Existence of Sign-Changing Solution with Least Energy for a Class of Schrödinger-Poisson Equations in R3 ()
1. Introduction
In this paper, the following nonlinear Schrödinger-Poisson system will be discussed
(1.1)
where the potential function
,
is a parameter and
. We can assume that f satisfies the following assumptions:
(f1)
;
(f2)
;
(f3)
, where
;
(f4)
is an increasing function of
.
To avoid involving too munch details for checking the compactness, we may assume that
and satisfies:
(V1)
for all
, where
is a positive constant; meanwhile, we set up the weak decay hypothesis on
:
(V2) there is
such that
.
We could also call system (1.1) as Schrödinger-Maxwell system, which is used in physics. In fact, the coupled nonlinear Schrödinger equation and Poisson equation can be used to describe the interaction of charged particles with electromagnetic fields. To learn more about the physical aspects of the Schrödinger-Poisson equation, the reader can read the related literature [1] [2] [3] and the references therein. What’s more, readers can also read the following articles, including [4] [5] [6], which show the mathematical and physical background of system (1.1).
In recent years, there has been a lot of research on the solutions of Schrödinger-Poisson equation, especially the existence of positive solutions, multiple solutions, sign-changing solutions, ground state solutions and semi-classical states, we can look at literatures [2] [5] [7] - [14] and references therein. In addition, the research on the existence of sign-changing solutions is in [15] - [20], etc.
As we can see, Wang and Shuai in [17] also studied problem (1.1) and they obtained the existence of sign-changing solution to problem (1.1). They assumed that
and
satisfies (f1), (f2) and the following conditions:
(f3)’
;
(f4)’
is an increasing function of
.
By introducing a parameter
, they show that any sign-changing solution for system (1.1) is strictly greater than twice the least energy solution. What’s more, they combine the constrained variational method with the quantitative deformation lemma to prove the existence of the least energy sign-changing solution. In addition, the energy doubling and asymptotic properties of the solution are also discussed. In contrast to Wang and Shuai’s proof, we refer to the truncation function, which is inspired by [21] [22] [23] [24].
In [13], the following system is considered
(1.2)
where
,
and
. The authors obtained some existence and nonexistence results of positive radial solutions by using variational method, depending on the parameters
and p. It turns out that
is the critical value for the existence and nonexistence of solutions. However, their study of the existence of positive radial solutions for system (1.2) is dependent on the parameter
, which seems difficult to be applied to similar systems with variable potential.
Zhang in [22] consider the following Schrödinger-Poisson equation
(1.3)
where
, f has a critical growth. The author obtained the existence of solutions for system (1.3) with a general nonlinearity in the critical growth by variational method. But he did not study the existence of sign-changing.
Sofiane Khoutir in [25] considered the following system
(1.4)
where
is a positive constant. By using variational methods in combination with the Pohožaev identity, Sofiane Khoutir proved that system (1.4) has the least energy sign-changing solution and a ground state solution provided that
is sufficiently small. However, if the potential is not a positive constant, for example, the potential is variable, that is
, it is very difficult to verify the Sobolev embedding compactness.
In our work, we consider variable potential
and put some constraints on it, and then study the least energy sign-changing solution and ground state solution of the Schrödinger-Poisson Equation (1.1).
We now need to introduce some symbolic notations. As usual, for
, let
(1.5)
Let
(1.6)
with the inner product and norm
(1.7)
Therefore, the embedding
↪
is continuous for
, moreover, there exists a constant
such that
(1.8)
Let
(1.9)
Then,
, for
, the embedding
↪
is compact.
Let
be the Sobolev space with norm
(1.10)
Then, the embedding
↪
is continuous (see [26] ) and the best Sobolev constant is
(1.11)
We have known that for any
, if
is the unique solution of
in
, then
(1.12)
What’s more, the properties of
are as follows (the detail proof can be seen in [27] ):
Lemma 1.1. For
, we have
(i)
;
(ii)
;
(iii) If
weakly in
, then
weakly in
and
(1.13)
(iv) There exists a constant
, by Hölder inequality, such that
(1.14)
(v) If u is a radial function, then
is radial.
Now, we consider a family of
defined by
(1.15)
Hence, by (f1), (f2), (V1) and (V2),
is well defined and
. For any
, there is
(1.16)
Note that
is a solution of problem (1.1) if and only if
is a critical point of
and
. Moreover, the critical points of
on H are the critical points of
on
by the critical principle of symmetry. So, finding the weak solution of problem (1.1) is equivalent to finding the critical point of the functional
.
In this paper, we denote
(1.17)
then
.
We define the Nehari manifold for the energy functional
of problem (1.1) as
(1.18)
and the nodal-Nehari manifold
(1.19)
What’s more, we denote
(1.20)
Moreover,
denote positive constants possibly different in different places. Strong convergence is expressed in terms of
and weak convergence is expressed in terms of
.
The main result of this paper is presented as follows.
Theorem 1.1. Assume that (f1)-(f4), (V1) and (V2) hold. Then there exists a positive
such that for all
, problem (1.1) has a least energy sign-changing solution
and a ground solution
which is constant sign. In addition, these two solutions satisfy the following relationship
Remark 1.1. It is easy to see that (f3) and (f4) are weaker than (f3)’ and (f4)’, respectively, so our result can be seen as a generalization of the result in [17]. Besides, we consider variable potential, from this point, our result can be seen as a slight generalization and improvement of [25].
The paper is organized as follows. In Section 2, we provide some lemmas, which are crucial to prove the main result of this paper. Section 3 is devoted to the proof of Theorem 1.1.
2. Preliminaries
We shall obtain a critical point of
by a mountain pass type argument, however, even though it is likely that critical point has a mountain pass geometry, showing that the (PS) sequence at the mountain-pass level are bounded seems out of reach under our weak assumptions on f. To overcome this difficulty, inspired by [21] [22] [23] [24], which consists in truncating the “rest” term of
outside of a ball centered at the origin and to show that, as
goes to zero, all (PS) sequences at the mountain-pass level lie in this ball, which is called truncated technique. Precisely, let
be the truncation radius and consider a smooth function
satisfying
,
,
and
is not increasing on
. Similar to [21] [22] [23] [24], for any positive constant
, we consider the truncated functional
defined by
(2.1)
where
. From (f1), (f2), (V1) and (V2), it is easy to check that
and
(2.2)
In the following, we try to find a critical point
of
on H for small
. Then, by showing that
, we will prove that
also solves the original problem (1.1). Similarly, we can define the Nehari manifold of
as
(2.3)
and the nodal-Nehari manifold
(2.4)
What’s more, we denote
(2.5)
We have the following result.
Theorem 2.1. Assume that (f1)-(f4), (V1) and (V2) hold. Then there exists
such that for all
, the functional
possesses one least energy critical point
which is constant sign and one least energy sign-changing critical point
. Moreover, the energy of the sign-changing critical point is strictly greater than the least energy, that is
Lemma 2.1. For each
with
, there exists a pair
with
such that
, moreover
Proof. For any
with
, define the function
by
and its gradient is
By (f1) and (f2), for any
and
, there exists
such that
(2.6)
By (1.8), (2.6), the conclusion (i) of Lemma 1.1 and the property of
, we obtain
where every constant
is non-negative and
. Then, for
small enough,
. On the other hand, we can get that from (f3), for
large enough, there exists a large
such that
(2.7)
Hence, for
sufficiently large, from (2.7), we have
Therefore, we can get
when
. We can infer that there is a pair of
such that
Then, we prove that
. Without loss of genreality, we assume that
is the maximum point of
. Hence, we have
From (f1), we can get
for
small enough, which implies
that
is increasing for t small. This contradicts with the fact that
is the maximum point of
. Therefore,
is a positive maximum point of
.
Finally, according to the definition of
, we note that
is equivalent to
for any
. Since the pair of
is a positive maximum point of
, we observe that
then,
which implies that
, because of
. This completes the proof. o
Corollary 2.2. For each
, there exists a
with
such that
, moreover
Lemma 2.3. (see [28] [29] ) Let
and
. If
is bounded in H and
then we have
in
for
.
Similar to [25], we have the following lemma.
Lemma 2.4. Assume that (f1)-(f4), (V1) and (V2) hold. Then, for any
with
, one has
Lemma 2.5. Let
be a minimum sequence of
, then
is bounded in H.
Proof. We prove this lemma by contradiction. Set the unit normal vector of the level surface of the functional
is
, and suppose
as
. Therefore, we have
. Going to a subsequence if necessary, we may assume that
Hence, we’re going to consider two cases:
or
.
Case (i).
. For
, then
(2.8)
By Lemma 2.3 and (2.8), we have
in
for
. Let
. By (f1) and (f2), for any
and
, there is
such that
(2.9)
Then, by (1.8), (2.8), (2.9), Lemma 2.4 and the property of
, for n sufficiently large such that
, one has
which is a contradiction by the arbitrariness of
.
Case (ii).
. There are
and a sequence
such that
(2.10)
Let
. Hence, for
, one has
as
. From (f3) and Fatou’s Lemma, we have
a contradiction. Therefore,
is bounded. o
Corollary 2.6. Let
be a minimizing sequence of
, then
is bounded in H. Hence, there exists a constant
such that
.
Lemma 2.7. If
, then
satisfies
condition.
Proof. In view of Corollary 2.6, let
be such that
Then,
is bounded in H. Since
, we have
(2.11)
Then, by (f1), (f2), (1.8) and (2.11), one has
(2.12)
Therefore, there exists a constant
such that
(2.13)
Let
be a
sequence for
, i.e.
(2.14)
We can derive from Lemma 2.3 that
is bounded in H, up to a subsequence, there exists
such that
(2.15)
From (1.8), (2.9), (2.15), Corollary 2.6 and Hölder inequality, we have
(2.16)
From (1.11), (2.15), Lemma 1.1 and Hölder inequality, we obtain
(2.17)
By (2.2), (2.14), (2.16), (2.17), for n large enough, we have
(2.18)
Hence, from (2.18), we have
. The proof is completed. o
Lemma 2.7. The
is achieved at some
for
small, which is a critical point of
in H.
Proof. By Lemma 2.6, we know that
satisfies
condition, then, there exists a
such that
(2.19)
in H as
. Then, by (2.12), one has
, likewise,
. It means that
. Since H is a Hilbert space and the project mapping
is continuous in H, we get
and
, then
is a sign-changing function. And then we show that
. Since
, one has
by (2.19) and passing to the limit, one has
which means that
and
. So the minimum value of
is achieved at u, therefore u is a nontrivial critical point of
in
.
We also need to show that u is the critical point of
in H. Because u is a critical point of
in
, we have that
in
. Then, there is a Lagrange multipiler
such that
(2.20)
where
. That’s enough to prove that
. By (2.20), one has
(2.21)
Taking
, meanwhile, for any
with
, we calculated that
And we can find out from (f4) that there is a positive constant
such that
Hence,
for
sufficiently small, which together with (2.21) shows that
. Thus, the proof is completed. o
Corollary 2.8. The
is achieved at some
, which is a critical of
in H.
Proof of Theorem 2.1. Through the lemmas and corollaries in this Section, we know that
has a least energy critical point and a least energy sign-changing critical point, which are
and
respectively. For
, by the above discussions, there exists a
such that
, then
Finally, we’re going to prove that
is a constant sign. By assuming indirectly, assume that
is sign-changing, then
and
it is absurd. We’ve done the proof.
3. Proof of the Main Result
First, an important identity is given, which will be used to prove that
and
are uniformly bounded in H. For details of Pohožaev identity, one can see [30].
Lemma 3.1. If
is a critical point of
, then for
small, u satisfies
Lemma 3.2. For
and
obtained in Theorem 2.1, if
large enough and
small enough, then we have
and
.
Proof. This part of proof is similar to [22]. However, it plays a key role in the proof of Theorem 1.1, so we give in detail here for completeness and convenience to the readers.
According to Hardy inequality [30], one has
(3.1)
Since
, by Lemma 3.1, Lemma 1.1 (iv), (V2) and (3.1), one has
therefore, we have
If
, then
. Therefore, the following inequality holds
(3.2)
By (1.16), (2.9) and
, we have
(3.3)
By
↪
, (3.3) and (V1), we have
(3.4)
Therefore, for
, according to (3.2) and (3.4), we have
(3.5)
We make the opposite hypothesis that
, then, by (3.5), we have
(3.6)
Choosing
and
, then (3.6) yields
which is impossible. Thus
, similarly, we can prove that
. This completes the proof. o
Proof of Theorem 1.1. Let T be large enough and
small. We know from Theorem 2.1 that
has a least energy critical
at level
and a least energy sign-changing critical point
at level
, and by Lemma 3.2 we have that
,
, therefore
and
and
are critical points of
with
and
. Hence, system (1.1) has a least energy sign-changing solution
and a ground state solution
which is constant sign. Moreover, since
, it follows from Lemma 2.1 that
The proof is completed.
4. Conclusion
In this paper, we firstly proved that the Schrödinger-Poisson equation has a sign-changing solution by using a truncation technique, and then prove that the minimum sequence
is bounded in H. What’s more, according to the condition that
satisfies (PS) sequence, we find out a critical point when the least energy sign-changing solution is achieved, and similarly find out a critical point when the ground state solution is achieved and prove that the sign-changing solution is strictly larger than the ground state solution. Finally, we prove that the critical points are uniformly bounded in H using the Pohožaev identity. It is obviously that the truncation function has been successfully applied to solve the least energy sign-changing solution of the Schrödinger-Poisson system. We hope that the truncation technique can be widely used in the study of sign-changing solutions of similar systems.
Acknowledgements
The authors would like to thank the referees for their useful suggestions which have significantly improved the paper.
Funding
This work is supported by the National Natural Science Foundation of China (No. 11961014, No. 61563013) and Guangxi Natural Science Foundation (2021GXNSFAA196040, 2018GXNSFAA281021).
Availability of Data and Materials
No data were used to support to the work.
Authors’ Contributions
The authors declare that the study was realized in collaboration with the same responsibility. All authors read and approved the final manuscript.