Volume 2022, Issue 1 6791308
Research Article
Open Access

Existence Solutions for a Class of Schrödinger-Maxwell Systems with Steep Well Potential

Guocui Yang

Guocui Yang

Department of Mathematics, Baoshan university, Baoshan, Yunnan 678000, China bsnc.cn

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Shengzhong Duan

Corresponding Author

Shengzhong Duan

Department of Mathematics, Baoshan university, Baoshan, Yunnan 678000, China bsnc.cn

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First published: 12 April 2022
Citations: 1
Academic Editor: Gisele Mophou

Abstract

In this paper, we are concerned with the system of the Schrödinger-Maxwell equations where λ, b > 0 are constants, and 3 < p < 6. Under appropriate assumptions on V and K, we prove the existence of positive solutions in the case 3 < p < 4 via the truncation technique. Moreover, suppose that V may change sign, we also obtain the multiplicity of solutions for the case 4 < p < 6.

1. Introduction and Main Results

Consider the following system of Schrödinger-Maxwell equations:
(1)
where λ, b > 0 are constants, and 3 < p < 6. Such a system is also called the Schrödinger-Poission equation which is obtained while looking for the existence of standing waves for nonlinear Schrödinger equations interacting with an unknown electrostatic field. For more details on the physical aspects, we refer the reader to [1] and the references therein.

On the potential V, we make the following assumptions:

(V1) VC(R3, R) and V is bounded below.

(V2) there exists a constant c > 0 such that the set {xR3 : V(x) ≤ c} is nonempty and meas{xR3 : V(x) ≤ c} < +∞, where meas denote the Lebesgue measure in R3.

(V3)Ω = intV−1(0) is nonempty and has smooth boundary and .

In recent years, system (1) has been widely studied under various conditions on V and K. The greatest part of the literature focuses on the study of the system for V and K being constants or radially symmetric functions. We refer the reader to [29].

When the potential V(x) is neither a constant nor radially symmetric, in [10, 11], the existence of ground state solutions is proved for 3 < p < 6. In [1214], the existence of nontrival solution is obtained via the variational techniques in a standard way under the following condition:

(V4) VC(R3, R), where a0 > 0 is a constant. Moreover, for any M > 0, meas{xR3 : V(x) ≤ M} < +∞, where meas denote the Lebesgue measure in R3.

It is worth mentioning that conditions (V4) were first introduced by Bartsch and Wang [15] to guarantee the compact embedding of the functional space. If replacing (V4) by more general assumptions (V1) and (V2), the compactness of the embedding fails and this situation becomes more complicated. Recently, [16, 17] considered this case. The authors studied the following problem
(2)
where λ > 0 is a parameter, the potential V may change sign and f is either the superlinear or sublinear in u as |u|⟶∞. Very recently, Liu and Mosconi [18] considered the following system with a coercive sign-changing potential and a 3-sublinear nonlinearity:
(3)

By using a linking theorem, the authors obtained the existence of nontrivial solutions. Nextly, Gu, Jin, and Zhang [19] investigated the existence of sign-changing solutions for system (3). By using the method of invariant sets of descending flow, the multiple radial sign-changing solutions are obtained in the subquadratic case as λ small. For more results about the Schrödinger-Poisson systems, we refer the reader to [2023] and the reference therein.

Here, we should point out that for the power-type nonlinearity f(u) = |u|p−2u, in order to get the boundedness of a (PS) sequence, the methods heavily rely on the restriction p ∈ (4, 6). Meanwile, the condition (V1) − (V3) cannot guarantee the compactness of the embedding of H1(R3) into the Lebesgue spaces Ls(R3), s ∈ [2, 6). This prevents from using the variational techniques in a standard way. Motivated by the works mentioned above and [2428], in the present paper, we are mostly interested in sign-changing potentials and consider system (1) with more general potential V, K, and the range of p. Our main results are as follows:

Theorem 1. Suppose that V ≥ 0, (V1)- (V3), , K(x) ≥ 0, and 3 < p < 4 hold. Then, system (1) possesses at least a nontrivial solution for b small and λ large.

Remark 2. It is known that it is difficult to get the boundedness of a (PS) sequence when dealing with the case p ∈ (3, 4). To overcome the difficulty, motivated by [24, 25], we use the truncation technique to obtain a bounded Cerami sequence for b small. In this case, the conditions (V4) and (K1) of Theorem 1.3 in [23] cannot be used. Moreover, in the process of proving the convergence of a bounded Cerami sequence, we use the observation that the condition KL3(R3) ∪ L(R3) makes the less strong influence of the nonlocal term K(x)ϕu (The conclusions remain valid if KL2(R3)). In this sense, Theorem 1 can be viewed as an improvement of Theorem 1.3 in Zhao et al. [23].

Theorem 3. Suppose that (V1), (V2), KL3(R3) ∪ L(R3), K(x) ≥ 0, and 4 < p < 6 hold. Then, system (1) possesses infinitely many distinct pairs of nontrivial solutions whenever λ > 0 is sufficiently large.

Remark 4. In Theorem 3, V is allowed to be sign-changing for p ∈ (4, 6). We obtain the multiplicity of solutions for (1).

2. Preliminaries

Let
(4)
is the usual Sobolev space with the standard inner product and norm
(5)
In our problem, we work in the space defined by
(6)
with the inner product and the norm
(7)
where V±(x) = max{±V(x), 0}, V(x) = V+(x) − V(x). Obviously, it follows from (V1), (V2) that the embedding EλH1(R3) is continuous.
As in [26], let
(8)
and denote the orthogonal complement of Fλ in Eλ by . Consider the eigenvalue problem
(9)

In view of (V1), (V2), the quadratic form is weakly continuous. We have the following proposition.

Proposition 5 ([26], Lemma 8). Suppose (V1), (V2), and V ≠ 0. Then, for each fixed j,

  • (i)

    μj(λ)⟶0 as λ⟶+∞

  • (ii)

    μj(λ) is a nonincreasing continuous function of λ, where is sequence of positive eigenvalues of problem (P) satisfying μ1(λ) ≤ μ2(λ) ≤ ⋯≤μj(λ)⟶∞ as j⟶∞ and the corresponding eigenfunctions .

Let

(10)

Then,

(11)

Moreover dim for every fixed λ > 0.

In the sequel, we denote the usual Lp-norm by ‖·‖p, and C stands for different positive constants. For any R > 0, BR(x) denotes the open ball of radius R centered at x. Since the continuity of the following embedding
(12)
there are constants as > 0 and a > 0 such that
(13)
It is well known that system (1) is the Euler-Lagrange equation of the functional J : Eλ × D1,2(R3)⟶R defined by
(14)

Evidently, the action functional J belongs to C1(Eλ × D1,2(R3), R) and its critical points are the solutions of system (1). It is easy to know that J exhibits a strong indefiniteness, namely, it is unbounded both from below and from above on infinitely dimensional subspaces. This indefiniteness can be removed using the reduction method described in [29], by which we are led to study a one variable functional that does not present such a strongly indefiniteness.

Actually, considering for all uEλ, the linear functional Lu defined in D1,2(R3) by
(15)
If KL(R3), the Hölder inequality and the Sobolev inequality imply
(16)
while for KL3(R3), we have
(17)
Hence, by the Lax-Milgram theorem, there exists a unique ϕuD1,2(R3) such that
(18)
Moreover, we can write an integral expression for ϕu in the form:
(19)
So, we can consider the functional Iλ : EλR defined by Iλ(u) = J(u, ϕu). Then,
(20)
It follows from (16), (17), (18), and the Sobolev inequality that
(21)
(22)
Thus, I is a well-defined C1 functional with derivative given by
(23)

By the proposition 2.3 in [12], we know that (u, ϕ) ∈ Eλ × D1,2(R3) is a critical point of J if and only if u is a critical point of I and ϕ = ϕu.

To complete the proof of our theorem, we need the following results.

Theorem 6 (see [30].)Let X be a real Banach space with its dual space X, and suppose that JC1(X, R) satisfies

(24)
for some μ < η, ρ > 0 and eX with ‖eX > ρ. Let cη be characterized by
(25)
where Γ = {γC([0, 1], X): γ(0) = 0, γ(1) = e} is the set of continuous paths joining 0 and e. Then, there exists a sequence {un} ⊂ X such that
(26)

Theorem 7 see ([31], Theorem 9.12). Let E be an infinite dimensional Banach space and let IC1(E, R) be even, satisfy (PS), and I(0) = 0. If E = VX, where V is finite dimensional and I satisfies.

(I1) there are constants ρ, α > 0 such that , and

(I2) for each finite dimensional subspace , there is an such that I ≤ 0 on ,

then, I possesses an unbounded sequence of critical values.

Lemma 8 ([17], Lemma 2.3). The function ϕu possess the following properties:

  • (1)

    The mapping uϕu maps bounded sets of Eλ into bounded sets of D1,2(R3);

  • (2)

    If unu in Eλ, then in D1,2(R3);

  • (3)

    The mapping uϕu : EλD1,2(R3) is continuous.

3. Proof Of Theorem 1

In this section, we give the proof of Theorem 2 by using Theorem 6. Let ηC([0, ∞), R) be a cut-off function satisfying 0 ≤ η ≤ 1, η(t) = 1 if 0 ≤ t ≤ 1, η(t) = 0 if t ≥ 2, maxt>0|η(t)| ≤ 2 and η(t) ≤ 0 for each t > 0. For every T > 0 we consider the truncated functional J : EλR defined by
(27)
It is easy to see that J is of C1. Moreover, for each u, vEλ, we have
(28)

Lemma 9. Suppose that 3 < p < 4 and (V1) − (V3),V(x) ≥ 0 and , K(x) ≥ 0 hold. Then,

  • (i)

    there exists a vEλ\{0},with J(v) < 0

  • (ii)

    there exists M > 0 independent of T, λ and b such that , where , Γ = {γC([0, 1], Eλ): γ(0) = 0, γ(1) = v}

Proof. For any t > 0, by (19), we have

(29)

Let x0Ω and for some r0 > 0. Let be such that . Set vt = t2u(t(xx0)), then for t > 1. Hence, it is easy to see that
(30)
Since p > 3, we have that J(vt)⟶−∞ as t⟶+∞. Taking v = vt for t large enough, we have J(v) < 0.
  • (iii)

    since there exists a constant M > 0 (independent of T, λ and b) such that .

Lemma 10. Suppose that 3 < p < 4 and (V1), (V2) hold. Then, there exists α > 0 such that .

Proof. For any uEλ, we have

(31)

Since p > 3, we conclude that there exists ρ > 0 such that J(u) ≥ α > 0 for all uEλ with

From Lemmas 9 and 10 and Theorem 6, we thus deduce that there exist a Cerami sequence {un} ⊂ Eλ such that

(32)

Lemma 11. Let 3 < p < 4, V(x) ≥ 0, (V1) − (V2), K(x) ≥ 0, and KL3(R3) ∪ L(R3) be satisfied. Then, there exists Λ > 1 such that for each λ ∈ (Λ, ∞), if {un} ⊂ Eλ is sequence satisfying (32), then {un} has a convergent subsequence in Eλ for b small enough.

Proof. Let {un} be a Cerami sequence satisfying (32). Let , we show that . We first prove that . Suppose by contradiction that there exist a subsequence of {un}, still denoted by {un}, such that , we obtain

(33)

which is a contradiction by Lemma 9. Suppose that there exists no subsequence of {un} which is uniformly bounded by T. Then, we deduce that . We handle the case of KL3(R3) (The conclusions remain valid if KL(R3)). By (22), we obtain

(34)

This is contradiction by choosing b sufficiently small. Since , passing to a subsequence if necessary, we can assume that there exists uEλ such that unu, inEλ . In view of the Sobolev embedding theorems and Lemma 8, unu inEλ implies.

(35)

Furthermore, for any φEλ, we have

(36)

In fact, if KL(R3), we just need to show that

(37)

Note that

(38)
The first limit on the right is 0 by the fact inL2(R3) and so is the second limit because inL6(R3) and uφL6/5(R3). While for KL3(R3), on the one hand, since Ku2L6/5(R3), using (35) (c), we have
(39)
as n⟶∞. On the other hand, since KL3(R3), for any ε1 > 0 there exists ρ1 = ρ1(ε1) > 0 such that
(40)

Moreover, in view of the Sobolev embedding theorem, unu implies that

(41)

Hence, for large n, we obtain

(42)

Consequently,

(43)

From above inequality and (39), one has

(44)

Then, J(un)⟶0 implies that

(45)

Let vnunu. It follows from (V1), (V2) that

(46)

Moreover, let 0 < α < min{(6 − p/2), 1}, 2 < p < 6. Then, 2 < 2(pα)/2 − α < 6. By the Sobolev inequalities and Hölder inequality, one has

(47)

We know

(48)

Letting Λ > 0 be so large that the term in the brackets above is positive when λΛ, we get vn⟶0 in Eλ. Since vn = unu and vn⟶0, it follows that unu in Eλ. This completes the proof.

Proof of Theorem 2. Note that {un} is also a Cerami sequence of Iλ satisfying , the conclusion follows from Lemmas 9, 10, and 11 and Theorem 6.

4. Proof Of Theorem 3

In this section, while V is sign-changing, we study the existence of solutions of (1) for the case 4 < p < 6 and give the proof of Theorem 3. Without loss of generality, we assume that b = 1.

Lemma 13. Let (V1), (V2), K(x) ≥ 0, and KL3(R3) ∪ L(R3) be satisfied. Then, there exist α, ρ > 0 such that Iλ(u) ≥ α for all with .

Proof. By proposition 5, for each fixed λ > Λ, there exists a positive integer nλ such that μj(λ) ≤ 1 for j < nλ and μj(λ) > 1 for jnλ. Thus, for any , we have

(49)

for all , where . Since p > 2, the conclusion follows by choosing ρ sufficiently small.

Lemma 14. Let (V1), (V2), K(x) ≥ 0, and KL3(R3) ∪ L(R3) be satisfied. Then, for any finite dimensional subspace , there is a large r > 0 such that Iλ(u) < 0 on .

Proof. Since all norms are equivalent in a finite dimensional space, there is a constant b1 > 0 such that

(50)

By (22), there is a constant CK > 0 such that

(51)

Hence, for all ,

(52)

Since p > 4, there is a large r > 0 such that I(u) < 0 on .

Lemma 15. Let (V1), (V2), K(x) ≥ 0, and KL3(R3) ∪ L(R3) be satisfied. Then, there exists Λ > 1 such that, for each λ ∈ (Λ, ∞), Iλ satisfies the (PS)c condition.

Proof. Let {un} be a (PS)c sequence, that is Iλ(un)⟶c and . If {un} is unbounded in Eλ, up to a subsequence, we can assume that

(53)
as n⟶∞, after passing to a subsequence. Set , we can assume that wnw in Eλ and wn(x)⟶w(x) a.e.xR3.

If w = 0, since is weakly continuous, we have

(54)

a contradiction. If w ≠ 0, then the set Ω = {xR3 : ω(x) ≠ 0} has the positive Lebesgue measure. For xΩ, one has|un(x)|⟶∞ as n⟶∞, Fatou’s lemma shows that as n⟶∞. Thus, by (22), we obtain

(55)

This is a contradiction. This implies {un} is bounded in Eλ. Going if necessary to a subsequence, we can assume that unu in Eλ. The following proof is similar to the proof of Lemma 11. We omit details of this.

Proof of Theorem 3. Obviously, Iλ(0) = 0. Furthermore, Iλ is even. The conclusion follows from Lemmas 13, 14, and 15, and Theorem 7.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgment

This work is supported partially by the Foundation of Baoshan University (BYPY202016).

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