Existence of Two Solutions for a Critical Elliptic Problem with Nonlocal Term in ℝ4
Abstract
In this paper, we prove the existence of two positive solutions for a critical elliptic problem with nonlocal term and Sobolev exponent in dimension four.
1. Introduction
In our setting, the Laplacian operator is associated to Kirchhoff term a∫Ω|∇u|2dx + b, which contains an integral over the entire domain Ω, this implies that the equation in is no longer a pointwise identity and so the problem turns to be nonlocal. This fact brings some mathematical difficulties in the search of the solution, and the solvability of this kind of problems has been under various authors’ attention; so, some classical investigations can be seen in the works [1, 2] and the references therein.
Such nonlinear Kirchhoff’s equations can be used for describing the dynamic for an axially moving string and was first formulated by Kirchhoff himself [3] in 1883, he take into account the changes in length of the strings produced by transverse vibrations, and his model can be seen as a generalization of the classical D’Alembert wave equation for free vibrations of elastic strings.
Problems which involve nonlocal operator have been widely studied due to their numerous and relevant applications in various fields of sciences. In particular, Kirchhoff type problems proved to be valuable tools for modeling several physical and biological phenomena, and many works have been made to ensure the existence of solutions for such problems; we quote in particular the article of Lions [4]. Since this famous paper, very fruitful development has given rise to many works on this advantageous axis, and in most of them, the used approach relies on topological methods. However, just few improvements were held concerning the multiplicity of solutions. In [5], Maia obtained a multiplicity of solutions for a class of p(x)-Choquard equations with a nonlocal and nondegenerate Kirchhoff term by using truncation arguments and Krasnoselskii’s genus. In [6], Vetro studied the existence of two different notions of solutions by using Galerkin approximation method, jointly with the theory of pseudomonotone operators.
With this regard, variational approach was solicited instead of topological methods to solve this kind of problems and also to prove the existence of multiple solutions; we refer interested readers to the works [7–10].
where Ω is a smooth bounded domain of ℝN, N ≥ 3.
Without nonlocal term (a = 0), much interest has grown on problems involving critical exponents, and there are many publications dealing with the existence of solutions, starting from the celebrated paper by Brézis and Nirenberg [11] when and 2∗ = 2N/N − 2 are the critical Sobolev exponent. For convenience of the reader, we give a brief summary of these results: they established existence results in dimension N = 3 when Ω is a ball namely, and they ensure the existence of a positive constant λ0 such that the problem admits a positive solution for λ ∈ (λ0/4, λ1), where λ1 is the first eigenvalue of the operator −Δ. In higher dimensions N ≥ 4, they proved the existence of a positive solution for λ sufficiently small, i.e., λ ≤ λ0 and no positive solution for λ > λ0 and Ω a star-shaped domain.
When 1 < q < 2∗, Ambrosetti et al. [12] established a multiplicity result in a bounded domain of ℝN, N ≥ 3 indeed, they ensured the existence of a positive constant λ0 such that the problem admits two positive solutions for λ ∈ (0, λ∗), a positive solution for λ = λ∗ and no positive solution for λ > λ∗.
for all
We emphases that the extension of the previous results to the nonlocal case, namely, for elliptic problems driven by Kirchhoff type operator are not obvious in high dimensions N ≥ 4. Therefore, no improvement was hold concerning the multiplicity of solutions in this case.
For the case a > 0 and , Naimen in [14] treated the problem for N = 3 and obtained homologous results than the ones obtained by Brézis and Niremberg [11] in the nonlocal case under a suitable condition on a.
In dimension four, Naimen in [15] used variational methods to explore problem and showed that admits a positive solution when a > 0, b ≥ 0.
In the same order of ideas and still in the nonlocal case (a > 0), Lei et al. in [16] and Liao et al. in [17] extended the findings of [12] to a more general setting, namely, with the Kirchhoff operator; they established a multiplicity result in dimensions three and four, respectively.
Benmansour and Bouchekif [8] generalized the results obtained by Tarantello [13] to the nonlocal case in dimension three. Indeed, they have shown the existence of two solutions under a sufficient condition on f by introducing the Nehari manifold.
A natural question is to know whether the multiplicity result persists in the case of dimension four.
In the current paper, our main purpose inspired by [8] is to see that the result obtained in [8] can be extended to dimension four. We emphases that our results are new and complement the above works.
The main results are concluded as the following theorems, which are news for the case when a ≠ 0.
Theorem 1. Assume that γa,b,f > 0. Then, the problem admits a local minimal solution u0 with I(u0) < 0. Furthermore u0 ≥ 0 for f ≥ 0.
Theorem 2. Assume that γa,b,f > 0. Then, the problem admits another solution u1 with I(u1) > 0. Furthermore, u1 ≥ 0 for f ≥ 0.
This paper is structured as follows: in Section 2, we give some basic results useful for what follows. Section 3 is devoted to the proofs of our main results.
2. Some Preliminary Results
Obviously, if u ∈ H is a critical point of the functional I; then, u is a weak solution of problem
Lemma 3. Assume that b > 0, a ≥ 0, and f ∈ H−1\{0}. Then, the functional I is coercive and bounded from below on
Proof. For , we have
Therefore
Thus, I is coercive and bounded from below on .
Let hu(t) = I(tu) for t ∈ ℝ∗ and u ∈ H. These maps are known as fibering maps and were first introduced by Drábek and Pohozaev [18]. The set is closely linked to the behavior of hu(t), for more details, see for example [19] or [20].
It is natural to split into three subsets:
where These subsets correspond to local minima, points of inflexion, and local maxima of I, respectively.
Definition 4. A sequence {un} ⊂ H is said to be a Palais Smale sequence at level c ((PS)c sequence in short) for I if
I verifies Palais Smale condition at level c ((PS)c condition in short) if any (PS)c sequence has a convergent subsequence in H.
This is crucial for the following.
Lemma 5. Assume that γa,b,f > 0, then,
Proof. Arguing by contradiction we assume that there exists i.e., u ≠ 0 verifies
From (24) and (25), we derive that
As 0 ≤ a < S−2, we get from (24), (26), and the definition of S
Thus, as u ≠ 0 and a < S−2, we derive that
therefore, from (25), we obtain ‖u‖4 ≥ t1, with
Then, from (23), (25), and (26), we get
Lemma 6. Assume that γa,b,f > 0, then, for all u ∈ H\{0}, there exists unique positive value such that
Moreover, if ∫Ωfudx > 0, then, there exists unique positive value such that
Proof. We have hu(t) = I(tu), and Φu is concave and achieves its maximum at the point If γa,b,f > 0; then, there exists a unique such that and , which implies that and for all Moreover, if ∫Ωfudx > 0, then, there exists a unique such that and , which implies that and for all
Set
In the following lemma, we prove that is closed and disconnects H in exactly two connected components E1 and E2.
Lemma 7. Assume that γa,b,f > 0. Then
- (i)
is closed
- (ii)
- (iii)
Proof. Let and then, . Assume by contradiction that then
So, this implies that
From (36) and the definition of S, we get so which yields to a contradiction.
Let and v = u/‖u‖, then, t+(u) = 1, and there exists unique t+(v) such that As
Thus if u ∈ H\{0} and,
Let then
Since t+(u) > t−(u), it follows that
So, ‖u‖ < t+(u/‖u‖), and we conclude that
Lemma 8. Assume that γa,b,f > 0, then, there exists t∗ > 0 such that
Proof. Let then
Thus .
Set u∗ ∈ H the unique solution of the equation −Δu = f, it follows
By Lemma 6, there exists a unique positive value such that . So
The following lemma is needed for prove the existence of Palais Smale sequences.
Lemma 9. Assume that γa,b,f > 0. Then, for any there exist ε > 0 and a differentiable function ζ : BH(0, ε)⟶ℝ+\{0} such that
Proof. Let and define:ℝ × H⟶ℝ as follows
Clearly, (1, 0) = 0. Moreover, from Lemma 5, we derive that
Thus, we get our result by a straightforward application of the implicit function theorem to the function at the point (1, 0).
Lemma 10. Let θ ∈ {θ0, θ1}. There exist a Palais Smale sequences such that
Proof. Assume θ = θ0, by Lemma 3, I is bounded from below in then by applying the Ekeland Variational Principle, we can obtain a minimizing sequence satisfying
By using Lemma 8, we get for n large enough
Consequently, un ≠ 0 and
Now, we show that ‖I′(un)‖ tend to 0 as n goes to +∞. Arguing by contradiction and fix n with ‖I′(un)‖ ≠ 0.
Then, by Lemma 9, there exist ε > 0 and a function ζn : BH(0, ε)⟶ℝ such that with vn = δI′(un)/‖I′(un)‖ and 0 < δ < ε. By (56) and the Taylor expansion of I, we have
Then
We have
This, together with (61) implies
Then, as a is a small enough positive number, we get
So from (61), we derive that
Also, as , we get from (68)
For θ = θ1, adopting exactly the same way as in the case where θ = θ0.
In the following, we will prove our results.
3. Proofs of the Main Results
The proof of our main results is divided in two parts.
3.1. Existence of a Solution in
In this subsection, we prove that I has a solution in
Proposition 11. Assume that γa,b,f > 0. Then, the minimization problem
Proof. By using Lemma 10, there exists a bounded minimizing sequence such that I(un)⟶θ0 and I′(un)⟶0 in H−1. So, we deduce that {un} is bounded in H.
Passing to a subsequence if necessary, we have un⇀u0 weakly in H, then, 〈I′(u0), w〉 = 0, for all w ∈ H. In addition, from (60), we get ∫Ωfu0dx > 0. So, u0 is a weak solution for and
Thus
To conclude that u0 is a local minimum of I, let us recall that we have from Lemma 6
Choose ε > 0 sufficiently small to have
If f ≥ 0, we have ∫Ω fu0dx ≤ ∫Ω f|u0|dx and clearly I(|u0|) ≤ I(u0), and from (75) necessarily t−(|u0|) ≥ 1. Therefore, as , we get
3.2. Existence of a Solution in
The following part is devoted to prove the existence of a second solution u1 such that First, we determine the good level for covering the Palais Smale condition.
We have the following important result.
Lemma 12. Assume that γa,b,f > 0. Then, I satisfies the (PS)c condition for with
Proof. Let {un} be a (PS)c sequence with then
Hence, {un} is a bounded sequence in H. Thus for a subsequence still denoted {un} and we can find u1 ∈ H such that un⇀u1 in H, ∫Ωfundx⟶∫Ωfu1dx and un⟶u1 a.e in Ω. Therefore, u1 ≠ 0, , and I(u1) ≥ θ1.
Let wn = un − u1. From Brézis-Lieb Lemma [21], one has
Assume that ‖wn‖⟶l with l > 0, then, by (85) and the Sobolev inequality, we obtain
Hence
On the other hand, we have
Now, it is natural to show that As it is well know, (seeHL), the best Sobolev constant S defined above is attained in ℝ4 by
For x0 ∈ Ω let such that ϕ(x) = 1 for for and 0 ≤ ϕ ≤ 1, |∇ϕ| ≤ C. Now, we shall give some useful estimates of the extremal functions Uε. Let and The following estimates are obtained in [22] as ε tends to 0
Let Ω′ ⊂ Ω a set of positive measure such that u0 > 0 on Ω′ (if not replace u0 and f by −u0 and −f, respectively).
Lemma 13. Let b > 0, 0 ≤ a < S−2 a small enough positive number. Assume that f ∈ H−1\{0} satisfies γa,b,f > 0; then, for every t > 0 and a.e. x0 ∈ Ω′ ⊂ Ω, there exists ε0 such that for every 0<ε < ε0.
Proof. For ε small enough, let us consider the functional g defined by
We have by (92)
On the other hand, since is a solution of problem , we have ‖u0‖ ≤ C, I(u0) = θ0 and 〈I′(u0), tuε〉 = 0.
Then we obtain by (92)
Since a < S−2, we have h(t)⟶−∞ as t goes to ∞ and h(t)⟶0 as t goes to 0. This implies that there exist 0 < T1 < T2 such that
Then, for a = εσ with σ > 1, we conclude
Therefore,
Proposition 14. Let b > 0, 0 ≤ a < S−2 a small enough positive number. Assume that f ∈ H−1\{0} satisfies γa,b,f > 0, then, I has a local minimizer u1 on such that I(u1) = θ1. Moreover u1 is a local minimizer for I on H.
Proof. By Lemma 6, for every u ∈ H such that ‖u‖ = 1, there exists unique t+(u) > 0 such that t+(u)u∈ and I(t+u) ≥ I(tu), for all Then, for suitable constant T3 > 0, we deduce that
Therefore for t0 > 0 carefully chosen, the estimate (97) holds for ε small enough.
Thus, we derive that
Then, from Lemma 7, we conclude that u0 + t0uε ∈ E2.
Set
It is obvious that ξ : [0, 1]⟶H given by ξ(t) = u0 + tt0uε belongs to Γ. We conclude from Lemma 13 that
As the range of any ξ∈Γ intersects one has
From Lemma 10, we can obtain a minimizing sequence such that
We also deduce that
Consequently, we obtain a subsequence still denoted {un}, and we can find u1 ∈ H such that
This implies that u1 is a critical point for I, and I(u1) = θ1.
Finally, for f ≥ 0, let t+(|u1|) > 0 satisfying From Lemma 6, we have
So we conclude that u1 ≥ 0.
4. Conclusion
In our work, we have searched the critical points as the minimizers of the energy functional associated to the problem on the constraint defined by the Nehari manifold , which is a solution of our problem. Under some sufficient conditions, we split in two disjoint subsets and . Thus, we consider the minimization problems on and , respectively. If γa,b,f > 0, then, the problem has a local minimal solution u0 with I(u0) < 0. Furthermore, u0 ≥ 0 for f ≥ 0 and if γa,b,f > 0. The problem has another solution u1 with I(u1) > 0. Furthermore, u1 ≥ 0 for f ≥ 0.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The authors gratefully acknowledge (1) Qassim University, represented by the Deanship of Scientific Research, on the material support for this research under the number (1124) during the academic year 1443AH/2022 AD and (2) Algerian Ministry of Higher Education and Scientific Research on the material support for this research under the number (1124) during the academic year 1443AH/2022 AD.
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Data Availability
The functions, functionals, and parameters used to support the findings of this study are included within the article.