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 Open Access
Existence of positive ground states for some nonlinear Schrödinger systems
 Hui Zhang^{1},
 Junxiang Xu^{1}Email author and
 Fubao Zhang^{1}
https://doi.org/10.1186/16872770201313
© Zhang et al.; licensee Springer. 2013
 Received: 26 March 2012
 Accepted: 12 January 2013
 Published: 28 January 2013
Abstract
We prove the existence of positive ground states for the nonlinear Schrödinger system
where a, b are periodic or asymptotically periodic and F satisfies some superlinear conditions in $(u,v)$. The proof is based on the method of Nehari manifold and the concentrationcompactness principle.
MSC:35J05, 35J50, 35J61.
Keywords
 nonlinear Schrödinger system
 Nehari manifold
 lack of compactness
 ground state
1 Introduction and statement of the main result
Following the work [2] by Lin and Wei about the existence of ground states for the problem (1.2), there are many results on the existence of ground states relevant to five parameters (${\lambda}_{1}$, ${\lambda}_{2}$, ${\mu}_{1}$, ${\mu}_{2}$ and β); see [3–9] and the references therein. Later in [10], assuming ${\lambda}_{1}={\lambda}_{2}=1$, Pomponio and Secchi established the existence of radially symmetric ground states for (1.2) with general nonlinearities ($f(u)$ and $g(v)$).
Moreover, in what follows, the notation inf (sup) is understood as the essential infimum (supremum). In the sequel, let $a,b\in {L}^{\mathrm{\infty}}({\mathbb{R}}^{N})$ and $F\in {C}^{1}({\mathbb{R}}^{2},\mathbb{R})$ with $F(0,0)=0$, we always assume that
(V1) ${inf}_{{\mathbb{R}}^{N}}\{1+a(x)\}>\lambda $, ${inf}_{{\mathbb{R}}^{N}}\{1+b(x)\}>\lambda $,
(F1) $\mathrm{\nabla}F(u,v)\le {C}_{0}(1+{(u,v)}^{q1})$ for some ${C}_{0}>0$ and $2<q<{2}^{\ast}$,
(F2) $\mathrm{\nabla}F(u,v)=o((u,v))$ as $(u,v)\to 0$,
(F3) $\mathrm{\forall}(u,v)\ne (0,0)$, $s>0$, $s\mapsto \frac{\mathrm{\nabla}F(su,sv)(u,v)}{s}$ is strictly increasing,
(F4) $\frac{F(u,v)}{{u}^{2}+{v}^{2}}\to \mathrm{\infty}$ as $(u,v)\to \mathrm{\infty}$,
(F5) ${F}_{u}(u,v)\ge 0$, ${F}_{v}(u,v)\ge 0$, ${F}_{u}(0,v)={F}_{v}(u,0)=0$, $u\ge 0$, $v\ge 0$,
(F6) $F(u,v)\le F(u,v)$, $u,v\in \mathbb{R}$.
(F1)(F4) are similar to the conditions of the nonlinearities for the periodic system (1.3) as considered in [11]. We divide the study of (NLS) into two cases as follows.
First, we consider the periodic case
(V2) $a(x)=a(x+y)$, $b(x)=b(x+y)$, $\mathrm{\forall}x\in {\mathbb{R}}^{N}$, $y\in {\mathbb{Z}}^{N}$.
We have the following result.
Theorem 1.1 Let (V1), (V2) and (F1)(F6) hold. Then the system (NLS) has a positive ground state.
and G is periodic in x. The problem (1.3) is mentioned in [11] when G is periodic in x. However, in [11] the conditions on the function G are not made explicit.
Next, we consider the asymptotically periodic case. We assume that there are functions ${a}_{p},{b}_{p}\in {L}^{\mathrm{\infty}}({\mathbb{R}}^{N})$ satisfying (V1) and (V2) and a, b satisfies that
(V3) ${lim}_{x\to \mathrm{\infty}}a(x){a}_{p}(x)=0$, ${lim}_{x\to \mathrm{\infty}}b(x){b}_{p}(x)=0$,
(V4) $a\le {a}_{p}$, $b\le {b}_{p}$.
We have the following result.
Theorem 1.2 Assume that ${a}_{p}$ and ${b}_{p}$ satisfy (V2). Let (V1), (V3), (V4) and (F1)(F6) hold. Then the system (NLS) has a positive ground state.
Remark 1.2 Conditions (V1) and (V4) imply that ${a}_{p}$ and ${b}_{p}$ satisfy (V1).
In addition, we consider the following conditions:
(V5) ${a}_{p}={b}_{p}:=V$, $a+b\le 2V$,
(F7) ${F}_{u}(u,v)={F}_{v}(v,u)$, $u>0$, $v>0$.
We have the following result.
Theorem 1.3 Suppose that ${a}_{p}$ and ${b}_{p}$ satisfy (V1) and (V2). Let (V1), (V3), (V5) and (F1)(F7) hold. Then the system (NLS) has a positive ground state.
We will prove Theorems 1.1, 1.2 and 1.3 using the method of Nehari manifold. We first reduce the problem of seeking for ground states of (NLS) into that of looking for minimizers of the functional constrained on the Nehari manifold. Then we apply the concentrationcompactness principle to solve the minimization problem. Since the Nehari manifold for (NLS) may not be smooth, in the same way as [11], we will make use of the differential structure of a unit sphere in ${W}^{1,2}({\mathbb{R}}^{N})\times {W}^{1,2}({\mathbb{R}}^{N})$ to find a ${(\mathit{PS})}_{c}$ sequence (c is the infimum of the functional constrained on the Nehari manifold). When (NLS) is periodic, we will use the invariance of the functional under translation to recover the compactness of the ${(\mathit{PS})}_{c}$ sequence. When the system (NLS) is asymptotically periodic, the difficulty is to recover the compactness for the ${(\mathit{PS})}_{c}$ sequence. By comparing c with the infimum of the functional of the related periodic limit system constrained on the corresponding Nehari manifold, we will restore the compactness.
The paper is organized as follows. In Section 2 we give some preliminaries. In Section 3 we introduce the variational setting. In Section 4 we consider the periodic case and prove Theorem 1.1. Section 5 is devoted to studying the asymptotically periodic case and showing Theorems 1.2 and 1.3.
2 Notation and preliminaries
We use the following notation:

For simplicity, we denote $\int h:={\int}_{{\mathbb{R}}^{N}}h(x)\phantom{\rule{0.2em}{0ex}}dx$ and ${\int}_{E}h:={\int}_{E}h(x)\phantom{\rule{0.2em}{0ex}}dx$, where $E\subset {\mathbb{R}}^{N}$ is measurable.

X denotes the Sobolev space ${W}^{1,2}({\mathbb{R}}^{N})$ ($N\ge 2$), with the standard scalar product ${\u3008u,v\u3009}_{X}=\int (\mathrm{\nabla}u\cdot \mathrm{\nabla}v+uv)$ and the norm ${\parallel u\parallel}_{X}^{2}={\u3008u,u\u3009}_{X}$. $H=X\times X$ with the norm ${\parallel (u,v)\parallel}_{H}^{2}={\parallel u\parallel}_{X}^{2}+{\parallel v\parallel}_{X}^{2}$. When there is no possible misunderstanding, the subscripts could be omitted.

The usual norm in ${L}^{r}({\mathbb{R}}^{N})$ ($2\le r\le \mathrm{\infty}$) will be denoted by ${\cdot }_{r}$.

$S=\{(u,v)\in H:{\parallel (u,v)\parallel}^{2}=1\}$.

For any $\varrho >0$ and $z\in {\mathbb{R}}^{N}$, ${B}_{\varrho}(z)$ denotes the ball of radius ϱ centered at z.
with $\mu ,\nu >0$.
A ground state $(u,v)$ such that $u>0$, $v>0$ ($u\ge 0$, $v\ge 0$) is called a positive (nonnegative) ground state. Below we give some lemmas useful for studying our problem.
Moreover, (F3) implies the function $\alpha (s)=\frac{1}{2}\mathrm{\nabla}F(su,sv)(su,sv)F(su,sv)$ is increasing in $(0,\mathrm{\infty})$ for all $u,v\in \mathbb{R}$.
□
Lemma 2.2 Let (F1) and (F2) hold. Then ${\mathrm{\Phi}}^{\prime}$ is weakly sequentially continuous. Namely, if $({u}_{n},{v}_{n})\rightharpoonup (u,v)$ in H, then ${\mathrm{\Phi}}^{\prime}({u}_{n},{v}_{n})\rightharpoonup {\mathrm{\Phi}}^{\prime}(u,v)$ in H.
Hence, ${\mathrm{\Phi}}^{\prime}({u}_{n},{v}_{n})$ is bounded in H. Combining with the fact that ${C}_{0}^{\mathrm{\infty}}({\mathbb{R}}^{N})\times {C}_{0}^{\mathrm{\infty}}({\mathbb{R}}^{N})$ is dense in H, we easily deduce that (2.4) holds for any $(\varphi ,\phi )\in H$. Therefore, ${\mathrm{\Phi}}^{\prime}({u}_{n},{v}_{n})\rightharpoonup {\mathrm{\Phi}}^{\prime}(u,v)$ in H. □
3 Variational setting
This section is devoted to describing the variational framework for the study of ground states for (NLS).
Below we investigate the main properties of Φ on M.
Lemma 3.1 Let (F2) and (F3) hold. Then Φ is bounded from below on M by 0.
Proof
By (2.3) we have $\mathrm{\Phi}{}_{M}(u,v)>0$. □
Define the least energy of (NLS) on M by $c:=inf\mathrm{\Phi}{}_{M}$, then $c\ge 0$. Next, we prove M is a manifold. First, we give the following two lemmas, which will be important when proving M is a manifold.
Moreover, $\mathrm{\Phi}({t}_{n}{u}_{n},{t}_{n}{v}_{n})\to \mathrm{\infty}$.
since $\{({u}_{n},{v}_{n})\}$ is bounded in H. □
 (i)
for each $(u,v)\in H\setminus \{(0,0)\}$, there exists ${t}_{(u,v)}$ such that if ${g}_{(u,v)}(t):=\mathrm{\Phi}(tu,tv)$, then ${g}_{(u,v)}^{\mathrm{\prime}}(t)>0$ for $0<t<{t}_{(u,v)}$ and ${g}_{(u,v)}^{\mathrm{\prime}}(t)<0$ for $t>{t}_{(u,v)}$;
 (ii)
there exists $\rho >0$ such that ${t}_{(w,z)}\ge \rho $ for all $(w,z)\in S$;
 (iii)
for each compact subset $W\subset S$, there exists a constant ${C}_{W}$ such that ${t}_{(u,v)}\le {C}_{W}$ for all $(u,v)\in W$.
 (i)Note that${g}_{(u,v)}^{\mathrm{\prime}}(t)=t[{\parallel u\parallel}^{2}+{\parallel v\parallel}^{2}+\int a(x){u}^{2}+\int b(x){v}^{2}2\lambda \int uv\int \frac{\mathrm{\nabla}F(tu,tv)(u,v)}{t}].$
 (ii)If $(u,v)\in M$, then${\parallel u\parallel}^{2}+{\parallel v\parallel}^{2}+\int a(x){u}^{2}+\int b(x){v}^{2}2\lambda \int uv=\int \mathrm{\nabla}F(u,v)(u,v).$
 (iii)
We argue by contradiction. Suppose that there exist a compact set W and a sequence $\{({u}_{n},{v}_{n})\}$ such that $\{({u}_{n},{v}_{n})\}\subset W\subset S$ and ${t}_{({u}_{n},{v}_{n})}\to \mathrm{\infty}$. Since W is compact, there exists $(u,v)\in W$ such that $({u}_{n},{v}_{n})\to (u,v)$ in H. Then Lemma 3.2 implies that $\mathrm{\Phi}({t}_{({u}_{n},{v}_{n})}({u}_{n},{v}_{n}))\to \mathrm{\infty}$. Contrary to Lemma 3.1 since ${t}_{({u}_{n},{v}_{n})}({u}_{n},{v}_{n})\in M$. This ends the proof. □
As a consequence of Lemma 3.3(i), we can define the mapping $m:S\to M$ by $m(u,v)={t}_{(u,v)}(u,v)$. By Lemma 3.3, [[11], Proposition 3.1(b)] yields the following result.
Lemma 3.4 If (V1) and (F1)(F4) are satisfied, then m is a homeomorphism between S and M, and M is a manifold.
If M is a ${C}^{1}$ manifold, we can make use of the differential structure of M to reduce the problem of finding a ground state for (NLS) into that of looking for a minimizer of $\mathrm{\Phi}{}_{M}$ and solve the minimizing problem. However, since $F\in {C}^{1}({\mathbb{R}}^{2},\mathbb{R})$, M may not be a ${C}^{1}$ manifold. Noting that M and S are homeomorphic, we will take advantage of the differential structure of S to seek for ground states for (NLS) as [11]. Therefore, as in [11], we introduce the functional $\mathrm{\Psi}:S\to \mathbb{R}$ defined by $\mathrm{\Psi}(u,v):=\mathrm{\Phi}(m(u,v))$, and we have the following conclusion.
 (i)
If $\{({w}_{n},{z}_{n})\}$ is a PS sequence for Ψ, then $\{m({w}_{n},{z}_{n})\}$ is a PS sequence for Φ.
 (ii)
$(w,z)$ is a critical point of Ψ if and only if $m(w,z)$ is a nontrivial critical point of Φ. Moreover, ${inf}_{S}\mathrm{\Psi}={inf}_{M}\mathrm{\Phi}$.
 (iii)
A minimizer of $\mathrm{\Phi}{}_{M}$ is a solution of (NLS).
Proof As in the proof of [[11], Corollary 3.3], we can show (i) and (ii). Now, we prove the conclusion (iii). Indeed, let $(u,v)\in M$ such that $\mathrm{\Phi}(u,v)=c$. Then $\mathrm{\Psi}(w,z)=c$, where $(w,z)={m}^{1}(u,v)\in S$. By the conclusion (ii), we have $\mathrm{\Psi}(w,z)={inf}_{S}\mathrm{\Psi}$. So, ${\mathrm{\Psi}}^{\prime}(w,z)=0$. Using the conclusion (ii) again, we deduce that ${\mathrm{\Phi}}^{\prime}(u,v)=0$. □
From the definition of a ground state, we translate the problem of looking for a ground state for (NLS) into that of seeking for a solution for (NLS) which is a minimizer of $\mathrm{\Phi}{}_{M}$. By Proposition 3.1(iii), in order to look for a ground state for (NLS), we just need to seek for a minimizer of $\mathrm{\Phi}{}_{M}$.
4 The periodic case
In this section, we consider the periodic case and prove Theorem 1.1. In [11], Szulkin and Weth considered the existence of ground states for periodic single Schrödinger equations. Treating as in [11], we find ground states for a periodic case for the system (NLS). In addition, under conditions (F5) and (F6), we deduce that there are positive ground states.
From the statement in Section 3, it suffices to solve the minimizing problem. By conclusions (i) and (ii) of Proposition 3.1, we first make use of the minimizing sequence of Ψ to obtain a ${(\mathit{PS})}_{c}$ sequence of Φ. Then we use the invariant of the functional under translation of the form $v\mapsto v(\cdot y)$, $y\in {\mathbb{Z}}^{N}$ to recover the compactness for the ${(\mathit{PS})}_{c}$ sequence.
Proof of Theorem 1.1 Let $({\overline{w}}_{n},{\overline{z}}_{n})\in S$ be a minimizing sequence of Ψ. By the Ekeland variational principle [[16], Theorem 8.5], we may assume that ${\mathrm{\Psi}}^{\prime}({\overline{w}}_{n},{\overline{z}}_{n})\to 0$. Using Proposition 3.1(i), we have that ${\mathrm{\Phi}}^{\prime}({u}_{n},{v}_{n})\to 0$, where $({u}_{n},{v}_{n})=m({\overline{w}}_{n},{\overline{z}}_{n})\in M$. Proposition 3.1(ii) implies that $\mathrm{\Phi}({u}_{n},{v}_{n})=\mathrm{\Psi}({\overline{w}}_{n},{\overline{z}}_{n})\to {inf}_{S}\mathrm{\Psi}=c$.
 (i)If $A=0$, then for any $\u03f5>0$, there exists $K\in \mathbb{N}$ such that ${{w}_{n}}_{q}^{q}+{{z}_{n}}_{q}^{q}<C\u03f5$, for $n>K$. Combining with (2.2), for $n>K$ and $s>0$, we have$\int F(s{w}_{n},s{z}_{n})\le \u03f5{s}^{2}({{w}_{n}}_{2}^{2}+{{z}_{n}}_{2}^{2})+{s}^{q}C{C}_{\u03f5}({{w}_{n}}_{q}^{q}+{{z}_{n}}_{q}^{q})<C\u03f5.$
 (ii)If $A\ne 0$, then we can assume that in ${L}^{q}({\mathbb{R}}^{N})$. From the Lions compactness lemma [[16], Lemma 1.21], it follows that there exist ${\delta}_{0}>0$ and ${x}_{n}\in {\mathbb{R}}^{N}$ such that${\int}_{{B}_{1}({x}_{n})}{{w}_{n}}^{2}>{\delta}_{0}.$(4.1)
Since Φ and M are invariant by translation of the form $v\mapsto v(\cdot y)$, $y\in {\mathbb{Z}}^{N}$, translating ${w}_{n}$ if necessary, we may assume $\{{x}_{n}\}$ is bounded. Since ${w}_{n}\to w$ in ${L}_{\mathrm{loc}}^{2}({\mathbb{R}}^{N})$, then (4.1) implies $w\ne 0$. Then from Lemma 3.2, we deduce that $\mathrm{\Phi}({s}_{n}{w}_{n},{s}_{n}{z}_{n})\to \mathrm{\infty}$. This is impossible since $\mathrm{\Phi}({s}_{n}{w}_{n},{s}_{n}{z}_{n})=\mathrm{\Phi}({u}_{n},{v}_{n})\to c$.
Hence, $\{({u}_{n},{v}_{n})\}$ is bounded in H. Suppose that $({u}_{n},{v}_{n})\rightharpoonup (\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})$ in H, $({u}_{n},{v}_{n})\to (\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})$ in ${L}_{\mathrm{loc}}^{2}({\mathbb{R}}^{N})\times {L}_{\mathrm{loc}}^{2}({\mathbb{R}}^{N})$ and $({u}_{n},{v}_{n})\to (\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})$ a.e. on ${\mathbb{R}}^{2N}$ for a subsequence. Since ${\mathrm{\Phi}}^{\prime}({u}_{n},{v}_{n})\to 0$, Lemma 2.2 yields ${\mathrm{\Phi}}^{\prime}(\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})=0$.
where (4.3) follows from the Fatou lemma and (2.3). Then $\mathrm{\Phi}(\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})\le c$. According to $(\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})\in M$, we have $\mathrm{\Phi}(\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})\ge c$. Thus, $\mathrm{\Phi}(\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})=c$. Consequently, $(\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})$ is a ground state of (NLS).
It remains to look for a positive ground state for (NLS). First, we can assume that $(\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})$ is nonnegative. In fact, note that ${\mathrm{\nabla}u}_{2}^{2}=\mathrm{\nabla}u{}_{2}^{2}$ and ${\mathrm{\nabla}v}_{2}^{2}=\mathrm{\nabla}v{}_{2}^{2}$ for all $(u,v)\in H$. Then $(\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})\in H$. Let $\tau >0$ be such that $\tau (\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})\in M$. By (F6) we easily have that $\mathrm{\Phi}(\tau \stackrel{\u02c7}{u},\tau \stackrel{\u02c7}{v})\le \mathrm{\Phi}(\tau \stackrel{\u02c7}{u},\tau \stackrel{\u02c7}{v})$. Moreover, $\mathrm{\Phi}(\tau \stackrel{\u02c7}{u},\tau \stackrel{\u02c7}{v})\le \mathrm{\Phi}(\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})$ since $(\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})\in M$. Then $\mathrm{\Phi}(\tau \stackrel{\u02c7}{u},\tau \stackrel{\u02c7}{v})\le \mathrm{\Phi}(\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})$. So, $(\tau \stackrel{\u02c7}{u},\tau \stackrel{\u02c7}{v})$ is also a minimizer of Φ on M. Then $(\tau \stackrel{\u02c7}{u},\tau \stackrel{\u02c7}{v})$ is also a ground state of (NLS). Thus we can assume that $(\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})$ is a nonnegative ground state for (NLS). Now, we claim that $\stackrel{\u02c7}{u}\ne 0$, $\stackrel{\u02c7}{v}\ne 0$. Indeed, if $\stackrel{\u02c7}{u}=0$, then from (F5) and $\lambda >0$, the first equation of (NLS) yields that $\stackrel{\u02c7}{v}=0$. Then $(\stackrel{\u02c7}{u},\stackrel{\u02c7}{v})=(0,0)$. This is impossible. So, $\stackrel{\u02c7}{u}\ne 0$. Similarly, $\stackrel{\u02c7}{v}\ne 0$. By (F5), applying the maximum principle to each equation of (NLS), we infer that $\stackrel{\u02c7}{u}>0$, $\stackrel{\u02c7}{v}>0$. The proof is complete. □
5 The asymptotically periodic case
As for c, we have ${c}_{p}\ge 0$.
Lemma 5.1 Suppose that ${a}_{p}$, ${b}_{p}$ satisfy (V1) and (V2). Let (F1)(F6) hold. Then the problem (NLS)_{ p } has a positive ground state $(u,v)\in {M}_{p}$ such that ${\mathrm{\Phi}}_{p}(u,v)={c}_{p}$.
Proof As a corollary of Theorem 1.1, we infer that the problem (NLS)_{ p } has a positive ground state. Moreover, from the argument of Theorem 1.1, we find that the ground state of the problem (NLS)_{ p } we obtained is a minimizer of ${\mathrm{\Phi}}_{p}$ on ${M}_{p}$. □
Next, we prove that $c<{c}_{p}$ under some conditions.
Lemma 5.2 Suppose that ${a}_{p}$, ${b}_{p}$ satisfy (V2). Let (V1), (V4), (5.2) and (F1)(F6) hold. Then $c<{c}_{p}$.
Then $\mathrm{\Phi}(t{u}_{0},t{v}_{0})\le {\mathrm{\Phi}}_{p}(t{u}_{0},t{v}_{0})$.
If $c<{c}_{p}$, we are done. Otherwise, $c={c}_{p}$. Then by (5.3) and (5.4), we get $t=1$ and $\mathrm{\Phi}({u}_{0},{v}_{0})=c$. Then $({u}_{0},{v}_{0})$ is a ground state for (NLS). Note that $({u}_{0},{v}_{0})$ is a solution of (NLS)_{ p }. From the first equations of (NLS) and (NLS)_{ p }, we infer that $a={a}_{p}$. Similarly, $b={b}_{p}$ contrary to (5.2). The proof is now complete. □
Lemma 5.3 Suppose that ${a}_{p}$, ${b}_{p}$ satisfy (V1) and (V2). Let (V1), (V5), (5.2) and (F1)(F7) hold. Then $c<{c}_{p}$.
Then $\mathrm{\Phi}(t{u}_{0},t{v}_{0})\le {\mathrm{\Phi}}_{p}(t{u}_{0},t{v}_{0})$. Below we argue analogously with the proof of Lemma 5.2 to infer that $c<{c}_{p}$. This ends the proof. □
Now, we are ready to prove Theorems 1.2 and 1.3. The proof is partially inspired by [17], where the authors dealt with SchrödingerPoisson equations.
Proof of Theorem 1.2 As the argument of Theorem 1.1, we infer that there exists a sequence $({u}_{n},{v}_{n})\in M$ such that ${\mathrm{\Phi}}^{\prime}({u}_{n},{v}_{n})\to 0$ and $\mathrm{\Phi}({u}_{n},{v}_{n})\to c$.
This is a contradiction.
Hence, $\{({u}_{n},{v}_{n})\}$ is bounded in H. Up to a subsequence, we assume that $({u}_{n},{v}_{n})\rightharpoonup (\tilde{u},\tilde{v})$ in H, $({u}_{n},{v}_{n})\to (\tilde{u},\tilde{v})$ in ${L}_{\mathrm{loc}}^{2}({\mathbb{R}}^{N})\times {L}_{\mathrm{loc}}^{2}({\mathbb{R}}^{N})$ and $({u}_{n},{v}_{n})\to (\tilde{u},\tilde{v})$ a.e. on ${\mathbb{R}}^{2N}$. By Lemma 2.2, we have ${\mathrm{\Phi}}^{\prime}(\tilde{u},\tilde{v})=0$. Namely, $(\tilde{u},\tilde{v})$ is a solution of (NLS).
Let ${s}_{n}>0$ be such that ${s}_{n}({u}_{n},{v}_{n})\in {M}_{p}$. We claim that ${s}_{n}\ge 1$ for large n and ${s}_{n}\to 1$.
which is impossible. Consequently, (5.7) holds.
Hence, ${\mathrm{\Phi}}_{p}({s}_{n}{u}_{n},{s}_{n}{v}_{n})={\mathrm{\Phi}}_{p}({u}_{n},{v}_{n})+{o}_{n}(1)$. Then using (5.6), we have ${c}_{p}\le {\mathrm{\Phi}}_{p}({s}_{n}{u}_{n},{s}_{n}{v}_{n})=c+{o}_{n}(1)$. Then ${c}_{p}\le c$. However, Lemma 5.2 implies that $c<{c}_{p}$. This is a contradiction. Note that this contradiction follows from the hypothesis that $(\tilde{u},\tilde{v})=(0,0)$. So, $(\tilde{u},\tilde{v})\ne (0,0)$. Then $(\tilde{u},\tilde{v})\in M$.
where the inequality (5.13) holds by (2.3) and the Fatou lemma. Then $\mathrm{\Phi}(\tilde{u},\tilde{v})\le c$. According to $(\tilde{u},\tilde{v})\in M$, we have $\mathrm{\Phi}(\tilde{u},\tilde{v})=c$. Then $(\tilde{u},\tilde{v})$ is a ground state for (NLS). Below we argue analogously with the proof of Theorem 1.1 to get a positive ground state for (NLS). The proof is complete. □
Proof of Theorem 1.3 By Lemma 5.3, repeating the argument of Theorem 1.2, we show the existence of a ground state for (NLS) and then look for a positive ground state as the argument of Theorem 1.1. □
Declarations
Acknowledgements
The authors would like to express their sincere gratitude to the referee for helpful and insightful comments. Hui Zhang was supported by the Research and Innovation Project for College Graduates of Jiangsu Province with contract number CXLX12_0069, Junxiang Xu and Fubao Zhang were supported by the National Natural Science Foundation of China with contract number 11071038.
Authors’ Affiliations
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