1. Introduction
When one looks for standing waves of the Schrödinger equation coupled with the Bopp-Podolsky theory of the electromagnetic field in the purely electrostatic situation, it is equivalent to consider the existence of solutions for the following Schrödinger-Bopp-Podolsky system
(1.1)
where
,
,
, and
. From the physical standpoint, u represents the modulus of the wave function and
is the electrostatic potential, the parameter q has the meaning of the electric charge and a is the parameter of the Bopp-Podolsky term [1]. As is known to all, the Bopp-Podolsky theory, a second-order gauge theory of the electromagnetic field, was developed by Bopp [2] and then independented by Podolsky [3]. According to Mie theory [4] and its generalizations in [5] [6] [7] [8], the Bopp-Podolsky theory was introduced to solve the alleged infinity problem in classical Maxwell theory.
As far as system (1.1) is concerned, there are very few papers related to the existence of solutions. Indeed, to the best of our knowledge, Siciliano and d’Avenia in [9] for the first time showed that system (1.1) possesses nontrivial solutions by means of splitting lemma and the monotonicity trick, when p and q belong to different scope. Meanwhile, they also demonstrated that in the radial case, as
, the solutions they found tend to solutions of the classical Schrödinger-Poisson system. At the same time, Silva and Siciliano [1] proved by the fibering approach that system (1.1) has no solutions at all for large values of q and has two radial solutions for small
, when
. In addition, if system (1.1) is dependent on potentials, that is, non-autonomous, or the corresponding nonlinearity is of more general case, the authors in [10] [11] considered the existence of nontrivial solutions, the main results obtained in [12] - [16] are related to the existence of ground state solutions for system (1.1) with critical growth.
To deal with system (1.1), in the light of its variational structure, it can be reduced to search for nontrivial critical points of the associated energy functional. Actually, define the Hilbert space
normed by
then, according to the Gauss law, it can be proved that for every
there is a unique solution
of the second equation in system (1.1). Here,
is the usual Sobolev space with standard norm
In other words, a unique element
satisfies
in the weak sense. Moreover, it turns out to be that
(1.2)
where
.
In view of the solvability of
, that is (1.2), system (1.1) can be naturally reduced to the following single equation
(1.3)
As for (1.3), the corresponding energy functional
is defined by
Based on the above arguments, if
is a critical point of J, we call that the pair
is a weak solution of system (1.1). For the simplicity of the notations, throughout this paper we just say
, instead of
, is a solution of system (1.1).
Just as mentioned above, all the existing results involving system (1.1) focus on the case that
is a fixed and assigned parameter. Nevertheless, as a model coupling the Schrödinger field and the electromagnetic field, the physicists are more interested in the existence of “normalized solutions”, that is, solutions with prescribed L2-norm. To this subject, we have not found any references dealing with system (1.1), except for the recent work [17], in which the authors investigated the normalized solutions to a Schrödinger-Bopp-Podolsky system defined on a connected, bounded, smooth open set under Neumann boundary conditions. However, it is must be pointed out that, although there are no results about the normalized solutions of system (1.1), as far as we know, many results concerning the existence or non-existence of normalized solutions to the elliptic problems have been extensively established, see [18] - [22] and the references therein.
Motivated by the above references, especially [1] [9] [18] [19], the purpose of this paper is to handle with the existence of normalized solutions for system
(1.1) when p belongs to the scope
. As usual, for any given
,
searching solution of (1.3) with
(normalized solution) is equivalent to consider nontrivial solution of following constraint problem
(1.4)
It is worth mentioning that in this situation the parameter
arises as a Lagrange multiplier, depending on the solution and is not a priori given. To solve the problem (1.4), it can be obtained as a critical point of the following
functional
(1.5)
constrained on the L2-spheres in
(1.6)
A direct strategy to deal with (1.5)-(1.6) is to consider the constraint minimizing problem
(1.7)
and verify that the minimizers are critical points of
.
Up to now, we can state our first result as follows.
Theorem 1.1. For
, there exists
(depending on p) such
that all the minimizing sequences for (7) are precompact in
up to translations provided that
That is, there exists a couple of
being solution of (1.4).
We take advantage of the techniques used in [18] [19] to finish the proof of Theorem 1.1. In fact, for any minimizing sequence
of (1.7), due to vanishing
(or the dichotomy situation
and
) may occur, which leads to the main difficulty, that is, the (bounded) minimizing sequence
is lack of compactness. To avoid the above two cases, the effective procedure is to verify that any minimizing sequence weakly converges, up to translation, to a function
which is different from zero, excluding the vanishing case; and then, to show that
, which illustrates that the dichotomy property does not occur. On account of the above discussions, we firstly check that the energy functional I defined in (1.5) for problem (1.4) satisfies the hypothesis of Lemma 2.1, which guarantees that the condition (MD) as introduced in Remark 2.2 can be recovered. Furthermore, with the help of Proposition 2.5, we can examine that
.
Here, it should be noted that, different from [18] [19], to find the existence of constraint minimizers for problem (1.7), the main difficulty is caused by the inhomogeneity of
defined in (1.2), which makes the calculation involving the energy functional I more complicated and leads to more difficult to prove strong subadditivity with the current method.
In addition, we prove the nonexistence of the solution when
.
Theorem 1.2. If
, for any
, (1.4) has no positive solution in
.
As previously said, our Theorem 1.1 is the first attempt to consider the existence of normalized solutions for system (1.1). Notice that, if
, system (1.1) reduces to the following Schrödinger-Poisson system
(1.8)
which has been widely studied in recent years, see [23] [24] and the references therein. It is well known that system (1.8) is equivalent to
(1.9)
where now
Evidently, if
satisfies Equation (1.9), we are readily going to obtain standing wave solutions being of the form
corresponding to the following problem dependent on t
In addition, there are also some papers dealing with the existence of normalized
solutions of Equation (1.9). For the case that
, normalized solutions
can be found by considering the minimization problem, since the functional
is bounded from below and coercive on
. Bellazzini and Siciliano in [18] [19] proved that
is achieved when
is small for
and when
is large for
, respectively. Subsequently, for the range
, Jeanjean and Luo in [25] explicated a threshold value of ρseparating
the existence and nonexistence of minimizers of
. Catto and Lions in [26]
showed that minimizers of
exist for
provided that
for
some suitable
small enough. When p is L2-supercritical and Sobolev
subcritical, that is,
, the existence of normalized solutions can be
generalized to the minimization problem in [27].
Since the normalized solution
obtained in Theorem 1.1 corresponds to the standing wave
of the following evolution equation
Therefore, the stability of standing wave is the second concerned problem in present paper. Explicitly, we will discuss the orbital stability of standing waves with L2-norm for the following initial problem
(1.10)
Definition 1.3. Define
Then,
is orbitally stable if for every
there exists
such that for any
with
we have
Here,
is the solution of initial problem (1.10).
We obtain the strong stability of standing waves for (1.10), which is shown in the following theorem.
Theorem 1.4. Let
. Then the set
is orbitally stable for
determined in Theorem 1.1.
This paper is organized as follows. In Section 2, various preliminary results are presented to be used in the sequel. In Section 3, we focus our attention on the proofs of Theorem 1.1 and Theorem 1.2. Finally, the orbital stability obtained in Theorem 1.4 is established in Section 4.
2. Preliminaries
To prove our main results, some preliminaries are in order during this section. We first recall an abstract framework introduced in [18], however we could not narrate it again in order to avoid the repetition. For the simplicity, we directly apply it to our variational framework. Explicitly, for our constrained minimization problem (1.7), we rewrite it as follows
where
(2.1)
Under suitable assumption on T defined in (2.1), we have the strong convergence of the weakly convergent minimizing sequence.
Lemma 2.1. [18] Let
. Let
and
be a minimizing sequence for
weakly convergent, up to translations, to a nonzero function
. Assume that the following inequality is satisfied
(2.2)
and that
(2.3)
(2.4)
then
.
Remark 2.2. In the above lemma, (2.2) is usually called strong subadditivity inequality. In order to ensure that any minimizing sequence on
is relatively compact, (2.2) is the necessary and sufficient condition and it is a stronger version of the following inequality
(2.5)
which is referred as the weak subadditivity inequality. It is worth mentioning that in [18] [19], checking (2.2) for the Schrödinger-Poisson system is the essential step to solve problem (1.7). As a matter of fact, this is also very important for us. However, the inhomogeneity of
makes this more difficult than
. To prove (2.2), we adopt the mediate approach which ensures that
(MD) the function
is monotone decreasing.
Indeed, assuming that (MD) holds for
, we get
Therefore, one has
In other words, verifying (MD) helps us to obtain (2.2), but it is not an easy work. In order to overcome this difficulty, the following Proposition 2.5 provides one criterion for (MD).
Before presenting it, we give some necessary definitions needed in the subsequence.
Definition 2.3. Let
with
. A continuous path
such that
is said to be a scaling path of u if
(2.6)
where the prime denotes the derivative. Furthermore,
is the set of scaling paths of u.
Definition 2.4. Let
be fixed and
. We say that the scaling path
is admissible for the functional (1.5) if
is a differentiable function.
Proposition 2.5. Let
satisfy the set of assumptions (2.3) and (2.4). Assume that for every
. All the minimizing sequences
for
have a weak limit up to translations different from zero. Assume finally (2.5) and the following conditions
(2.7)
(2.8)
(2.9)
Then, for every
, the set
is nonempty. If, in addition,
(2.10)
then (MD) holds. Moreover, if
is a minimizing sequence for
weakly convergent, up to translations, to a nonzero function
,
(2.11)
(2.12)
then
. In particular, it follows that
.
Next, we need to consider the local well posedness for the Cauchy problem (1.10). The framework established in [28] helps us to achieve this fact. To make our problem better correspond to the abstract results in [28], we will give the following details. In fact, the local well posedness considered in the following is applicable to more general nonlinearity
(2.13)
For the nonlinearity being of the form
, we assume that there exist
(
) such that
(2.14)
In addition,
, there exist some
such that
(2.15)
and for every
there exists
such that
(2.16)
with
, where
represent the conjugate exponent of
. Finally, we assume that for every
(2.17)
Let
and define the energy E by
for
. Then, according to ( [28], Theorem 4.3.1), the following proposition is a direct consequence.
Proposition 2.6. [28] If g is defined as above. the initial-value problem (2.13) is locally well posed in
. Furthermore. there is conservation of charge and energy i.e.
for all
, where
is the solution of (2.13).
3. Proofs of Theorem 1.1 and Theorem 1.2
Firstly, we focus our attention on verifying the hypotheses of Proposition 2.5 to finish the proof of Theorem 1.1. We start this section to give some properties of
(see [9], Lemma 3.4), which will be used frequently in the later.
Lemma 3.1. ( [9], Lemma 3.4]) For any
.
has following properties:
1)
,
;
2)
;
3) if
in
, then
in
;
4)
.
Hereafter, we use
to denote suitable positive constants whose value may also change from line to line. The following lemma shows that problem
(1.7) is well-defined for
.
Lemma 3.2. For every
and
. the functional I as shown in
(1.5) is bounded from below and coercive on
.
Proof. In view of Lemma 3.1 and using the Gagliardo-Nirenberg inequality (see [29], Proposition 1.16), we have
(3.1)
where
. Since
, it results
, which concludes
the proof. □
Remark 3.3. For
, observing the above inequality (3.1) yields that the functional I is bounded from below and coercive on
. That is, Lemma 3.2
is effective for
. In addition, as a consequence of this lemma,
whenever
is fixed and
is a minimizing sequence for
, we obtain that
is bounded in
and exists a weakly convergent subsequence.
In order to verify all the hypothesis of Proposition 2.5, we begin with the weak subadditivity inequality (2.5).
Lemma 3.4. For the functional I defined in (1.5). The weak subadditivity inequality (2.5) is satisfied.
Proof. According to the definition of infimum, for any
, there are
such that
(3.2)
where
. Denote
, where
is some given unit vector in
. By ( [9], Lemma B.5), we get
in
and
a.e. in
, up to a subsequence if necessary. And, according to Brézis-Lieb Lemma (see [30], Lemma 1.32), we have
by ( [9], Lemma B.2), we derive that
Moreover, since
and
are translation-invariant, we can infer from (3.2) that
and
As a result, according to the definition of infimum for
, it is achieved that
□
Lemma 3.5. The functional T defined in (2.1) satisfies (2.3) and (2.4).
Proof. For the convenience of notation, we redefine (2.1)
It is obvious that
. Therefore, we only need to verify that both M and N hold true for the relationships (2.3) and (2.4). By Lemma 3.2, for
being an arbitrary minimization sequence for
,
is bounded in
. Due to the Sobolev embedding theorem,
is also bounded in
-norm for
and there is
such that
.
Note that, M and N satisfy the condition (2.3) which were proved by ( [9], Lemma B.2). Therefore, it is sufficient to verify that the condition (2.4) is satisfied for M and N. Actually, by Hölder inequality and Lemma 3.1, we have
Then,
is bounded. In addition, once
, we conclude
the proof from
and
Indeed, since
in
, up to a subsequence if necessary, by Brézis-Lieb Lemma we get
Hence,
which implies that
and
. So we deduce immediately that
and complete the proof. □
We are now concentrating on testing that conditions (2.7)-(2.9) are achievable.
Lemma 3.6. If
. then condition (2.7) is satisfied.
Proof. By Lemma 3.2, we have
for all
. Hence, it only needs to prove that
for every
. Let
and choose the family of scaling paths of u parameterized with
given by
such that
and
, where
and
is given in Definition 2.3. For the simplicity of notations, we introduce the following quantities
which gives that
Meanwhile, some direct calculations bring the equalities as follows
Taking
, we readily see that
as
, since
and
. This signifies that there is a small
such that
Letting
, then for every
, we derive from Lemma 3.4 that
since
. That is to say,
for s in the larger interval
. Iterating this procedure gives that
for every
and finishes the proof of this lemma. □
Lemma 3.7. If
, then the function
satisfies (2.8) and (2.9).
Proof. Firstly, we consider (2.8). Assume that
as
, it is equivalent to show
. For every
, let
such that
(3.3)
By the Gagliardo-Nirenberg inequality (see [29], Proposition 1.16), we have
Since
and
is bounded, we see that
is bounded in
. Thus,
,
and
are bounded sequences. So, by Lemma 3.1 and (3.3), using the fact that
as
, it leads to
(3.4)
On the other hand, given a minimizing sequence
for
, the following inequality holds
(3.5)
Combining (3.4) and (3.5), one has
, namely, condition (2.8) is established.
Next, we deal with
. Note that (2.7) implies that
where
Therefore, it is sufficient to verify that
as
. Indeed,
is the functional associated to the following pure Schrödinger equation
with prescribed L2-norm
. It is known that, if
, then for every
, there exists
such that
. For the details, we refer the reader to [31] [32].
For the minimizer
, by the Gagliardo-Nirenberg inequality, there holds
(3.6)
which implies that the sequence
is bounded in
for
. On the other hand, since the minimizer
for
satisfies the following equation in weakly sense
(3.7)
we infer form (3.6) that
(3.8)
where
is the Lagrange multiplier associated to the minimizer
. Observe (3.8), it reduces to prove that
.
We argue by contradiction assuming that there exists a sequence
such that
for some
. Since the minimizers
satisfy (3.7), we are led to
which yields that there exists
such that
due to
. However, in view of (3.6), it holds that
which is meaningless. Thus, we completed the proof. □
Based on Lemma 3.4-Lemma 3.7, we have shown that
.
Lemma 3.8. For every
. all the minimizing sequences
for
have a weak limit. up to translations. different from zero. Furthermore. the weak limit defined in Proposition 2.5 is contained in
.
Proof. Let
be a minimizing sequence in
for
. Notice that for any sequence
, the translation invariance guarantees that
is still a minimizing sequence for
. Thus, we only need to prove the existence of one sequence
such that the weak limit of
is different from zero. By the Lions’ lemma (see [30], Lemma 1.21), it follows that if
then
for any
, where
. So
as
, and then
, which contradicts to Lemma 3.6. Therefore, we must have
In this case we can choose
such that
Due to the compactness of the embedding
↪
, we deduce that the weak limit of the sequence
, let us call it v, is nontrivial.
In the next moment, we verify that
. Indeed, if
, it is trivial. Thus, we only discuss the case of
. If
, then using ( [19], Proposition 3.1), we have
and
, which indicates that
. □
As previously stated, the strong subadditivity inequality (2.2) is only used to ensure that (MD) holds, which is the purpose of the following lemma.
Lemma 3.9. For small
. the function
defined in Definition 2.4 satisfies (2.10).
Proof. For
, that is
and
, we set
for
. It is obvious that
.
Furthermore, a simple calculation gives that
Since the map
is differentiable and u is the minimizer of
on
, we infer that
which means that
(3.9)
Next, for
we compute explicitly
by choosing the family of scaling paths of u parameterized with
given by
Evidently, all the paths of this family have the associated function
where
is defined in (2.6). Denote by
can be rewritten as follows
(3.10)
Obviously,
is differentiable for every
, i.e., the paths in
are admissible.
Meanwhile, for
Hence, it remains to demonstrate that the admissible scaling path satisfies
, which can be chosen in
. To prove this point, we argue by contradiction. Assume that there exists a sequence
with
such that for all
Then, combining just the above with (3.9), we deduce that
(3.11)
As a result, it gives that
(3.12)
In the sequel, we derive the contradiction to four different situations by
showing that the relationships (3.12) are impossible for
and
is
small. Actually, by continuity, we know that
(3.13)
Case 1:
.
By the Hardy-Littlewood-Sobolev inequality (see [33], Theorem 4.3) and the interpolation inequality (see [34], Lemma 6.32), we have
where
. Then, according to Sobolev embedding theorem and
(3.11), one has
Since
, it results in
. Therefore, we are able to deduce that
which is a contradiction with (3.13).
Case 2:
.
Due to (3.11) and Lemma (3.1), we obtain that
which is impossible, since
.
Case 3:
.
Using the interpolation inequality, it follows that
where
. Since
, we infer that
,
which contradicts (3.13).
Case 4:
.
To this situation, one has
where
. Noting that
, we also get a contradiction
evidently. □
Remark 3.10. It is worth mentioning that in the above lemmas, except for Lemma 3.9, all the conclusions are effective for
. Unfortunately, we could not say anything more when
as did in Lemma 3.9. In fact, when
, as usual, we want to establish the strong subadditivity (2.2). However, the appearance of
in
, see (3.10), makes it impossible for us. In addition, since the functional
is unbounded from below on
if
, the above minimizing method is invalid any more.
Lemma 3.11. The functional T defined in (2.1) satisfies (2.11) and (2.12).
Proof. Based on Lemma 3.2, any minimizing sequence
is bounded in
. Hence
is bounded in all
norms for
and up to a subsequence, by Lemma 2.1, there exists
such that
. According to the Gagliardo-Nirenberg inequality, using Lemma 2.1 and Lemma 3.5, we have
where
and
discussed in Remark 3.10. As a result,
using the Hölder inequality, we obtain that
and then
(3.14)
Additionally, by the Hölder inequality and Lemma 3.1, it holds that
(3.15)
On the basis of (3.14), (3.15) and
(2.12) is a direct consequence. Moreover, the boundedness of
in Ls-norm for
brings that
Thus, (2.11) is achieved. □
Proof of Theorem 1.1.
Proof. Summing up, we have verified all the hypotheses of Lemma 2.1 and Proposition 2.5. Therefore, the limit
of the minimizing sequence
makes problem (1.7) solved. In other words,
and the corresponding Lagrange multiplier
is a couple of solution for problem (1.4). □
Next, to prove Theorem 1.2, we first recall a Liouville-type result, see ( [35], Theorem 2.1).
Proposition 3.12. Assume that
and the nonlinearity
is continuous and satisfies
Then the differential inequality
has no positive solution in any exterior domain of
.
Proof of Theorem 1.2. First of all, we assert that if
is a minimizer to problem (1.7) for
, then the associated Lagrange multiplier
is negative. Indeed, firstly we have the following Pohozaev type identity ( [9], A.3)
which can be rewritten as
(3.16)
In addition, it is easy to see that
(3.17)
Thus, (3.16) together with (3.17) gives that
(3.18)
As a result, substituting (3.17) into (3.18), it derives that
(3.19)
For
, obviously one has
which, combining with (3.19), brings that
.
By Lemma 3.1, we know that
(3.20)
Moreover, note that ( [36], Lemma 2.3) gives that
(3.21)
Thus, together with (3.20) and (3.21) means that
(3.22)
Assume that
is positive satisfying problem (1.7). In view of (3.22) and
, there exists
large enough such that
Therefore, we infer that
By applying Proposition 3.12 with
, we have
and
reach to a contradiction. □
4. Proof of Theorem 1.4
In this section, we consider Cauchy problem (1.10). To do this, the first task is to establish the local well posedness with aid of Proposition 2.6. In addition, to discuss the orbital stability, the solution with given initial value should exist globally.
According to the framework discussed in Section 2, for problem (1.10) our nonlinearity is of the following form
Observe that
defined in (1.2) satisfies the following conditions:
is an even real-valued potential and
. Then, by ( [28], Proposition 3.2.9),
holds (2.14)-(2.17). Moreover, according to the discussion in ( [28], Remark 3.2.6), we see that
satisfies (2.14)-(2.17) evidently. So, g fulfils the hypotheses of Proposition 2.6, which means that the local well posedness is established.
Lemma 4.1.
. i.e. the solution of problem (1.10) is global.
Proof. Let
be the solution of (1.10) and
is its maximal time of existence. Then we either have
or
If
, due to
where
and
, we infer that
as
. However, this contradicts the conservation of energy
,
. Hence,
. □
Proof of Theorem 1.4. Observe that
is invariant by translation, namely, if
then also
for any
. Assume that for some
small enough
is orbitally unstable, that is, there exist
, a sequence of initial value
and
such that the solution
, which is global and
, satisfying
(4.1)
Consequently, there exists
minimizer of
and
such that
,
Indeed, we can assume that
, because there exists
such that
and
. In other words,
can be replaced by
. Thus,
is a minimizing sequence for
. Since
, we know that
is also a minimizing sequence for
. However, we have proved that every minimizing sequence has a subsequence converging (up to translation) in
-norm to a minimizer on the sphere
, which leads to a contradiction with (4.1). □