Volume 2022, Issue 1 2187810
Research Article
Open Access

Existence of Two Solutions for a Critical Elliptic Problem with Nonlocal Term in 4

Khadidja Sabri

Khadidja Sabri

University of Oran 2, Laboratory of Analysis and Control of Partial Differential Equations of Sidi Bel Abbes-Algeria, Algeria

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Atika Matallah

Atika Matallah

Higher School of Management-Tlemcen, Laboratory of Analysis and Control of Partial Differential Equations of Sidi Bel Abbes-Algeria, Algeria

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Safia Benmansour

Safia Benmansour

Higher School of Management-Tlemcen, Laboratory of Analysis and Control of Partial Differential Equations of Sidi Bel Abbes-Algeria, Algeria

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Mohammed El Mokhtar Ould El Mokhtar

Corresponding Author

Mohammed El Mokhtar Ould El Mokhtar

Qassim University, College of Science, Department of Mathematics, BO 6644, Buraidah: 51452, Saudi Arabia qu.edu.sa

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First published: 20 June 2022
Citations: 1
Academic Editor: Pasquale Vetro

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 this work, we are mainly concerned by the existence and the multiplicity of solutions for the following critical elliptic nonlocal problem:
(1)
where Ω is a smooth bounded domain of 4, a and b are positive constants, and f belongs to H−1 satisfying suitable condition specified afterward.

In our setting, the Laplacian operator is associated to Kirchhoff term aΩ|∇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 [710].

We begin by giving an overview about the previous research related to the problem which can be written in the more general form,
(2)

where Ω is a smooth bounded domain of N, 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., λλ0 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 N, 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 the nonhomogeneous case, namely, when Tarantello [13] proved the existence of at least two solutions when f satisfies
(3)

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.

In order to study we shall work with the space endowed with the norm we use also the following notation: for 1 ≤ p < ∞, C and Ci denote generic positive constants whose exact values are not important, BH(0, r)≔{uH : ‖u‖ < r} is the ball of center 0 and radius r, on(1) denotes any quantity which tends to zero as n tends to infinity, and S is the best Sobolev constant defined by
(4)
To state the main results, we define
(5)
where b > 0, a < S−2 a small enough positive number and f belongs to H−1\{0}.

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.

Notice that γa,b,f ≥ 0 if
(6)
Moreover, the assumption γa,b,f > 0 certainly holds if f satisfies certain conditions, for example,
(7)
for all uH with ‖u4 = 1. Indeed, we have γ0,b,f is achieved and strictly positive if f satisfies (f)2 (see Lemma 2.2 in [13]). In order as
(8)
then, γa,b,fγ0,b,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

We consider the energy functional associated to problem defined for uH and given by
(9)
Observe that IC1(H, ), whose derivative at the point uH is given by
(10)

Obviously, if uH is a critical point of the functional I; then, u is a weak solution of problem

In general, I is not bounded from below on H, to overcome this and achieve a multiplicity result, the key argument is to use an appropriate manifold called in mathematical literature the Nehari manifold, it is a suitable manifold who has a pertinent property to prove the distinction of two solutions. Indeed, a minimizer in this set may give rise to solution of the corresponding equation. This so called Nehari manifold is defined by
(11)

Lemma 3. Assume that b > 0, a ≥ 0, and fH−1\{0}. Then, the functional I is coercive and bounded from below on

Proof. For , we have

(12)

Therefore

(13)

Thus, I is coercive and bounded from below on .

Let hu(t) = I(tu) for t and uH. 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:

(14)
(15)

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

(16)

I verifies Palais Smale condition at level c ((PS)c condition in short) if any (PS)c sequence has a convergent subsequence in H.

Next, for u ≠ 0, b > 0, and a, a small enough positive number set
(17)
then Easy computations show that Φu is concave and achieves its maximum at the point where
(18)
That is,
(19)
(20)
Now, for uH\{0} set , that is
(21)
Fix t1 > 0, then, for tt1, we have Ψ(tu) = tΨ(u),
(22)
so
(23)

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

(24)
(25)

From (24) and (25), we derive that

(26)

As 0 ≤ a < S−2, we get from (24), (26), and the definition of S

(27)
that is
(28)

Thus, as u ≠ 0 and a < S−2, we derive that

(29)

therefore, from (25), we obtain ‖u4t1, with

(30)

Then, from (23), (25), and (26), we get

(31)
which yields to a contradiction.

Lemma 6. Assume that γa,b,f > 0, then, for all uH\{0}, there exists unique positive value such that

(32)

Moreover, if ∫Ωfudx > 0, then, there exists unique positive value such that

(33)

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

(34)
(35)

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

(36)
(37)

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

(38)
then
(39)

Thus if uH\{0} and,

(40)
then, u and
(41)

Let then

(42)

Since t+(u) > t(u), it follows that

(43)

So, ‖u‖ < t+(u/‖u‖), and we conclude that

By Lemma 6, we know that and are not empty, so we can define θ0θ1 with
(44)

Lemma 8. Assume that γa,b,f > 0, then, there exists t > 0 such that

(45)

Proof. Let then

(46)

Thus .

Set uH the unique solution of the equation −Δu = f, it follows

(47)

By Lemma 6, there exists a unique positive value such that . So

(48)
consequently
(49)

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)⟶+\{0} such that

(50)
(51)

Proof. Let and define: × H as follows

(52)

Clearly, (1, 0) = 0. Moreover, from Lemma 5, we derive that

(53)

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

(54)

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

(55)
(56)
for all Thus, I(un)⟶θ0.

By using Lemma 8, we get for n large enough

(57)
this implies that
(58)
then
(59)
and by Holder inequality, we get
(60)

Consequently, un ≠ 0 and

(61)

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)⟶ such that with vn = δI(un)/‖I(un)‖ and 0 < δ < ε. By (56) and the Taylor expansion of I, we have

(62)

Then

(63)

We have

(64)
(65)

This, together with (61) implies

(66)
for a suitable constant C3 > 0. Now, we must show that is uniformly bounded in n: indeed, since {un} is a bounded sequence, we have from Lemma 8
(67)
for a suitable constant C4 > 0. Assume by contradiction that for a subsequence still called {un}, we have
(68)

Then, as a is a small enough positive number, we get

(69)

So from (61), we derive that

(70)

Also, as , we get from (68)

(71)
then
(72)
which is absurd. At this point, we conclude that I(un)⟶0 in H−1.

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

(73)
attaints its infimum at a point . Moreover, u0 is a local minimizer for I in H.

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 unu0 weakly in H, then, 〈I(u0), w〉 = 0, for all wH. In addition, from (60), we get ∫Ωfu0dx > 0. So, u0 is a weak solution for and

Thus

(74)
then It follows that {un} converges strongly to u0 in H, then, and necessarily
(75)

To conclude that u0 is a local minimum of I, let us recall that we have from Lemma 6

(76)

Choose ε > 0 sufficiently small to have

(77)
and t(w) satisfying for every ‖w‖ < ε. Since t(w)⟶1 as ‖w‖⟶0, we can always assume that
(78)
so and for , we have
(79)
from (75), we can take s = 1 and conclude that I(u0w) ≥ I(u0), for all wH such that ‖w‖ < ε. Thus, u0 is a local minimum of I.

If f ≥ 0, we have ∫Ωfu0dx ≤ ∫Ωf|u0|dx and clearly I(|u0|) ≤ I(u0), and from (75) necessarily t(|u0|) ≥ 1. Therefore, as , we get

(80)
so, we can always take u0 ≥ 0.

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

(81)

Proof. Let {un} be a (PS)c sequence with then

(82)

Hence, {un} is a bounded sequence in H. Thus for a subsequence still denoted {un} and we can find u1H such that unu1 in H, ∫Ωfundx⟶∫Ωfu1dx and unu1 a.e in Ω. Therefore, u1 ≠ 0, , and I(u1) ≥ θ1.

Let wn = unu1. From Brézis-Lieb Lemma [21], one has

(83)
(84)
this implies that
(85)

Assume that ‖wn‖⟶l with l > 0, then, by (85) and the Sobolev inequality, we obtain

(86)
this implies that
(87)

Hence

(88)

On the other hand, we have

(89)
consequently, we obtain
(90)
which is a contradiction. Therefore, l = 0 and unu1 strongly in H.

Now, it is natural to show that As it is well know, (seeHL), the best Sobolev constant S defined above is attained in 4 by

(91)

For x0Ω 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

(92)

Let ΩΩ 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 fH−1\{0} satisfies γa,b,f > 0; then, for every t > 0 and a.e. x0ΩΩ, there exists ε0 such that for every 0<ε < ε0.

Proof. For ε small enough, let us consider the functional g defined by

(93)

We have by (92)

(94)

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)

(95)
where
(96)

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

(97)

Then, for a = εσ with σ > 1, we conclude

(98)

Therefore,

(99)
for a small enough positive number.

Proposition 14. Let b > 0, 0 ≤ a < S−2 a small enough positive number. Assume that fH−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 uH 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

(100)

Therefore for t0 > 0 carefully chosen, the estimate (97) holds for ε small enough.

Thus, we derive that

(101)

Then, from Lemma 7, we conclude that u0 + t0uεE2.

Set

(102)

It is obvious that ξ : [0, 1]⟶H given by ξ(t) = u0 + tt0uε belongs to Γ. We conclude from Lemma 13 that

(103)

As the range of any ξΓ intersects one has

(104)

From Lemma 10, we can obtain a minimizing sequence such that

(105)

We also deduce that

Consequently, we obtain a subsequence still denoted {un}, and we can find u1H such that

(106)

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

(107)

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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