 0\) such that

$$\begin{aligned} \Vert u^\pm \Vert \ge \rho \quad \text{ for } \text{ all }\ \ u \in \mathcal {M}_b^{\lambda }. \end{aligned}$$
(3.17)

On the other hand, for any \(u \in \mathcal {M}_b^{\lambda }\), it is obvious that \(\langle (I_b^{\lambda })'(u), u \rangle = 0\). Then, we have

$$\begin{aligned} I_b^{\lambda }(u)&= I_b^{\lambda }(u) -\frac{1}{q}\langle (I_b^{\lambda })'(u), u\rangle \\&= \left( \frac{1}{p} - \frac{1}{q}\right) a\Vert u\Vert ^p + \left( \frac{1}{2p} - \frac{1}{q}\right) b\Vert u\Vert ^{2p} + \left( \frac{1}{q}-\frac{1}{p_s^{*}}\right) \int _{\Omega }|u|^{p_s^{*}}dx\\&\quad + \frac{2\lambda }{q^2}\int _\Omega |u|^{q}dx \ge \left( \frac{1}{p} - \frac{1}{q}\right) a\Vert u\Vert ^p. \end{aligned}$$

From above discussions, we have that \(I_b^{\lambda }(u) > 0\) for all \(u \in \mathcal {M}_b^{\lambda }\). Therefore, \(I_b^{\lambda }\) is bounded below on \(\mathcal {M}_b^{\lambda }\), that is \(c_b^{\lambda } = \inf _{u \in \mathcal {M}_b^{\lambda }}I_b^{\lambda }(u)\) is well defined.

Let \(u \in W^{s,p}_0(\Omega )\) with \(u^\pm \ne 0\) be fixed. By Lemma 3.1, for each \(\lambda > 0\), there exist \(\alpha _\lambda , \beta _\lambda > 0\) such that \(\alpha _\lambda u^+ +\beta _\lambda u^- \in \mathcal {M}_b^{\lambda }\). By using Lemma 3.1 again, we have that

$$\begin{aligned} 0 \le c_b^{\lambda }&= \inf _{u \in \mathcal {M}_b^{\lambda }}I_b^{\lambda }(u) \le I_b^{\lambda }(\alpha _\lambda u^+ +\beta _\lambda u^-)\\&\le \frac{1}{p}a\Vert \alpha _\lambda u^+ +\beta _\lambda u^-\Vert ^p + \frac{b}{2p}\Vert \alpha _\lambda u^+ +\beta _\lambda u^-\Vert ^{2p}\\&\le a\alpha _\lambda ^p\Vert u^+\Vert ^p + a\beta _\lambda ^p\Vert u^-\Vert ^p + pb\alpha _\lambda ^{2p}\Vert u^+\Vert ^{2p} + pb\beta _\lambda ^{2p}\Vert u^-\Vert ^{2p}. \end{aligned}$$

To our end, we just prove that \(\alpha _\lambda \rightarrow 0\) and \(\beta _\lambda \rightarrow 0\) as \(\lambda \rightarrow \infty \).

Let

$$\begin{aligned} \mathcal {T}_u = \left\{ (\alpha _\lambda , \beta _\lambda ) \in [0, \infty )\times [0, \infty ) : T_u(\alpha _\lambda , \beta _\lambda ) = (0, 0), \lambda > 0\right\} , \end{aligned}$$

where \(T_u\) is defined as (3.2). By (3.3), we have that

$$\begin{aligned}&\alpha _\lambda ^{p_s^{*}}\int _{\Omega }|u^+|^{p_s^{*}}dx + \beta _\lambda ^{p_s^{*}}\int _{\Omega }|u^-|^{p_s^{*}}dx \\&\quad \le \alpha _\lambda ^{p_s^{*}}\int _{\Omega }|u^+|^{p_s^{*}}dx + \beta _\lambda ^{p_s^{*}}\int _{\Omega }|u^-|^{p_s^{*}}dx + \lambda \alpha _\lambda ^q\int _\Omega |u^+|^{q}\ln |u^+|^2dx \\&\quad \quad +\, \lambda \beta _\lambda ^q \int _\Omega |u^+|^{q}\ln |u^+|^2dx\\&\quad = \Vert \alpha _\lambda u^+ + \beta _\lambda u^-\Vert ^p + b\Vert \alpha _\lambda u^+ +\beta _\lambda u^-\Vert ^{2p}\\&\quad \le p\alpha _\lambda ^p\Vert u^+\Vert ^p + p\beta _\lambda ^p\Vert u^-\Vert ^p + 2pb\alpha _\lambda ^{2p}\Vert u^+\Vert ^{2p} + 2pb\beta _\lambda ^{2p}\Vert u^-\Vert ^{2p}. \end{aligned}$$

Hence, \(\mathcal {T}_u\) is bounded since \(2p < p_s^{*}\). Let \(\{\lambda _n\} \subset (0, \infty )\) be such that \(\lambda _n \rightarrow \infty \) as \(n \rightarrow \infty \). Then, there exist \(\alpha _0\) and \(\beta _0\) such that \((\alpha _{\lambda _n}, \beta _{\lambda _n}) \rightarrow (\alpha _0, \beta _0)\) as \(n \rightarrow \infty \).

Now, we claim \(\alpha _0 = \beta _0 = 0\). Suppose, by contradiction, that \(\alpha _0 > 0\) or \(\beta _0 > 0\). By \(\alpha _{\lambda _n} u^+ +\beta _{\lambda _n} u^- \in \mathcal {M}_b^{\lambda _n}\), for any \(n \in {\mathbb {N}}\), we have

$$\begin{aligned}&\nonumber \Vert \alpha _{\lambda _n} u^+ +\beta _{\lambda _n} u^-\Vert ^p + b\Vert \alpha _{\lambda _n} u^+ +\beta _{\lambda _n} u^-\Vert ^{2p} \\&\quad = \lambda _n \int _\Omega |\alpha _{\lambda _n} u^+ +\beta _{\lambda _n} u^-|^{q}\ln |\alpha _{\lambda _n} u^+ +\beta _{\lambda _n} u^-|^2dx + \int _{\Omega }|\alpha _{\lambda _n} u^+ +\beta _{\lambda _n} u^-|^{p_s^{*}}dx. \end{aligned}$$
(3.18)

Thanks to \(\alpha _{\lambda _n} u^+ \rightarrow \alpha _0 u^+\) and \(\beta _{\lambda _n} u^- \rightarrow \beta _0 u^+\) in \(W^{s,p}_0(\Omega )\), (3.4) and the Lebesgue dominated convergence theorem, we have that

$$\begin{aligned}&\int _\Omega |\alpha _{\lambda _n} u^+ +\beta _{\lambda _n} u^-|^{q}\ln |\alpha _{\lambda _n} u^+ +\beta _{\lambda _n} u^-|^2dx \\&\quad \rightarrow \int _\Omega |\alpha _{0} u^+ +\beta _{0} u^-|^{q}\ln |\alpha _{0} u^+ +\beta _{0} u^-|^2dx > 0 \end{aligned}$$

as \(n \rightarrow \infty \). It follows from \(\lambda _n \rightarrow \infty \) as \(n \rightarrow \infty \) and \(\{\alpha _{\lambda _n} u^+ +\beta _{\lambda _n} u^-\}\) is bounded in \(W^{s,p}_0(\Omega )\) that we have a contradiction with equality (3.18). Hence, \(\alpha _0 = \beta _0 = 0\).

Hence, we conclude that \(\lim _{\lambda \rightarrow \infty } c_b^{\lambda } = 0\). \(\square \)

Lemma 3.4

There exists \(\lambda ^*> 0\) such that for all \(\lambda \ge \lambda ^*\), the infimum \(c_b^{\lambda }\) is achieved.

Proof

By the definition of \(c_b^{\lambda }\), there exists a sequence \(\{u_n\} \subset \mathcal {M}_b^{\lambda }\) such that

$$\begin{aligned} \lim _{n\rightarrow \infty }I_b^{\lambda }(u_n) = c_b^{\lambda }. \end{aligned}$$

Obviously, \(\{u_n\}\) is bounded in \(W^{s,p}_0(\Omega )\). Then, up to a subsequence, still denoted by \(\{u_n\}\), there exists \(u \in W^{s,p}_0(\Omega )\) such that \(u_n \rightharpoonup u\). Since the embedding \(W^{s,p}_0(\Omega )\hookrightarrow L^t(\Omega )\) is compact, for all \(t \in (p, p_s^{*})\), we have

$$\begin{aligned} u_n \rightarrow u \quad \text{ in }\ \ L^t(\Omega ), \quad u_n \rightarrow u \quad \text{ a.e. } \ \ x \in \Omega . \end{aligned}$$

Hence

$$\begin{aligned}&u_n^\pm \rightharpoonup u^\pm \ \ \text{ in }\ W^{s,p}_0(\Omega ), \\&u_n^\pm \rightarrow u^\pm \ \ \text{ in }\ L^t(\Omega ), \\&u_n^\pm \rightarrow u^\pm \ \ \text{ a.e. } \ x \in \Omega . \end{aligned}$$

By Lemma 3.1, we have that

$$\begin{aligned} I_b^{\lambda }(\alpha u_n^++ \beta u_n^-)\le I_b^{\lambda }(u_n) \end{aligned}$$

for all \(\alpha , \beta \ge 0\).

On the one hand, the Vitali convergence theorem yields that

$$\begin{aligned} \lim _{n\rightarrow \infty }\int _\Omega |u_n|^{q}\ln |u_n|^2dx \rightarrow \int _\Omega |u|^{q}\ln |u|^2dx. \end{aligned}$$
(3.19)

On the other hand, since \(u_n \rightarrow u\) in \(L^q(\Omega )\), we have

$$\begin{aligned} \lim _{n\rightarrow \infty }\int _\Omega |u_n|^{q}dx \rightarrow \int _\Omega |u|^{q}dx. \end{aligned}$$
(3.20)

Then, by (3.19), (3.20), Brézis–Lieb lemma and the weak semicontinuity of norm, we have

$$\begin{aligned}&\liminf _{n\rightarrow \infty }I_b^{\lambda }(\alpha u_n^++ \beta u_n^-)\\&\quad \ge \frac{a\alpha ^p}{p} \lim _{n\rightarrow \infty }( \Vert u_n^+-u^+\Vert ^p+\Vert u^+\Vert ^p) +\frac{a\beta ^p}{p}\lim _{n\rightarrow \infty }(\Vert u_n^- - u^-\Vert ^p+\Vert u^-\Vert ^p)\\&\qquad + \frac{b\alpha ^{2p}}{2p}\left[ \lim _{n\rightarrow \infty }( \Vert u_n^+-u^+\Vert ^p+\Vert u^+\Vert ^p)\right] ^2 +\frac{b\beta ^{2p}}{2p}\left[ \lim _{n\rightarrow \infty }(\Vert u_n^- - u^-\Vert ^p+\Vert u^-\Vert ^p)\right] ^2\\&\qquad - \frac{\alpha ^{p_s^{*}}}{p_s^{*}}\lim _{n\rightarrow \infty }\left[ \int _{\Omega }|u_{n}^+-u^+|^{p_s^{*}}dx + \int _{\Omega }|u^+|^{p_s^{*}}dx \right] \\&\qquad -\frac{\beta ^{p_s^{*}}}{p_s^{*}}\lim _{n\rightarrow \infty }\left[ \int _{\Omega }|u_{n}^--u^-|^{p_s^{*}}dx + \int _{\Omega }|u^-|^{p_s^{*}}dx \right] \\&\qquad + \frac{2\lambda }{q^2}\int _\Omega |u|^{q}dx - \frac{\lambda }{q}\int _\Omega |u|^{q}\ln |u|^2dx + \frac{b\alpha ^2\beta ^2}{2}\liminf _{n\rightarrow \infty }(\Vert u_n^+\Vert ^2 \Vert u_n^-\Vert ^2)\\&\quad \ge I_b^{\lambda }(\alpha u^++\beta u^-) + \frac{a\alpha ^p}{p} \lim _{n\rightarrow \infty } \Vert u_n^+-u^+\Vert ^p + \frac{a\beta ^p}{p}\lim _{n\rightarrow \infty }\Vert u_n^- - u^-\Vert ^p\\&\qquad + \frac{b\alpha ^{2p}}{p}\lim _{n\rightarrow \infty } \Vert u_n^+-u^+\Vert ^p \Vert u^+\Vert ^p +\frac{b\beta ^{2p}}{p}\lim _{n\rightarrow \infty } \Vert u_n^- - u^-\Vert ^p \Vert u^-\Vert ^p \\&\qquad + \frac{b\alpha ^{2p}}{2p}(\lim _{n\rightarrow \infty } \Vert u_n^+-u^+\Vert ^p )^2 +\frac{bt^{2p}}{2p}(\lim _{n\rightarrow \infty } \Vert u_n^- - u^-\Vert ^p)^2 \\&\qquad - \frac{\alpha ^{p_s^{*}}}{p_s^{*}}\int _{\Omega }|u_{n}^+-u^+|^{p_s^{*}}dx - \frac{\beta ^{p_s^{*}}}{p_s^{*}}\int _{\Omega }|u_{n}^--u^-|^{p_s^{*}}dx\\&\quad \ge I_b^{\lambda }(\alpha u^++\beta u^-)+\frac{a\alpha ^p}{p}A_1 + \frac{b\alpha ^{2p}}{p}A_1\Vert u^+\Vert ^p+ \frac{b\alpha ^{2p}}{2p}A_1^p - \frac{a\alpha ^{p_s^{*}}}{p_s^{*}}B_1\\&\qquad + \frac{a\beta ^p}{p}A_2 + \frac{b\beta ^{2p}}{p}A_2\Vert u^-\Vert ^p+ \frac{b\beta ^{2p}}{2p}A_2^p - \frac{\beta ^{p_s^{*}}}{p_s^{*}}B_2, \end{aligned}$$

where

$$\begin{aligned}&A_1=\lim _{n\rightarrow \infty } \Vert u_n^+-u^+\Vert ^p, \quad A_2=\lim _{n\rightarrow \infty }\Vert u_n^- - u^-\Vert ^p,\\&B_1=\lim _{n\rightarrow \infty } |u_n^+-u^+|^{p_s^{*}}_{p_s^{*}},\quad B_2=\lim _{n\rightarrow \infty }|u_n^- - u^-|^{p_s^{*}}_{p_s^{*}}. \end{aligned}$$

That is, one has that

$$\begin{aligned}&I_b^{\lambda }(\alpha u^++\beta u^-)+\frac{a\alpha ^p}{p}A_1 + \frac{b\alpha ^{2p}}{p}A_1\Vert u^+\Vert ^p+ \frac{b\alpha ^{2p}}{2p}A_1^p - \frac{\alpha ^{p_s^{*}}}{p_s^{*}}B_1\nonumber \\&+ \frac{a\beta ^p}{p}A_2 + \frac{b\beta ^{2p}}{p}A_2\Vert u^-\Vert ^p+ \frac{b\beta ^{2p}}{2p}A_2^p - \frac{\beta ^{p_s^{*}}}{p_s^{*}}B_2 \le c_b^{\lambda } \end{aligned}$$
(3.21)

for all \(\alpha \ge 0\) and all \(\beta \ge 0\).

Now, we claim that \(u^\pm \ne 0\).

In fact, since the situation \(u^- \ne 0\) is analogous, we just prove \(u^+ \ne 0\). By contradiction, we suppose \(u^+ = 0\). Hence, let \(\beta = 0\) in (3.17) and we have that

$$\begin{aligned} \frac{a\alpha ^p}{p}A_1 + \frac{b\alpha ^{2p}}{2p}A_1^2- \frac{\alpha ^{p_s^{*}}}{p_s^{*}}B_1\le c_b^{\lambda } \end{aligned}$$
(3.22)

for all \(\alpha \ge 0\).

Case 1: \(B_1=0\).

If \(A_1=0\), that is, \(u_n^+ \rightarrow u^+\) in \(W^{s,p}_0(\Omega )\). In view of Lemma (3.22), we obtain \(\Vert u^+\Vert >0\), which contradicts our supposition. If \(A_1>0\), by (3.22), we have that

$$\begin{aligned} \frac{a\alpha ^p}{p}A_1 + \frac{b\alpha ^{2p}}{2p}A_1^2 \le c_b^{\lambda } \end{aligned}$$

for all \(\alpha \ge 0\), which is absurd by Lemma 3.3. Anyway, we have a contradiction.

Case 2: \(B_1>0\).

One one hand, by Lemma 3.3, there exists \(\lambda ^*> 0\) such that

$$\begin{aligned} c_b^{\lambda } < \frac{s}{N}(aS)^{N/ps} \quad \text{ for } \text{ all } \ \lambda \ge \lambda ^*, \end{aligned}$$
(3.23)

where \(S>0\) is given by (2.2).

On the other hand, since \(B_1>0\), we obtain \(A_1>0\). Hence, in view of (3.22), we have that

$$\begin{aligned} \frac{s}{N}(aS)^{N/ps} \le \max _{\alpha \ge 0}\left\{ \frac{a\alpha ^p}{p}A_1 - \frac{\alpha ^{p_s^{*}}}{p_s^{*}}B_1\right\} \le \max _{\alpha \ge 0}\left\{ \frac{a\alpha ^p}{p}A_1 + \frac{b\alpha ^{2p}}{2p}A_1^2- \frac{\alpha ^{p_s^{*}}}{p_s^{*}}B_1\right\} \le c_b^{\lambda }, \end{aligned}$$

which is a contradiction. That is, we deduce that \(u^\pm \ne 0\).

Second, we prove \(B_1=B_2=0\).

Since the situation \(B_2=0\) is analogous, we only prove \(B_1=0\). By contradiction, we suppose that \(B_1>0\).

Case 1: \(B_2>0\).

According to \(B_1,B_2>0\) and Sobolev embedding, we obtain that \(A_1,A_2>0\). Let

$$\begin{aligned} \phi (\alpha )= \frac{a\alpha ^p}{p}A_1 + \frac{b\alpha ^{2p}}{2p}A_1^2- \frac{\alpha ^{p_s^{*}}}{p_s^{*}}B_1 \quad \text{ for } \text{ all }\ \ \alpha \ge 0. \end{aligned}$$

It is easy to see that \(\phi (\alpha )>0\) for \(\alpha > 0\) small enough and \(\phi (\alpha ) <0\) for \(\alpha < 0\) large enough. Hence, by continuous of \(\phi (\alpha )\), there exists \({\hat{\alpha }}>0\) such that

$$\begin{aligned} \frac{a{\hat{\alpha }}^p}{p}A_1 +\frac{b{\hat{\alpha }}^{2p}}{2p}A_1^2 - \frac{{\hat{\alpha }}^{p_s^{*}}}{p_s^{*}}B_1 =\max _{\alpha \ge 0}\left\{ \frac{a\alpha ^p}{p}A_1 + \frac{b\alpha ^{2p}}{2p}A_1^2- \frac{\alpha ^{p_s^{*}}}{p_s^{*}}B_1\right\} . \end{aligned}$$

Similarly, there exists \({\hat{\beta }}>0\) such that

$$\begin{aligned} \frac{a{\hat{\beta }}^p}{p}A_2 +\frac{b{\hat{\beta }}^{2p}}{2p}A_2^2 - \frac{{\hat{\beta }}^{p_s^{*}}}{p_s^{*}}B_2 = \max _{\alpha \ge 0}\left\{ \frac{a\alpha ^p}{p}B_1 + \frac{b\alpha ^{2p}}{2p}B_1^2- \frac{\alpha ^{p_s^{*}}}{p_s^{*}}B_1\right\} . \end{aligned}$$

Since \([0,{\hat{\alpha }}]\times [0, {\hat{\beta }}]\) is compact and \(\phi \) is continuous, there exists \((\alpha _u, \beta _u)\in [0,{\hat{\alpha }}]\times [0, {\hat{\beta }}]\) such that

$$\begin{aligned} \phi (\alpha _u, \beta _u)=\max _{(\alpha ,\beta )\in [0,{\hat{\alpha }}]\times [0, {\hat{\beta }}]}\phi (\alpha ,\beta ). \end{aligned}$$

Now, we prove that \((\alpha _u, \beta _u) \in (0,{\hat{\alpha }})\times (0, {\hat{\beta }})\).

Note that, if \(\beta \) is small enough, we have that

$$\begin{aligned} \phi (\alpha ,0)= I_b^{\lambda }(\alpha u^+)< I_b^{\lambda }(\alpha u^+) + I_b^{\lambda }(\beta u^-)\le I_b^{\lambda }(\alpha u^+ + \beta u^-) = \phi (\alpha , \beta ) \end{aligned}$$

for all \(\alpha \in [0,{\hat{\alpha }}]\).

Hence, there exists \(\beta _0\in [0, {\hat{\beta }}]\) such that

$$\begin{aligned} \phi (\alpha ,0) \le \phi (\alpha , \beta _0) \quad \text{ for } \text{ all }\ \alpha \in [0, {\hat{\alpha }}]. \end{aligned}$$

That is, any point of \((\alpha , 0)\) with \(0\le \alpha \le {\hat{\alpha }}\) is not the maximizer of \(\phi \). Hence, \((\alpha _u, \beta _u)\notin [0,{\hat{\alpha }}]\times \{0\}\). Similarly, we obtain \((\alpha _u,\beta _u)\notin \{0\}\times [0,{\hat{\alpha }}]\).

On the other hand, it is easy to see that

$$\begin{aligned} \frac{a\alpha ^p}{p}A_1 + \frac{b\alpha ^{2p}}{p}A_1\Vert u^+\Vert ^p+ \frac{b\alpha ^{2p}}{2p}A_1^2 - \frac{\alpha ^{p_s^{*}}}{p_s^{*}}B_1 >0 \end{aligned}$$
(3.24)

and

$$\begin{aligned} \frac{a\beta ^p}{p}A_2 + \frac{b\beta ^{2p}}{p}A_2\Vert u^-\Vert ^p+ \frac{b\beta ^{2p}}{2p}A_2^2 - \frac{\beta ^{p_s^{*}}}{p_s^{*}}B_2 > 0 \end{aligned}$$
(3.25)

for \(\alpha \in (0,{\hat{\alpha }}]\), \(\beta \in (0, {\hat{\beta }}]\).

Then, we have that

$$\begin{aligned} \frac{s}{N}(aS)^{N/ps}&\le \frac{a{\hat{\alpha }}^p}{p}A_1 +\frac{b{\hat{\alpha }}^{2p}}{2p}A_1^2- \frac{{\hat{\alpha }}^{p_s^{*}}}{p_s^{*}}B_1+\frac{b{\hat{\alpha }}^{2p}}{p}A_1\Vert u^+\Vert ^p\\&\quad + \frac{a\beta ^p}{p}A_2 + \frac{b\beta ^{2p}}{p}A_2\Vert u^-\Vert ^p+ \frac{b\beta ^{2p}}{2p}A_2^2 - \frac{\beta ^{p_s^{*}}}{p_s^{*}}B_2 \end{aligned}$$

and

$$\begin{aligned} \frac{s}{N}(aS)^{N/ps}&\le \frac{a\tilde{\beta }^p}{p}A_2 +\frac{b\tilde{\beta }^{2p}}{2p}A_2^p- \frac{\tilde{\beta }^{p_s^{*}}}{p_s^{*}}B_2 +\frac{b\tilde{\beta }^{2p}}{p}A_2\Vert u^-\Vert ^p\\&\quad + \frac{a\alpha ^p}{p}A_1 + \frac{b\alpha ^{2p}}{p}A_1\Vert u^+\Vert ^p+ \frac{b\alpha ^{2p}}{2p}A_1^2 - \frac{\alpha ^{p_s^{*}}}{p_s^{*}}B_1 \end{aligned}$$

for all \(\alpha \in [0,{\hat{\alpha }}]\) and all \(\beta \in [0, {\hat{\beta }}]\).

Therefore, according to (3.21), we conclude that

$$\begin{aligned} \psi (\alpha ,{\hat{\beta }})\le 0,\quad \psi ({\hat{\alpha }}, \beta )\le 0 \end{aligned}$$

for all \(\alpha \in [0,{\hat{\alpha }}]\) and all \(\beta \in [0, {\hat{\beta }}]\).

Hence,\((\alpha _u, \beta _u)\notin \{{\hat{\alpha }}\}\times [0, {\hat{\beta }}]\) and \((\alpha _u, \beta _u)\notin [0,{\hat{\alpha }}]\times \{{\hat{\beta }}\}\).

Finally, we get that \((\alpha _u, \beta _u)\in (0,{\hat{\alpha }})\times (0, {\hat{\beta }})\). Hence, it follows that \((\alpha _u, \beta _u)\) is a critical point of \(\psi \).

Hence, \(\alpha _u u^+ + \beta _u u^-\in {\mathcal {M}}_b^{\lambda }\). From (3.17), (3.20), and (3.21), we have that

$$\begin{aligned} c_b^{\lambda }&\ge I_b^{\lambda }(\alpha _u u^+ + \beta _u u^-) +\frac{a{\alpha _u^p}}{p}A_1 +\frac{b{\alpha _u^{2p}}}{p}A_1\Vert u^+\Vert ^p+ \frac{b{\alpha _u^{2p}}}{2p}A_1^2- \frac{\alpha ^{p_s^{*}}}{p_s^{*}}B_1\\&\quad + \frac{a{\beta ^p_u}}{p}A_2 +\frac{b{\beta ^{2p}_u}}{p}A_2\Vert u^-\Vert ^p+\frac{b{\beta ^{2p}_u}}{2p}A_2^p- \frac{\beta ^{p_s^{*}}}{p_s^{*}}B_2\\&> I_b^\lambda (\alpha _u u^+ + \beta _u u^-)\ge c_b^{\lambda }, \end{aligned}$$

which is a contradiction.

Case 2: \(B_2 = 0\).

In this case, we can maximize in \([0, {\hat{\alpha }}]\times [0, \infty )\). Indeed, it is possible to show that there exist \(\beta _0\in [0, \infty )\) such that

$$\begin{aligned} I_b^{\lambda }(\alpha _u u^+ + \beta _u u^-)\le 0 \quad \text{ for } \text{ all }\ (\alpha , \beta )\in [0,{\hat{\alpha }}]\times [\beta _0, \infty ). \end{aligned}$$

Hence, there is \((\alpha _u, \beta _u)\in [0, {\hat{\alpha }}]\times [0, \infty )\) such that

$$\begin{aligned} \phi (\alpha _u, \beta _u) = \max _{(\alpha , \beta )\in [0,{\hat{\alpha }}]\times [0, \infty )}\phi (\alpha ,\beta ). \end{aligned}$$

In the following, we prove that \((\alpha _u, \beta _u)\in (0, {\hat{\alpha }})\times (0, \infty )\).

It is noted that \(\phi (\alpha ,0) < \phi (\alpha , \beta )\) for \(\alpha \in [0,{\hat{\alpha }}]\) and \(\beta \) small enough, so we have \((\alpha _u, \beta _u)\notin [0,{\hat{\alpha }}]\times \{0\}\).

Meanwhile, \(\phi (0,\beta ) < \phi (\alpha , \beta )\) for \(\beta \in [0, \infty )\) and \(\alpha \) small enough, then we have \((\alpha _u, \beta _u)\notin \{0\}\times [0, \infty )\).

On the other hand, it is obvious that

$$\begin{aligned} \frac{s}{N}(aS)^{N/ps}&\le \frac{a{\hat{\alpha }}^p}{p}A_1 +\frac{b{\hat{\alpha }}^{2p}}{2p}A_1^2- \frac{\alpha ^{p_s^{*}}}{p_s^{*}}B_1 +\frac{b{\hat{\alpha }}^{2p}}{p}A_2\Vert u^+\Vert ^p\\&\quad + \frac{a\beta ^p}{p}A_2 +\frac{b\beta ^{2p}}{p}A_2\Vert u^-\Vert ^p+ \frac{b\beta ^{2p}}{2p}A_2^2 \end{aligned}$$

for all \(\beta \in [0, \infty )\).

Hence, we have that \(\phi ({\hat{\alpha }}, \beta )\le 0\) for all \(\beta \in [0, \infty )\). Thus, \((\alpha _u, \beta _u)\notin \{{\hat{\alpha }}\}\times [0, \infty )\). Hence, \((\alpha _u, \beta _u)\in (0,{\hat{\alpha }})\times (0, \infty )\). That is, \((\alpha _u, \beta _u)\) is an inner maximizer of \(\phi \) in \([0,{\hat{\alpha }}) \times [0, \infty )\). Hence, \(\alpha _u u^+ + \beta _u u^-\in {\mathcal {M}}_b^{\lambda }\).

Hence, in view of (3.24), we have that

$$\begin{aligned} c_b^{\lambda }&\ge I_b^{\lambda }(\alpha _u u^+ + \beta _u u^-) +\frac{a{\alpha _u^p}}{p}A_1 +\frac{b{\alpha _u^{2p}}}{p}A_1\Vert u^+\Vert ^p+ \frac{b{\alpha _u^{2p}}}{2p}A_1^2- \frac{\alpha ^{p_s^{*}}}{p_s^{*}}B_1\\&\quad + \frac{a{\beta ^p_u}}{p}A_2 +\frac{b{\beta ^{2p}_u}}{p}A_2\Vert u^-\Vert ^p+\frac{b{\beta ^{2p}_u}}{2p}A_2^2\\&> I_b^{\lambda }(\alpha _u u^+ + \beta _u u^-)\ge c_b^{\lambda }, \end{aligned}$$

which is a contradiction.

Therefore, from the above arguments, we have that \(B_1 = B_2 = 0\).

Finally, we prove that \(c_b^{\lambda }\) is achieved.

Since \(u^\pm \ne 0\), by Lemma 3.1, there exist \(\alpha _u, \beta _u > 0\) such that

$$\begin{aligned} \bar{u}:= \alpha _u u^+ + \beta _u u^-\in {\mathcal {M}}_b^{\lambda }. \end{aligned}$$

Furthermore, it is easy to see that

$$\begin{aligned} \langle (I_b^{\lambda })'(u),u^\pm \rangle \le 0. \end{aligned}$$

By Lemma 3.2, we obtain \(0<\alpha _u, \beta _u \le 1\).

Since \(u_n\in {\mathcal {M}}_b^{\lambda }\), according to Lemma 3.3, we get

$$\begin{aligned} I_b^{\lambda }(\alpha _u u_n^+ + \beta _u u_n^-)\le I_b^{\lambda }(u_n^+ + u_n^-)= I_b^{\lambda }(u_n). \end{aligned}$$

Thanks to \(B_1 = B_2 = 0\) and the norm in \(W^{s,p}_0(\Omega )\) is lower semicontinuous, and we have that

$$\begin{aligned} c_b^{\lambda }&\le I_b^{\lambda }(\bar{u})-\frac{1}{q}\langle (I_b^{\lambda })'(\bar{u}),\bar{u}\rangle \\&\le \left( \frac{1}{p}-\frac{1}{q}\right) a\Vert \bar{u}\Vert ^p + \left( \frac{1}{2p} - \frac{1}{q}\right) b\Vert \bar{u}\Vert ^{2p} + \frac{2\lambda }{q^2}\int _\Omega |\bar{u}|^{q}dx + \left( \frac{1}{q} -\frac{1}{p_s^{*}}\right) \int _{\Omega }|\bar{u}|^{p_s^{*}}dx \\&= \left( \frac{1}{p}-\frac{1}{q}\right) a(\Vert \alpha _u u^+\Vert ^p +\Vert \beta _u u^-\Vert ^p) + \left( \frac{1}{2p} - \frac{1}{q}\right) b(\Vert \alpha _u u^+\Vert ^p +\Vert \beta _u u^-\Vert ^p)^2 \\&\quad + \frac{2\lambda }{q^2} \left[ \int _{\Omega }|\alpha _uu^+|^{q}dx+ \int _{\Omega }|\beta _uu^-|^{q}dx\right] + \left( \frac{1}{q}-\frac{1}{p_s^{*}}\right) \left[ \int _{\Omega }|\alpha _uu^+|^{p_s^{*}}dx\right. \\&\quad \left. + \int _{\Omega }|\beta _uu^-|^{p_s^{*}}dx\right] \\&\le \left( \frac{1}{p}-\frac{1}{q}\right) a\Vert u\Vert ^p + \left( \frac{1}{2p} - \frac{1}{q}\right) b\Vert u\Vert ^{2p} + \frac{2\lambda }{q^2}\int _\Omega |u|^{q}dx + \left( \frac{1}{q}-\frac{1}{p_s^{*}}\right) \int _{\Omega }|u|^{p_s^{*}}dx \\&\le \liminf _{n\rightarrow \infty }\left[ I_b^{\lambda }(u_n)-\frac{1}{q}\langle (I_b^{\lambda })'(u_n),u_n\rangle \right] \le c_b^{\lambda }. \end{aligned}$$

Therefore, \(\alpha _u=\beta _u=1\), and \(c_b^{\lambda }\) is achieved by \(u_b:= u^+ + u^-\in {\mathcal {M}}_b^{\lambda }\). \(\square \)

4 Proof of Theorems

In this section, we prove our main results. First, we prove Theorem 2.1. In fact, thanks to Lemma 3.4, we just prove that the minimizer \(u_b\) for \(c_b^{\lambda }\) is indeed a sign-changing solution of problem (1.1).

Proof of Theorem 2.1

Since \(u_b\in {\mathcal {M}}_b^{\lambda }\), we have \(\langle (I_b^{\lambda })'(u_b), u_b^+\rangle =\langle (I_b^{\lambda })'(u_b),u_b^-\rangle =0\). By Lemma 3.4, for \((\alpha ,\beta )\in ({\mathbb {R}}_+\times {\mathbb {R}}_+)\backslash (1,1)\), we have

$$\begin{aligned} I_b^{\lambda }(\alpha u_b^+ + \beta u_b^-)<I_b^{\lambda }(u_b^+ + u_b^-)=c_b^{\lambda }. \end{aligned}$$
(4.1)

Arguing by contradiction, we assume that \((I_b^{\lambda })'(u_b)\ne 0\), then there exist \(\delta > 0\) and \(\iota > 0\) such that

$$\begin{aligned} \Vert (I_b^{\lambda })'(v)\Vert \ge \iota \quad \text{ for } \text{ all }\, \,\Vert v-u_b\Vert \ge 3\delta . \end{aligned}$$

Choose \(\tau \in (0,\min \{1/2,\frac{\delta }{\sqrt{2}\Vert u_b\Vert }\})\). Let

$$\begin{aligned} D:= (1-\tau ,1+\tau )\times (1-\tau ,1+\tau ) \end{aligned}$$

and

$$\begin{aligned} g(\alpha ,\beta ) = \alpha u_b^+ + \beta u_b^-\quad \text{ for } \text{ all }\ \ (\alpha ,\beta )\in D. \end{aligned}$$

In view of (4.1), it is easy to see that

$$\begin{aligned} \bar{c}_\lambda :=\max _{\partial \Omega }I_b^{\lambda }\circ g<c_{b,\lambda }. \end{aligned}$$
(4.2)

Let \(\varepsilon := \min \{(c_b^{\lambda } -\bar{c}_\lambda )/3,\iota \delta /8\}\) and \(S_\delta := B(u_b,\delta )\), according to Lemma 2.3 in [46], there exists a deformation \(\eta \in C([0,1]\times D,D)\) such that

  1. (a)

    \(\eta (1,v) = v\) if \(v\notin (I_b^{\lambda })^{-1}([c_b^{\lambda }-2\varepsilon , c_b^{\lambda }+2\varepsilon ]\cap S_{2\delta })\),

  2. (b)

    \(\eta (1,(I_b^{\lambda })^{c_b^{\lambda }+\varepsilon }\cap S_{\delta }) \subset (I_b^{\lambda })^{c_{b,\lambda }-\varepsilon }\),

  3. (c)

    \(I_b^{\lambda }(\eta (1,v))\le I_b^{\lambda })(v)\) for all \(v\in W^{s,p}_0(\Omega )\).

First, from (b) and Lemma 3.2, it is easy to see that

$$\begin{aligned} \max _{(\alpha ,\beta )\in \bar{D}}I_b^{\lambda }(\eta (1,g(\alpha ,\beta )))< c_b^{\lambda }. \end{aligned}$$
(4.3)

Next, we prove that \(\eta (1,g(D))\cap {\mathcal {M}}_b^{\lambda } \ne \emptyset \) , which contradicts the definition of \(c_b^{\lambda }\).

Let \(\gamma (\alpha ,\beta ):=\eta (1,g(\alpha ,\beta ))\) and

$$\begin{aligned} \Psi _0(\alpha ,\beta )&:=(\langle (I_b^{\lambda })'(g(\alpha ,\beta )),u_b^+\rangle ,\langle (I_b^{\lambda })'(g(\alpha ,\beta )), u_b^-\rangle )\\&= (\langle (I_b^{\lambda })'(\alpha u_b^+ + \beta u_b^-), u_b^+\rangle , \langle (I_b^{\lambda })'(\alpha u_b^+ + \beta u_b^-),u_b^-\rangle )\\&:= (\varphi _u^1(\alpha ,\beta ),\varphi _u^2(\alpha ,\beta )) \end{aligned}$$

and

$$\begin{aligned} \Psi _1(\alpha ,\beta ) := \left( \frac{1}{\alpha }\langle (I_b^{\lambda })'(\gamma (\alpha ,\beta )),\, (\gamma (\alpha ,\beta ))^+\rangle ,\frac{1}{\beta }\langle (I_b^{\lambda })'(\gamma (\alpha ,\beta )),(\gamma (\alpha ,\beta ))^-\rangle \right) . \end{aligned}$$

Since \(u_b \in {\mathcal {M}}_b^{\lambda }\), by the direct calculation, we have

$$\begin{aligned} \frac{\varphi _u^1(\alpha ,\beta )}{\partial \alpha }\mid _{(1,1)}&= a(p-1)\Vert u_b^+\Vert ^p +b(2p-1)\Vert u_b^+\Vert ^{2p} +b(p-1)\Vert u_b^+\Vert ^p\Vert u_b^-\Vert ^p\\&\quad -\lambda (q-1)\int _{\Omega }|u_b^+|^{q}\ln |u_b^+|^2dx - 2\int _{\Omega }|u_b^+|^{q}dx - (p_s^{*}-1)\int _{\Omega }|u_b^+|^{p_s^{*}}dx \\&= bp\Vert u_b^+\Vert ^{2p} -\lambda (q-p)\int _{\Omega }|u_b^+|^{q}\ln |u_b^+|^2dx - 2\int _{\Omega }|u_b^+|^{q}dx \\&\quad - (p_s^{*}-p)\int _{\Omega }|u_b^+|^{p_s^{*}}dx \end{aligned}$$

and

$$\begin{aligned} \frac{\varphi _u^1(\alpha ,\beta )}{\partial \beta }\mid _{(1,1)} = pb\Vert u_b^+\Vert ^p\Vert u_b^-\Vert ^p. \end{aligned}$$

Similarly, we have

$$\begin{aligned} \frac{\varphi _u^2(\alpha ,\beta )}{\partial \beta }\mid _{(1,1)}&= bp\Vert u_b^-\Vert ^{2p} -\lambda (q-p)\int _{\Omega }|u_b^-|^{q}\ln |u_b^-|^2dx - 2\int _{\Omega }|u_b^-|^{q}dx\\&\quad - (p_s^{*}-p)\int _{\Omega }|u_b^-|^{p_s^{*}}dx \end{aligned}$$

and

$$\begin{aligned} \frac{\varphi _u^2(\alpha ,\beta )}{\partial \alpha }\mid _{(1,1)} = pb\Vert u_b^+\Vert ^p\Vert u_b^-\Vert ^p. \end{aligned}$$

Let

$$\begin{aligned} M=\begin{bmatrix} \frac{\varphi _u^1(\alpha ,\beta )}{\partial \alpha }\mid _{(1,1)}&{} \frac{\varphi _u^2(\alpha ,\beta )}{\partial \alpha }\mid _{(1,1)} \\ \frac{\varphi _u^1(\alpha ,\beta )}{\partial \beta }\mid _{(1,1)}&{} \frac{\varphi _u^2(\alpha ,\beta )}{\partial \beta }\mid _{(1,1)} \end{bmatrix}. \end{aligned}$$

Then, we have that

$$\begin{aligned} \det M&= \frac{\varphi _u^1(\alpha ,\beta )}{\partial \alpha }\mid _{(1,1)} \times \frac{\varphi _u^2(\alpha ,\beta )}{\partial \beta }\mid _{(1,1)} - \frac{\varphi _u^1(\alpha ,\beta )}{\partial \beta }\mid _{(1,1)} \times \frac{\varphi _u^2(\alpha ,\beta )}{\partial \alpha }\mid _{(1,1)} \ne 0. \end{aligned}$$

Since \(\Psi _0(\alpha ,\beta )\) is a \(C^1\) function and (1, 1) is the unique isolated zero point of \(\Psi _0\), by using the degree theory, we deduce that \(\deg (\Psi _0 ,D,0) = 1\).

Hence, combining (4.3) with (a), we obtain

$$\begin{aligned} g(\alpha ,\beta ) = \gamma (\alpha ,\beta )\quad \text{ on }\ \partial D. \end{aligned}$$

Consequently, we obtain \(\deg (\Psi _1 ,D,0) = 1\). Therefore, \(\Psi _1(\alpha _0, \beta _0) = 0\) for some \((\alpha _0, \beta _0)\in D\) so that

$$\begin{aligned} \eta (1,g(\alpha _0, \beta _0))=\gamma (\alpha _0, \beta _0)\in {\mathcal {M}}_b^{\lambda }, \end{aligned}$$

which is contradicted to (4.3).

From the above discussions, we deduce that \(u_b\) is a sign-changing solution for problem (1.1).

Finally, we prove that u has exactly two nodal domains. To this end, we assume by contradiction that

$$\begin{aligned} u_b=u_1+u_2+u_3, \end{aligned}$$

where

$$\begin{aligned}&u_i\ne 0,\ u_1\ge 0,\ u_2\le 0, \ \Omega _1\cap \Omega _2 = \emptyset , \ u_1|_{\Omega {\setminus }\Omega _1\cup \Omega _2} = u_2|_{\Omega {\setminus }\Omega _1\cup \Omega _2} = u_3|_{\Omega _1\cap \Omega _2} = 0,\\&\Omega _1 := \{x \in \Omega : u_1(x) > 0\} \quad \text{ and }\quad \Omega _2 := \{x \in \Omega : u_2(x) < 0\} \quad \text{ for }\,\,i\ne j,\ i, j=1, 2, 3 \end{aligned}$$

are two connected open subsets of \(\Omega \), and

$$\begin{aligned} \langle (I_b^{\lambda })'(u),u_i\rangle = 0\quad \text{ for }\,\, i=1, 2, 3. \end{aligned}$$

Setting \(v:= u_1 + u_2\) , we see that \(v^+ = u_1\) and \(v^- = u_2\), i.e., \(v^\pm \ne 0\). Then, there exist a unique pair \((\alpha _v, \beta _v)\) of positive numbers such that

$$\begin{aligned} \alpha _v u_1 + \beta _vu_2\in {\mathcal {M}}_b^{\lambda }. \end{aligned}$$

Hence

$$\begin{aligned} I_b^{\lambda }(\alpha _v u_1 + \beta _vu_2)\ge c^\lambda _b. \end{aligned}$$

Moreover, using the fact that \(\langle (I_b^{\lambda })'(u),u_i\rangle = 0\), we obtain \(\langle (I_b^{\lambda })'(v),v^\pm \rangle = -b\Vert v^\pm \Vert ^p\Vert u_3\Vert ^p < 0\).

From Lemma 3.1 (ii), we have that

$$\begin{aligned} (\alpha _v, \beta _v)\in (0,1]\times (0,1]. \end{aligned}$$

On the other hand, we have that

$$\begin{aligned} 0 = \frac{1}{2p}\langle (I_b^{\lambda })'(u),u_3\rangle&= \frac{1}{2p}a\Vert u_3\Vert ^p +\frac{b}{2p}\Vert u_1\Vert ^p\Vert u_3\Vert ^p + \frac{b}{2p}\Vert u_2\Vert ^p\Vert u_3\Vert ^p + \frac{b}{2p}\Vert u_3\Vert ^{2p}\\&\quad - \frac{1}{2p}\int _\Omega |u_3|^{p_s^{*}} dx - \frac{\lambda }{2p}\int _{\Omega }|u_3^+|^{q}\ln |u_3^+|^2dx\\&< I_b^{\lambda }(u_3) + \frac{b}{2p}\Vert u_1\Vert ^2\Vert u_3\Vert ^p + \frac{b}{2p}\Vert u_2\Vert ^2\Vert u_3\Vert ^p. \end{aligned}$$

Hence, by (3.15), we can obtain that

$$\begin{aligned} c_b^{\lambda }&\le I_b^{\lambda }(\alpha _v u_1 + \beta _vu_2)= I_b^{\lambda }(\alpha _v u_1 + \beta _vu_2) - \frac{1}{2p}\langle (I_b^{\lambda })'(\alpha _v u_1 + \beta _vu_2),(\alpha _v u_1 + \beta _vu_2)\rangle \\&= \frac{a}{2p}(\Vert \alpha _v u_1\Vert ^p+\Vert \beta _v u_2\Vert ^p) + \frac{\lambda }{q^2}\left[ \int _\Omega |\alpha _v u_1|^qdx + \int _\Omega |\beta _v u_2|^qdx\right] \\&\quad + \left( \frac{1}{2p}-\frac{1}{q}\right) \lambda \left[ \int _\Omega [|\alpha _v u_1|^q\ln |\alpha _v u_1|^2dx + \int _\Omega |\beta _v u_2|^q\ln |\beta _v u_2|^2 dx\right] \\&\quad + \left( \frac{1}{2p}-\frac{1}{p_s^{*}}\right) \int _\Omega \alpha _v^{p_s^{*}}|u_1|^{p_s^{*}} dx + \left( \frac{1}{2p}-\frac{1}{p_s^{*}}\right) \int _\Omega \beta _v^{p_s^{*}} |u_2|^{p_s^{*}} dx\\&\le \frac{1}{2p}(\Vert u_1\Vert ^p+\Vert u_2\Vert ^p) + \frac{\lambda }{q^2}\left[ \int _\Omega |u_1|^qdx + \int _\Omega |u_2|^qdx\right] \\&\quad + \left( \frac{1}{2p}-\frac{1}{q}\right) \lambda \left[ \int _\Omega |u_1|^q\ln |u_1|^2dx + \int _\Omega |u_2|^q\ln |u_2|^2 dx\right] \\&\quad + \left( \frac{1}{2p}-\frac{1}{p_s^{*}}\right) \int _\Omega |u_1|^{p_s^{*}} dx + \left( \frac{1}{2p}-\frac{1}{p_s^{*}}\right) \int _\Omega |u_2|^{p_s^{*}} dx\\&= I_b^{\lambda }(u_1 + u_2) - \frac{1}{2p}\langle (I_b^{\lambda })'(u_1 + u_2),(u_1 + u_2)\rangle \\&= I_b^{\lambda }(u_1 + u_2) + \frac{1}{2p}\langle (I_b^{\lambda })'(u),u_3\rangle +\frac{b}{2p}\Vert u_1\Vert ^p\Vert u_3\Vert ^p + \frac{b}{2p}\Vert u_2\Vert ^p\Vert u_3\Vert ^p\\&< I_b^{\lambda }(u_1) + I_b^{\lambda }(u_2) + I_b^{\lambda }(u_3) + \frac{b}{2p}(\Vert u_2\Vert ^p+\Vert u_3\Vert ^p)\Vert u_1\Vert ^p\\&\quad + \frac{b}{2p}(\Vert u_1\Vert ^p+\Vert u_3\Vert ^p)\Vert u_2\Vert ^p + \frac{b}{2p}(\Vert u_1\Vert ^p+\Vert u_2\Vert ^p)\Vert u_3\Vert ^p\\&= I_b^{\lambda }(u) = c_b^{\lambda }, \end{aligned}$$

which is a contradiction, that is, \(u_3 = 0\) and \(u_b\) has exactly two nodal domains. \(\square \)

By Theorem 2.1, we obtain a least energy sign-changing solution \(u_b\) of problem (1.1). Next,we prove that the energy of \(u_b\) is strictly larger than two times the ground state energy.

Proof of Theorem 2.2

Similar to Proof of Lemma 3.3, there exists \(\lambda ^*_1 > 0\) such that for all \(\lambda \ge \lambda ^*_1\), and for each \(b > 0\), there exists \(v_b\in {\mathcal {N}}_b^{\lambda }\) such that \(I_b^{\lambda }(v_b)=c^*>0\). By standard arguments (see Corollary 2.13 in Ref. [22]), the critical points of the functional \(I_b^{\lambda }\) on \({\mathcal {N}}_b^{\lambda }\) are critical points of \(I_b^{\lambda }\) in \(W^{s,p}_0(\Omega )\), and we obtain \((I_b^{\lambda })'(v_b)=0\). That is, \(v_b\) is a ground state solution of (1.1).

According to Theorem 2.1, we know that the problem (1.1) has a least energy sign-changing solution \(u_b\), which changes sign only once when \(\lambda \ge \lambda ^*\).

Let

$$\begin{aligned} \lambda ^{**}=\max \{\lambda ^*,\lambda ^*_1\}. \end{aligned}$$

Suppose that \(u_b= u_b^+ + u_b^-\). As Proof of Lemma 3.1, there exist \(\alpha _{u_b^+} > 0\) and \(\beta _{u_b^-} > 0\) such that

$$\begin{aligned} \alpha _{u_b^+}u_b^+\in {\mathcal {N}}_b^{\lambda }, \quad \beta _{u_b^-}u_b^-\in {\mathcal {N}}_b^{\lambda }. \end{aligned}$$

Furthermore, Lemma 3.2 implies that \(\alpha _{u_b^+}, \beta _{u_b^-} \in (0, 1)\).

Therefore, in view of Lemma 3.1, we have that

$$\begin{aligned} 2c^*\le I_b^{\lambda }(\alpha _{u_b^+}u_b^+) + I_b^{\lambda }(\beta _{u_b^-}u_b^-) \le I_b^{\lambda }(\alpha _{u_b^+}u_b^+ + \beta _{u_b^-}u_b^-)<I_b^{\lambda }(u_b^+ + u_b^-) = c_b^\lambda . \end{aligned}$$

Hence, it follows that \(c^* > 0\) cannot be achieved by a sign-changing function. \(\square \)

Finally, we close this section with the proof of Theorem 2.3. In the following, we regard \(b > 0\) as a parameter in problem (1.1).

Proof of Theorem 2.3

We shall proceed through several steps on analyzing the convergence property of \(u_b\) as \(b \rightarrow 0\), where \(u_b\) is the least energy sign-changing solution obtained in Theorem 2.1.

Step 1 For any sequence \(\{b_n\}\) as \(b_n \searrow 0\), \(\{u_{b_n}\}\) is bounded in \(W^{s,p}_0(\Omega )\).

Choose a nonzero function \(\omega \in C_0^\infty (\Omega )\) with \(\omega ^\pm = 0\). Similar to discussion as in Lemma 3.2, for any \(\lambda \in [0, 1]\), there exists a pair positive numbers \((\lambda _1, \lambda _2)\) independent of \(\lambda \), such that

$$\begin{aligned} \langle (I_b^{\lambda })'(\lambda _1\omega ^+ + \lambda _2\omega ^-), \lambda _1\omega ^+ \rangle< 0, \quad \langle (I_b^{\lambda })'(\lambda _1\omega ^+ + \lambda _2\omega ^-), \lambda _2\omega ^- \rangle < 0. \end{aligned}$$

Then by virtue of Lemma 3.1, we get that, for any \(b \in [0, 1]\), there exists a unique pair \((\alpha _\omega (b), \beta _\omega (b)) \in (0, 1] \times (0, 1]\) such that

$$\begin{aligned} \overline{\omega } := \alpha _\omega (b) \lambda _1\omega ^+ + \beta _\omega (b))\lambda _2\omega ^- \in {\mathcal {M}}_b^{\lambda }. \end{aligned}$$
(4.4)

Thus, for any \(\lambda \in [0, 1]\), we have

$$\begin{aligned} I_b^{\lambda }(u_b)&= I_b^{\lambda }(\overline{\omega }) = I_b^{\lambda }(\overline{\omega }) - \frac{1}{2p}\langle (I_b^{\lambda })'(\overline{\omega }), \overline{\omega }\rangle \\&= \frac{a}{2p} \Vert \overline{\omega }\Vert ^p + \left( \frac{1}{2p}-\frac{1}{p_s^{*}}\right) \int _{\Omega }|\overline{\omega }|^{p_s^{*}}dx \\&\quad + \frac{\lambda }{q^2}\int _\Omega |\overline{\omega }|^qdx + \left( \frac{1}{2p}-\frac{1}{q}\right) \lambda \int _\Omega |\overline{\omega }|^q\ln |\overline{\omega }|^2dx\\&\le \frac{a}{2p} \Vert \overline{\omega }\Vert ^p + \left( \frac{1}{2p}-\frac{1}{p_s^{*}}\right) \int _{\Omega }|\overline{\omega }|^{p_s^{*}}dx\\&\quad + \frac{\lambda }{q^2}\int _\Omega |\overline{\omega }|^qdx + \left( \frac{1}{2p}-\frac{1}{q}\right) \lambda \int _{\Omega }\left( C_1|\overline{\omega }|^p + C_2|\overline{\omega }|^r\right) dx \\&\le \frac{a}{2p} \Vert \lambda _1\omega ^+\Vert ^p + \frac{1}{2p} \Vert \lambda _2\omega ^-\Vert ^p + \left( \frac{1}{2p}-\frac{1}{p_s^{*}}\right) \int _{\Omega }|\lambda _1\omega ^+|^{p_s^{*}}dx\\&\quad + \left( \frac{1}{2p}-\frac{1}{p_s^{*}}\right) \int _{\Omega }|\lambda _2\omega ^-|^{p_s^{*}}dx\\&\quad + \frac{\lambda }{q^2}\int _\Omega |\lambda _1\omega ^+|^qdx + \frac{\lambda }{q^2}\int _\Omega |\lambda _2\omega ^-|^qdx \\&\quad + \left( \frac{1}{2p}-\frac{1}{q}\right) \lambda \int _{\Omega }\left( C_1\lambda _1^2|\omega ^+|^2 + C_2\lambda _2^{r}|\omega ^+|^r\right) dx \\&\quad + \left( \frac{1}{2p}-\frac{1}{q}\right) \lambda \int _{\Omega }\left( C_1\lambda _1^2|\omega ^-|^2 + C_2\lambda _2^{r}|\omega ^-|^r\right) dx := C^*, \end{aligned}$$

where \(C^*> 0\) is a constant independent of \(\lambda \). So, let \(n \rightarrow \infty \), it follows that

$$\begin{aligned} C^*+ 1 \ge I_{b_n}^{\lambda }(u_{b_n}) = I_{b_n}^{\lambda }(u_{b_n}) - \frac{1}{2p}\langle (I_{b_n}^{\lambda })'(u_{b_n}), u_{b_n}\rangle \ge \frac{1}{2p} \Vert u_{b_n}\Vert ^2, \end{aligned}$$

which implies that \(\{u_{b_n}\}\) is bounded in \(W^{s,p}_0(\Omega )\).

Step 2 Problem (2.12) possesses one sign-changing solution \(u_0\).

Since \(\{u_{b_n}\}\) is bounded in \(W^{s,p}_0(\Omega )\), according to Step 1, going if necessary to a subsequence, there exists \(u_0 \in W^{s,p}_0(\Omega )\) such that

$$\begin{aligned}&\nonumber u_{b_n} \rightharpoonup u_0 \ \ \text{ in }\ W^{s,p}_0(\Omega ), \\&u_{b_n} \rightarrow u_0 \ \ \text{ in }\ \ L^t(\Omega )\ \text{ for }\ t \in [p, p_s^{*}), \\&\nonumber u_{b_n} \rightarrow u_0 \ \ \text{ a.e. } \ x \in \Omega . \end{aligned}$$
(4.5)

Since \(\{u_{b_n}\}\) is a weak solution of (1.1) with \(b = b_n\), we have

$$\begin{aligned} \left( a + b_n[u]_{s,p}^p\right) L(u,v) = \lambda \int _\Omega |u|^{q-2}uv\ln |u|^2dx + \int _\Omega |u|^{p_s^{*}-2}uv\,dx \end{aligned}$$
(4.6)

for all \(v \in C_0^\infty (\Omega )\), L(uv) is defined by (2.5).

From (4.5), (4.6) and Step 1, we find that

$$\begin{aligned} \left( a + b_n[u_0]_{s,p}^p\right) L(u_0,v) = \lambda \int _\Omega |u|^{q-2}u_0v\ln |u_0|^2dx + \int _\Omega |u_0|^{p_s^{*}-2}u_0v\,dx \end{aligned}$$
(4.7)

for all \(\nu \in C_0^\infty (\Omega )\), which in turn implies that \(u_0\) is a weak solution of problem (2.12). By a similar argument as in the proof of Lemma 3.3, we conclude that \(u_0^{\pm } \ne 0\). Therefore, we complete the proof of the Step 2.

Step 3 Problem (2.12) possesses a least energy sign-changing solution \(v_0\), and there exists a unique pair \((\alpha _{b_n}, \beta _{b_n}) \in [0, \infty ) \times [0, \infty )\) such that \(\alpha _{b_n}v_0^+ + \beta _{b_n}v_0^- \in {\mathcal {M}}_{b_n}^{\lambda }\). Moreover, \((\alpha _{b_n}, \beta _{b_n}) \rightarrow (1, 1)\) as \(n \rightarrow \infty \).

By a similar argument to the proof of Theorem 2.1, we have that problem (2.12) possesses a least energy sign-changing solution \(v_0\), where \(I_0^{\lambda }(v_0) = c_0\) nod and \((I_0^{\lambda })'(v_0) = 0\). Then, by Lemma 3.1, we can easily obtain the existence and uniqueness of the pair \((\alpha _{b_n}, \beta _{b_n})\) such that \(\alpha _{b_n}v_0^+ + \beta _{b_n}v_0^- \in {\mathcal {M}}_{b_n}^{\lambda }\). Moreover, we have \(\alpha _{b_n} > 0\) and \(\beta _{b_n} > 0\). Then the claim will follow once we can prove that \((\alpha _{b_n}, \beta _{b_n}) \rightarrow (1, 1)\) as \(n \rightarrow \infty \). In fact, since \(\alpha _{b_n}v_0^+ + \beta _{b_n}v_0^- \in {\mathcal {M}}_{b_n}^{\lambda }\), we have that

$$\begin{aligned}&\alpha _{b_n}^pa\Vert v_0^+\Vert ^p + b_n\alpha _{b_n}^{2p}\Vert v_0^+\Vert ^{2p} + b_n\alpha _{b_n}^p\beta _{b_n}^p\Vert v_0^+\Vert ^p\Vert v_0^-\Vert ^p \nonumber \\&\quad = \lambda \int _\Omega |\alpha _{b_n}v_0^+|^{q}\ln |\alpha _{b_n}v_0^+|^2dx + \int _{\Omega }|\alpha _{b_n}v_0^+|^{p_s^{*}}dx \end{aligned}$$
(4.8)

and

$$\begin{aligned}&\beta _{b_n}^pa\Vert v_0^-\Vert ^p + b_n\beta _{b_n}^{2p}\Vert v_0^-\Vert ^{2p} + b_n\alpha _{b_n}^p\beta _{b_n}^p\Vert v_0^+\Vert ^p\Vert v_0^-\Vert ^p \nonumber \\&\quad = \lambda \int _\Omega |\beta _{b_n}v_0^+|^{q}\ln |\beta _{b_n}v_0^+|^2dx + \int _{\Omega }|\beta _{b_n}v_0^-|^{p_s^{*}}dx. \end{aligned}$$
(4.9)

From the convergence of \(b_n\) as \(n \rightarrow \infty \), we deduce that the sequences \(\{\alpha _{b_n}\}\) and \(\{\beta _{b_n}\}\) are bounded. Up to a subsequence, suppose that \(\alpha _{b_n} \rightarrow \alpha _0\) and \(\beta _{b_n} \rightarrow \beta _0\). Then it follows from (4.8) and (4.9) that

$$\begin{aligned} \alpha _{0}^pa\Vert v_0^+\Vert ^p = \lambda \int _\Omega |\alpha _{0}v_0^+|^{q}\ln |\alpha _{0}v_0^+|^2dx + \int _{\Omega }|\alpha _{0}v_0^+|^{2^{**}}dx \end{aligned}$$
(4.10)

and

$$\begin{aligned} \beta _{0}^pa\Vert v_0^-\Vert ^p = \lambda \int _\Omega |\beta _{0}v_0^-|^{q}\ln |\beta _{0}v_0^-|^2dx + \int _{\Omega }|\beta _{0}v_0^-|^{2^{**}}dx. \end{aligned}$$
(4.11)

Thanks to \(v_0\) is a sign-changing solution of problem (2.12), we get

$$\begin{aligned} a\Vert v_0^\pm \Vert ^2 = \lambda \int _\Omega |v_0^\pm |^{q}\ln |v_0^\pm |^2dx + \int _{\Omega }|v_0^\pm |^{p_s^{*}}dx. \end{aligned}$$
(4.12)

Hence, in view of (4.10)–(4.12), we can easily obtain that \((\alpha _{0}, \beta _{0}) = (1, 1)\), and the Step 3 follows.

Now, we can now give the proof of Theorem 2.3. We assert that \(u_0\) obtained in Step 2 is a least energy solution of problem (2.12). In fact, by virtue of Step 3 and Lemma 3.1, we find that

$$\begin{aligned} I_0^{\lambda }(v_0) \le I_0^{\lambda }(u_0)&= \lim _{n\rightarrow \infty } I_{b_n}^{\lambda }(u_{b_n}) \le \lim _{n\rightarrow \infty } I_{b_n}^{\lambda }(\alpha _{b_n}v_0^+ + \beta _{b_n}v_0^-) \\&= \lim _{n\rightarrow \infty } I_0^{\lambda }(v_0^+ + v_0^-) = I_0^{\lambda }(v_0). \end{aligned}$$

Hence, the proof of Theorem 2.3 is completed. \(\square \)