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Existence of solutions for semilinear elliptic equations on {\mathbb{R}}^{N}
Boundary Value Problems volume 2013, Article number: 163 (2013)
Abstract
In this paper, the existence of at least one nontrivial solution for a class of semilinear elliptic equations on {\mathbb{R}}^{N} is established by using the linking methods.
1 Introduction
In this paper we consider the question of the existence of solutions for a class of semilinear equations of the form
where \lambda >0 is a parameter and the nonlinearity g\in C({\mathbb{R}}^{N}\times \mathbb{R}) is asymptotically linear, i.e.,
for some V(x)\in C({\mathbb{R}}^{N},\mathbb{R}) and {v}_{\mathrm{\infty}}\in \mathbb{R}. In case this equation is considered in a bounded domain \mathrm{\Omega}\subset {\mathbb{R}}^{N} (with, say, the Dirichlet boundary condition), there is a large amount of literature on existence and multiplicity results, with the case of resonance being of particular interest (see [1–3]). We recall that the problem is said to be at resonance if \lambda \in \sigma (S), where \sigma (S) denotes the spectrum of S, the ‘asymptotic linearization’ of the problem. In other words, S:D(S)\subset {L}^{2}(\mathrm{\Omega})\to {L}^{2}(\mathrm{\Omega}) is the operator given by
On the other hand, a systematic study of such asymptotically linear problems set in unbounded domains or the whole space {\mathbb{R}}^{N} is more recent and presents a number of mathematical difficulties (see [4, 5]). As an example, we note that in the case of problem ({P}_{\lambda}), the asymptotic linearization operator S (now defined on D(S)={H}^{2}({\mathbb{R}}^{N})) has a much more complicated spectrum (including an essential part [{v}_{\mathrm{\infty}},\mathrm{\infty})), which in turn makes the study of this problem more challenging. In [4], motivated by the paper [5], Tehrani and Costa studied the existence of positive solutions to ({P}_{\lambda}) by using the mountain pass theorem if g(x,u) satisfies some strong asymptotically linear conditions. Comparing with previous paper [4], in [6], Tehrani obtained the existence of a (possibly signchanging) solution for problem ({P}_{\lambda}) under essentially condition (1.1) only. In fact, he proved the following.
Theorem 1.0 [6]
Let {g}_{0}(x,s):=g(x,s)V(x)s and assume that

(G)
for every \u03f5>0, there exists 0\le {b}_{\u03f5}(x)\in {L}^{2}({\mathbb{R}}^{N}) such that
{g}_{0}(x,s)\le {b}_{\u03f5}(x)+\u03f5s\phantom{\rule{1em}{0ex}}\mathit{\text{a.e.}}\phantom{\rule{0.1em}{0ex}}x\in {\mathbb{R}}^{N},s\in \mathbb{R}.
If \mathrm{\Lambda}\ge 0 or max\{0,{v}_{\mathrm{\infty}}\}<\lambda <\mathrm{\Lambda} and \lambda \notin {\sigma}_{p}(S), then ({P}_{\lambda}) has a solution in {H}^{1}({\mathbb{R}}^{N}).
Now, one naturally asks: Are there nontrivial solutions for problem ({P}_{\lambda}) if \lambda \in \sigma (S) in the above theorem? Obviously, this case is resonance. But, this problem is not easy because we face the difficulties of verifying that the energy functional satisfies the (PS) condition if we still follow the idea of [6]. Here, there is still an interesting problem: Are there nontrivial solutions for problem ({P}_{\lambda}) if \lambda \in \sigma (S) and {g}_{0}(x,s) (in Theorem 1.0) is more generalized superlinear? We will answer the above problems affirmatively by using Li and Willem’s local linking methods (see [7]).
Next, we recall a few basic facts in the theory of Schrödinger operators which are relevant to our discussion (see [6]).

1.
Since {lim}_{x\to \mathrm{\infty}}V(x)={v}_{\mathrm{\infty}}, one has {\sigma}_{\mathrm{ess}}(S)=[{v}_{\mathrm{\infty}},\mathrm{\infty}).

2.
The bottom of the spectrum \sigma (S) of the operator S is given by
\mathrm{\Lambda}={\lambda}_{0}=\underset{0\ne u\in {H}^{2}({\mathbb{R}}^{N})}{inf}\frac{\int {\mathrm{\nabla}u}^{2}V(x){u}^{2}}{\int {u}^{2}}.
Therefore we clearly have \mathrm{\Lambda}\le {v}_{\mathrm{\infty}}. If \mathrm{\Lambda}<{v}_{\mathrm{\infty}}, then by using the concentration compactness principle of Lions, one shows that Λ is the principle eigenvalue of S with a positive eigenfunction {\mathrm{\Phi}}_{0}:

3.
The spectrum of S in (\mathrm{\infty},{v}_{\mathrm{\infty}}), namely \sigma (S)\cap (\mathrm{\infty},{v}_{\mathrm{\infty}}), is at most a countable set, which we denote by
\mathrm{\Lambda}={\lambda}_{0}<{\lambda}_{1}<{\lambda}_{2}<\cdots ,
where each {\lambda}_{k} is an isolated eigenvalue of S of the finite multiplicity. Let {E}_{{\lambda}_{j}}denote the eigenspace of S corresponding to the eigenvalue {\lambda}_{j}.
Now, we state our main results. In this paper, we always assume that {lim}_{x\to \mathrm{\infty}}V(x)={v}_{\mathrm{\infty}} and {v}_{\mathrm{\infty}}<0. The conditions imposed on {g}_{0}(x,t) (see Theorem 1.0) are as follows:
(H_{1}) {g}_{0}\in C({\mathbb{R}}^{N}\times \mathbb{R},\mathbb{R}), and there are constants {C}_{1},{C}_{2}\ge 0 such that
where {p}^{\ast}=\frac{2N}{N2};
(H_{2}) {g}_{0}(x,t)=\circ (t), t\to 0, uniformly on {\mathbb{R}}^{N};
(H_{3}) {lim}_{t\to \mathrm{\infty}}\frac{{g}_{0}(x,t)}{t}=+\mathrm{\infty} uniformly on {\mathbb{R}}^{N};
(H_{4}) There is a constant \theta \ge 1 such that for all (x,t)\in {\mathbb{R}}^{N}\times R and s\in [0,1],
where {G}_{0}(x,t)={\int}_{0}^{t}{g}_{0}(x,s)\phantom{\rule{0.2em}{0ex}}ds;
(H_{5}) For some \delta >0, either
or
(H_{6}) {lim}_{x\to \mathrm{\infty}}{sup}_{t\le r}\frac{{g}_{0}(x,t)}{t}=0 for every r>0.
Theorem 1.1 Assume that conditions (H_{1})(H_{4}) hold. If −λ is an eigenvalue of S(\lambda <{v}_{\mathrm{\infty}}), assume also that (H_{5}) and (H_{6}) hold. Then the problem ({P}_{\lambda}) has at least one nontrivial solution.
Remark 1.1 It follows from the condition (H_{3}) that our nonlinearity {g}_{0}(x,t) does not satisfy the classical condition of Ambrosetti and Rabinowitz:
(AR) There is \mu >2 such that 0<\mu {G}_{0}(x,u)\le u{g}_{0}(x,u) for all x\in {\mathbb{R}}^{N} and u\ne 0.
In recent years, there have been some papers devoted to replacing (AR) with more natural conditions (see [8–10]). But our methods are different from the references therein.
We also consider asymptotically quadratic functions. We assume that:
(H_{7}) For every \u03f5>0, there exists 0\le {b}_{\u03f5}(x)\in {L}^{2}({\mathbb{R}}^{N}) such that
and {\lambda}_{k}<\lambda <{\lambda}_{k+1}.
Theorem 1.2 Assume that conditions (H_{2}), (H_{6}), (H_{7}) and one of the following conditions hold:
(A_{1}) {\lambda}_{j}<0<{\lambda}_{j+1}, j\ne k;
(A_{2}) {\lambda}_{j}=0<{\lambda}_{j+1}, j\ne k for some \delta >0,
(A_{3}) {\lambda}_{j}<0={\lambda}_{j+1}, j\ne k for some \delta >0,
Then problem ({P}_{\lambda}) has at least one nontrivial solution.
2 Preliminaries
Let X be a Banach space with a direct sum decomposition
Consider two sequences of subspaces
such that
For every multiindex \alpha =({\alpha}_{1},{\alpha}_{2})\in {N}^{2}, let {X}_{\alpha}={X}_{{\alpha}_{1}}\oplus {X}_{{\alpha}_{2}}. We know that
A sequence ({\alpha}_{n})\subset {N}^{2} is admissible if, for every \alpha \in {N}^{2}, there is m\in N such that n\ge m\Rightarrow {\alpha}_{n}\ge \alpha. For every I:X\to R, we denote by {I}_{\alpha} the function I restricted {X}_{\alpha}.
Definition 2.1 Let I be locally Lipschitz on X and c\in R. The functional I satisfies the {(C)}_{c}^{\ast} condition if every sequence ({u}_{{\alpha}_{n}}) such that ({\alpha}_{n}) is admissible and
contains a subsequence which converges to a critical point of I.
Definition 2.2 Let I be locally Lipschitz on X and c\in R. The functional I satisfies the {(C)}^{\ast} condition if every sequence ({u}_{{\alpha}_{n}}) such that ({\alpha}_{n}) is admissible and
contains a subsequence which converges to a critical point of I.
Remark 2.1 1. The {(C)}^{\ast} condition implies the {(C)}_{c}^{\ast} condition for every c\in R.

2.
When the {(C)}_{c}^{\ast} sequence is bounded, then the sequence is a {(\mathrm{PS})}_{c}^{\ast} sequence (see [11]).

3.
Without loss of generality, we assume that the norm in X satisfies
{\parallel {u}_{1}+{u}_{2}\parallel}^{2}={\parallel {u}_{1}\parallel}^{2}+{\parallel {u}_{2}\parallel}^{2},\phantom{\rule{1em}{0ex}}{u}_{j}\in {X}_{j},j=1,2.
Definition 2.3 Let X be a Banach space with a direct sum decomposition
The function I\in {C}^{1}(X,R) has a local linking at 0, with respect to ({X}^{1},{X}^{2}) if, for some r>0,
Lemma 2.1 (see [7])
Suppose that I\in {C}^{1}(X,R) satisfies the following assumptions:
(B_{1}) I has a local linking at 0 and {X}^{1}\ne \{0\};
(B_{2}) I satisfies {(\mathrm{PS})}^{\ast};
(B_{3}) I maps bounded sets into bounded sets;
(B_{4}) For every m\in N, I(u)\to \mathrm{\infty}, \parallel u\parallel \to \mathrm{\infty}, u\in X={X}_{m}^{1}\oplus {X}^{2}. Then I has at least two critical points.
Remark 2.2 Assume that I satisfies the {(C)}_{c}^{\ast} condition. Then this theorem still holds.
Let X be a real Hilbert space and let I\in {C}^{1}(X,R). The gradient of I has the form
where A is a bounded selfadjoint operator, 0 is not the essential spectrum of A, and B is a nonlinear compact mapping.
We assume that there exist an orthogonal decomposition,
and two sequences of finitedimensional subspaces,
such that
For every multiindex \alpha =({\alpha}_{1},{\alpha}_{2})\in {N}^{2}, we denote by {X}_{\alpha} the space
by {p}_{\alpha}:X\to {X}_{\alpha} the orthogonal projector onto {X}_{\alpha}, and by {M}^{}(L) the Morse index of a selfadjoint operator L.
Lemma 2.2 (see [7])
I satisfies the following assumptions:

(i)
I has a local linking at 0 with respect to ({X}^{1},{X}^{2});

(ii)
There exists a compact selfadjoint operator {B}_{\mathrm{\infty}} such that
B(u)={B}_{\mathrm{\infty}}(u)+\circ (\parallel u\parallel ),\phantom{\rule{1em}{0ex}}\parallel u\parallel \to \mathrm{\infty}; 
(iii)
A+{B}_{\mathrm{\infty}} is invertible;

(vi)
For infinitely many multipleindices \alpha :=(n,n),
{M}^{}((A+{P}_{\alpha}{B}_{\mathrm{\infty}}){}_{{X}_{\alpha}})\ne dim{X}_{n}^{2}.
Then I has at least two critical points.
3 The proof of main results
Proof of Theorem 1.1 (1) We shall apply Lemma 2.1 to the functional
defined on X={H}^{1}({\mathbb{R}}^{N}). We consider only the case \lambda \in \sigma (S), and
Then other case is similar and simple.
Let {X}^{2} be a finite dimensional space spanned by the eigenfunctions corresponding to negative eigenvalues of S+\lambda and let {X}^{1} be its orthogonal complement in X. Choose a Hilbertian basis {e}_{n} (n\ge 0) for X and define
By the condition (H_{1}) and Sobolev inequalities, it is easy to see that the functional I belongs to {C}^{1}(X,R) and maps bounded sets to bounded sets.

(2)
We claim that I has a local linking at 0 with respect to ({X}^{1},{X}^{2}). Decompose {X}^{1} into V+W when V={E}_{\lambda}, W={({X}^{2}+V)}^{\mathrm{\perp}}. Also, set u=v+w, u\in {X}^{1}, v\in V, w\in W.
For the convenience of our proof, we state some facts for the norm of the whole space X. It is well known that there is an equivalent norm \parallel \cdot \parallel on X={H}^{1}({\mathbb{R}}^{N}) such that
and
By the equivalence of norm in the finitedimensional space, there exists C>0 such that
It follows from (H_{1}) and (H_{2}) that for any \u03f5>0, there exists {C}_{\u03f5} such that
Hence, we obtain
where m>0, {c}^{\ast} is a constant and hence, for r>0 small enough,
Let u=v+w\in {X}^{1} be such that {\parallel u\parallel}_{X}\le {r}_{1}=\frac{\delta}{2C} and let
From (3.2), we have
for all \parallel u\parallel \le {r}_{1} and x\in {\mathbb{R}}^{N}. On the one hand, one has u(x)\le v(x)+w(x)\le {\parallel v\parallel}_{\mathrm{\infty}}+\frac{\delta}{2}\le \delta for all x\in {\mathbb{A}}_{1}. Hence, from (H_{5}), we obtain
On the other hand, we have
for all x\in {\mathbb{A}}_{2}. It follows from (3.3) that
for all x\in {\mathbb{A}}_{2} and all u\in {X}_{1} with \parallel u\parallel \le {r}_{1}, which implies that
where {C}_{3} is a constant. Hence, there exist positive constants {C}^{\ast \ast}, {C}_{4} and {C}_{5} such that
for all u\in {X}^{1} with \parallel u\parallel \le {r}_{1}, which implies that
for 0<r small enough.

(3)
We claim that I satisfies {(C)}_{c}^{\ast}. Consider a sequence ({u}_{{\alpha}_{n}}) such that ({u}_{{\alpha}_{n}}) is admissible and
{u}_{{\alpha}_{n}}\in {X}_{{\alpha}_{n}},\phantom{\rule{1em}{0ex}}I({u}_{{\alpha}_{n}})\to c,\phantom{\rule{2em}{0ex}}(1+\parallel {u}_{{\alpha}_{n}}\parallel ){I}^{\prime}({u}_{{\alpha}_{n}})\to 0(3.4)
and
Let {w}_{{\alpha}_{n}}={\parallel {u}_{{\alpha}_{n}}\parallel}^{1}{u}_{{\alpha}_{n}}. Up to a subsequence, we have
If w=0, we choose a sequence \{{t}_{n}\}\subset [0,1] such that
For any m>0, let {v}_{{\alpha}_{n}}=2\sqrt{m}{w}_{{\alpha}_{n}}. Now, we claim that
Let \u03f5>0; for r\ge 1, then,
Since p<{2}^{\ast}, we may fix r large enough such that
for all n. Moreover, by (H_{6}), there exists R>0 such that
for all n. Finally, since {v}_{{\alpha}_{n}}\to 0 in {L}^{s}({B}_{R}(0)) for s\in [2,{2}^{\ast}), we can use (H_{1}) again to derive
for n large enough. Combining the above three formulas, our claim holds.
So, for n large enough, 2\sqrt{m}{\parallel {u}_{{\alpha}_{n}}\parallel}^{1}\in (0,1), we have
where ϵ is a small enough constant.
That is, I({t}_{n}{u}_{{\alpha}_{n}})\to \mathrm{\infty}. Now, I(0)=0, I({u}_{{\alpha}_{n}})\to c, we know that {t}_{n}\in [0,1] and
Therefore, using (H_{4}), we have
This contradicts (3.5).
If w\ne 0, then the set \u229d=\{x\in {\mathbb{R}}^{N}:w(x)\ne 0\} has a positive Lebesgue measure. For x\in \u229d, we have {u}_{{\alpha}_{n}}(x)\to \mathrm{\infty}. Hence, by (H_{3}), we have
From (3.4), we obtain
By (3.8), the righthand side of (3.9) \to +\mathrm{\infty}. This is a contradiction.
In any case, we obtain a contradiction. Therefore, \{{u}_{{\alpha}_{n}}\} is bounded.
Next, we denote \{{u}_{{\alpha}_{n}}\} as \{{u}_{n}\} and prove \{{u}_{n}\} contains a convergent subsequence.
In fact, we know that \{{u}_{n}\} is bounded in X. Passing to a subsequence, we may assume that {u}_{n}\rightharpoonup u in X. In order to establish strong convergence, it suffices to show that
By the condition (H_{6}) and \u3008{I}^{\prime}({u}_{n}),{u}_{n}u\u3009\to 0, we can similarly conclude it according to the above proof of our claim.
Finally, we claim that for every m\in N,
By (H_{2}) and (H_{3}), there exist large enough M and some positive constant T such that
So, for any u\in {X}_{m}^{1}\oplus {X}^{2}, we have
Hence, our claim holds. □
Proof of Theorem 1.2 We omit the proof which depends on Lemma 2.2 and is similar to the preceding one since our result is a variant of Ding Yanheng’s Theorem 1.2 (see [12]). □
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Acknowledgements
The authors would like to thank the referees for valuable comments and suggestions in improving this paper. This work was supported by the National NSF (Grant No. 10671156) of China.
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Pei, R., Zhang, J. Existence of solutions for semilinear elliptic equations on {\mathbb{R}}^{N}. Bound Value Probl 2013, 163 (2013). https://doi.org/10.1186/168727702013163
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DOI: https://doi.org/10.1186/168727702013163