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Some elliptic system and reduction method
Journal of Inequalities and Applications volume 2015, Article number: 96 (2015)
Abstract
We get a theorem which shows the existence of at least three solutions for some elliptic system with Dirichlet boundary condition. We obtain this result by using the finite dimensional reduction method for the dimension of the system which reduces the infinite dimensional problem to the finite dimensional one. We also use critical point theory on the reduced finite dimensional subspace.
1 Introduction
In this paper we are concerned with multiple solutions for a class of systems of elliptic equations with Dirichlet boundary condition
where Ω is a bounded subset of \(R^{n}\) with smooth boundary ∂Ω, \(n\ge3\), \(u_{i}(x)\in W^{1,2}_{0}(\Omega)\), \(F:R^{n}\times R^{n}\to R\) is a \(C^{2}\) function such that \(F(x,\theta )=0\), \(\theta=(0,\ldots,0)\) and \(F_{u_{i}}(x,u_{1},\ldots ,u_{n})=\frac{\partial F(x,u_{1},\ldots,u_{n})}{\partial u_{i}}\), \(i=1,\ldots,n\). Let \(U=(u_{1},\ldots,u_{n})\) and \(\|\cdot\|_{R^{n}}\) denote the Euclidean norm in \(R^{n}\). Let us define
and
Let \(\lambda_{1}<\lambda_{2}\le\cdots\le\lambda_{k}\le\cdots\) be eigenvalues of the eigenvalue problem \(-\Delta u=\lambda u\) in Ω, \(u=0\) on ∂Ω, and \(\phi_{k}\) be an eigenfunction belonging to the eigenvalue \(\lambda_{k}\), \(k\ge1\).
We assume that F satisfies the following conditions:
-
(F1)
\(F\in C^{2}(R^{n}\times R^{n},R)\), \(F(x,\theta)=0\), \(F_{U}(x,\theta)=\theta\), \(x\in\Omega\), \(\theta=(0,\ldots,0)\).
-
(F2)
There exist constants α and β (α, β are not eigenvalues of the elliptic eigenvalue problem) such that \(\alpha<\beta\) and
$$\alpha I\le d^{2}_{U}F(x,U)\le\beta I \quad\forall(x, U)\in R^{n}\times R^{n}, $$and there exists \(k\in N^{*}\) such that \(\alpha I<\lambda_{k} I< d^{2}_{U}F(x,U)<\lambda_{k+1}I<\beta I\) for every U, where I is the \(n\times n\) identity matrix.
-
(F3)
There exist eigenvalues \(\lambda_{h+1}, \ldots, \lambda_{h+m}\) such that
$$\lambda_{h}< \alpha<\lambda_{h+1}<\cdots< \lambda_{h+m}<\beta <\lambda_{h+m+1}, $$where \(h\ge1\), \(m\ge1\).
-
(F4)
There exist γ and C such that \(\lambda_{h+m}<\gamma <\beta\) and
$$F(x,U)\ge\frac{1}{2}\gamma\|U\|^{2}_{R^{n}}-C, \quad \forall (x,U)\in R^{n}\times R^{n}. $$
Some papers of Lee [1–4] concerning the semilinear elliptic system and some papers of the other several authors [5, 6] have treated the system of this like nonlinear elliptic equations. Some papers of Chang [7] and Choi and Jung [8] considered the existence and multiplicity of weak solutions for nonlinear boundary value problems with asymptotically linear term. The authors obtained some results for those problems by approaching the variational method, critical point theory and the topological method.
Let \(W^{1,2}_{0}(\Omega,R)\) be the Sobolev space with the norm
and the scalar product
Let E be a cartesian product of the Sobolev spaces \(W^{1,2}_{0}(\Omega,R)\), i.e.,
We endow the Hilbert space E with the norm
where \(\|u_{i}\|^{2}_{W^{1,2}_{0}(\Omega,R)}=\int_{\Omega}|\nabla u_{i}(x)|^{2}\,dx\). From now on we shall denote by \(W^{1,2}_{0}(\Omega)\) instead of \(W^{1,2}_{0}(\Omega,R)\).
System (1.1) can be rewritten by
where \(-\Delta U=(-\Delta u_{1},\ldots,-\Delta u_{n})\) and \(\theta =(0,\ldots,0)\). In this paper we are looking for weak solutions of system (1.1) in E, that is, \(U=(u_{1},\ldots,u_{n})\in E\) such that
Our main result is the following.
Theorem 1.1
Assume that F satisfies conditions (F1)-(F4). Then system (1.1) has at least three nontrivial weak solutions.
The proof of Theorem 1.1 is organized as follows: We approach the variational method and use the finite dimensional reduction method for the dimension of the system, which reduces the infinite dimensional problem to the finite dimensional one, and we get critical points of the functional on the infinite dimensional space E from that of the reduced functional on the finite dimensional subspace of E. We also use critical point theory on the reduced finite dimensional subspace. In Section 2, we approach the variational method and the reduction method. We show that the reduced functional satisfies the (PS) condition. In Section 3, we prove Theorem 1.1.
2 Reduction approach
We assume that \(F\in C^{2}(R^{n}\times R^{n},R)\), \(F(x,\theta)=0\), \(F_{U}(x,\theta)=\theta\), \(\theta=(0,\ldots,0)\) and there exist constants α and β (α, β are not eigenvalues of the elliptic eigenvalue problem) such that \(\alpha <\beta\) and
and there exists \(k\in N^{*}\) such that \(\alpha I<\lambda_{k}I< d^{2}_{U}F(x,U)<\lambda_{k+1}I<\beta I\) for every U, where \(U=(u_{1},\ldots,u_{n})\) and there exist eigenvalues \(\lambda _{h+1}, \ldots, \lambda_{h+m}\) such that
where \(h\ge1\), \(m\ge1\).
Lemma 2.1
Let \(F_{u_{i}}(x,U)\in L^{2}(\Omega)\), \(U=(u_{1},\ldots ,u_{i},\ldots,u_{n})\), \(i=1,\ldots,n\). Then all the solutions of
belong to E.
Proof
Let \(F_{u_{i}}(x,U)\in L^{2}(\Omega)\). We note that \(\{ \lambda_{n}: |\lambda_{n}|<|c|\}\) is finite. Then \(F_{u_{i}}(x,u_{1},\ldots ,u_{n})\in L^{2}(\Omega)\), \(i=1,\ldots, n\), can be expressed by
Then
Hence we have the inequality
which means that
so \(\|u_{i}\|_{W^{1,2}_{0}(\Omega)}<\infty\). Thus
 □
By the following Lemma 2.2, weak solutions of system (1.1) coincide with critical points of the associated functional I
where \(U=(u_{1},\ldots,u_{n})\) and \(\int_{\Omega}|\nabla U|^{2}\,dx=\sum^{n}_{i=1}\int_{\Omega}|\nabla u_{i}|^{2}\,dx\), \(n\ge1\).
Lemma 2.2
Assume that F satisfies conditions (F1)-(F4). Then the functional \(I(U)\) is continuous, Fréchet differentiable with Fréchet derivative
Moreover, \(DI\in C\). That is, \(I\in C^{1}\).
Proof
First we shall prove that \(I(U)\) is continuous. For \(U, V\in E\),
We have
Thus we have
Next we shall prove that \(I(U)\) is Fréchet differentiable. For \(U, V\in E\),
By (2.2),
Thus \(I\in C^{1}\). □
Let \(E=W^{1,2}_{0}(\Omega,R)\times\cdots\times W^{1,2}_{0}(\Omega ,R)\) and let \(\{e_{1},e_{2},\ldots,e_{n}\}\) be an orthonormal basis in \(R^{n}\). Then
where N is a natural number and \(M(\lambda_{m})=\operatorname{span}\{\phi _{m} e_{1},\ldots,\phi_{m}e_{n}\}\) is the eigenspace of −Δ with eigenvalue \(\lambda_{m}\), \(\dim M(\lambda_{m})=n\), \(m=1, 2,\ldots\) . Let
Then
For each \(X\in L^{2}(\Omega,R^{n})\), we have the composition
where \(X_{1}\in L_{1}\), \(X_{0}\in L_{0}\), \(X_{2}\in L_{2}\). Let \(P_{1}\) be the orthogonal projection from \(L^{2}(\Omega,R^{n})\) onto \(L_{1}\), \(P_{0}\) be that from \(L^{2}(\Omega,R^{n})\) onto \(L_{0}\) and \(P_{2}\) be that from \(L^{2}(\Omega,R^{n})\) onto \(L_{2}\). Let
Then \(E=V\oplus W_{1}\oplus W_{2}\), and for \(U\in E\), U has the decomposition \(U=Y+Z_{1}+Z_{2}\in E\), where
Let \(W=W_{1}\oplus W_{2}\). Then W is the orthogonal complement of V in E. Let \(P:E\to V\) be the orthogonal projection of E onto V and \(I-P:E\to W\) denote that of E onto W. Then every element \(U\in E\) is expressed by \(U=Y+Z\), \(Y=PU\), \(Z=(I-P)U\). Then (1.2) is equivalent to the two systems in the two unknowns Y and Z:
Let \(Y\in V\) be fixed and consider the function \(h:W_{1}\times W_{2}\to R\) defined by
The function h has continuous partial Fréchet derivatives \(D_{1}h\) and \(D_{2}h\) with respect to its first and second variables given by
for \(X_{i}\in W_{i}\), \(i=1, 2\). By Lemma 2.2, I is a functional of class \(C^{1}\).
By the following Lemma 2.3, we can get critical points of the functional \(I(U)\) on the infinite dimensional space E from that of the functional on the finite dimensional subspace V.
Lemma 2.3
(Reduction lemma)
Assume that F satisfies conditions (F1)-(F4). Then
(i) there exists a unique solution \(Z\in W\) of the equation
If we put \(Z=\Theta(Y)\), then Θ is continuous on V and satisfies a uniform Lipschitz condition in V with respect to the \(L^{2}\) norm (also norm \(\| \cdot\|\)). Moreover,
(ii) There exists \(m_{1}<0\) such that if \(Z_{1}\) and \(X_{1}\) are in \(W_{1}\) and \(Z_{2}\in W_{2}\), then
(iii) There exists \(m_{2}>0\) such that if \(Z_{2}\) and \(X_{2}\) are in \(W_{2}\) and \(Z_{1}\in W_{1}\), then
(iv) If \(\tilde{I}:V\to R\) is defined by \(\tilde{I}(Y)=I(Y+\Theta (Y))\), then \(\tilde{I}\) has a continuous Fréchet derivative \(D\tilde{I}\) with respect to Y, and
(v) \(Y_{0}\in V\) is a critical point of \(\tilde{I}\) if and only if \(Y_{0}+\Theta(Y_{0})\) is a critical point of I.
Proof
(i) Let \(\delta=\frac{\alpha+\beta}{2}\). Equation (2.7) is equivalent to
System (2.10) can be rewritten as
where \(u_{i}=y_{i}+z_{i}\), \(i=1, 2,\ldots, n\), \(U=(u_{1},\ldots ,u_{n})\), \(Y=(y_{1},\ldots,y_{n})\), \(Z=(z_{1},\ldots,z_{n})\). The operator \((-\Delta-\delta)^{-1}(I-P)\) is a self-adjoint, compact and linear map from \((I-P)L^{2}(\Omega,R)\) into itself, and by condition (F3) its \(L_{2}\) norm is \(\min\{\lambda_{h+m+1}-\delta,\delta -\lambda_{h}\}^{-1}\). Let \(U_{1}, U_{2}\in E\). Since
it follows that the right-hand side of (2.11) defines, for fixed \(Y\in V\), a Lipschitz mapping of \((I-P)L^{2}(\Omega,R)\) into itself with Lipschitz constant \(r=(\min\{\lambda_{h+m+1}-\delta,\delta -\lambda_{h}\})^{-1}\times\frac{\alpha+\beta}{2}<1\) because \(\lambda_{h+m+1}-\delta>\frac{\alpha+\beta}{2}\) and \(\delta-\lambda_{h}>\frac{\alpha+\beta}{2}\). Therefore, by the contraction mapping principle, for each given \(Y\in V\), \(i=1, 2,\ldots , n\), there exists a unique \(z_{i}\in(I-P)L^{2}(\Omega,R)\) which satisfies (2.11). Thus, for fixed \(Y\in V\), there exists a unique \(Z\in(I-P)L^{2}(\Omega ,R^{n})\) which satisfies (2.10). If \(\Theta(Y)\) denotes the unique \(Z\in(I-P)L^{2}(\Omega,R^{n})\) which solves (2.10), then Θ is continuous and satisfies a uniform Lipschitz condition in Y with respect to the \(L^{2}\) norm (also norm \(\| \cdot\|_{E}\)). In fact, if \(Z_{1}=\Theta (Y_{1})\) and \(Z_{2}=\Theta(Y_{2})\), then
Hence
Let \(U=Y+Z\), \(Y\in V\) and \(Z=\Theta(Y)\). If \(X\in(I-P)L^{2}(\Omega ,R^{n})\cap E\),
It follows from (2.7) that
Since
we have
(ii) If \(Z_{1}\) and \(X_{1}\) are in \(W_{1}\) and \(Z_{2}\in W_{2}\), then
Since \(\int_{\Omega}|\nabla(Z_{1}-X_{1})|^{2}=\|Z_{1}-X_{1}\| ^{2}_{E}\le\lambda_{h}\|Z_{1}-X_{1}\|^{2}_{L^{2}(\Omega,R^{n})}\) and
where \(1-\frac{\alpha}{\lambda_{h}}<0\).
(iii) Similarly, using the fact that \(\int_{\Omega}|\nabla (Z_{2}-X_{2})|^{2}\,dx=\|Z_{2}-X_{2}\|^{2}_{E}\ge\lambda_{h+m+1}\| Z_{2}-X_{2}\|^{2}_{L^{2}(\Omega,R^{n})}\) and
we see that if \(Z_{2}\) and \(X_{2}\) are in \(W_{2}\) and \(Z_{1}\in W_{1}\), then
where \((1-\frac{\beta}{\lambda_{h+m+1}})>0\).
(iv) Since the functional I has a continuous Fréchet derivative DI, \(\tilde{I}\) has a continuous Fréchet derivative \(D\tilde{I}\) with respect to Y.
(v) Suppose that there exists \(Y_{0}\in V\) such that \(D\tilde{I}(Y_{0})=0\). From \(D\tilde{I}(Y)\cdot B=DI(Y+\Theta(Y))\cdot B\) for all \(Y, B\in V\), \(DI(Y_{0}+\Theta(Y_{0}))(B)=D\tilde{I}(Y_{0})(B)=0\) for all \(B\in V\). Since \(DI(Y+\Theta(Y))\cdot B=0\) for all \(B\in W\) and E is the direct sum of V and W, it follows that \(DI(Y_{0}+\Theta(Y_{0}))=0\). Thus \(Y_{0}+\Theta(Y_{0})\) is a solution of (1.1). Conversely, if U is a solution of (1.1) and \(Y=PU\), then \(D\tilde{I}(Y)=0\). □
Remark
We note that if \(Y\in V\), then \(\Theta(Y)=0\).
3 Proof of Theorem 1.1
Lemma 3.1
((PS) condition)
Assume that F satisfies conditions (F1)-(F4). Then \(-\tilde {I}(v)\) is bounded from below and \(\tilde{I}(v)\) satisfies the Palais-Smale condition.
Proof
We have
We claim that
In fact, we note that
by (1.1). We also note that
Thus we have
We note that
which leads to
That is,
Thus we have
Thus we have
by condition (F2). Thus by condition (F3) we have
Thus \(-\tilde{I(Y)}\) is bounded from below and satisfies the (PS) condition. □
Lemma 3.2
Assume that F satisfies conditions (F1)-(F4). Then \(Y=\theta\), \(\theta=(0,\ldots,0)\) is neither minimum nor degenerate.
Proof
By (3.1) we have
Since
we have that
Thus we have
Since \(\Theta_{1}\in C^{1}(V,W_{1})\), it follows that if \(\|Y\|\to0\), then \(\|\Theta_{1}(Y)\|_{E}=O(\|Y\|_{E})\) because \(\Theta_{1}(\theta)=\theta\). Thus
Since \(F_{U}(x,\theta)=\theta\), there exists a bounded self-adjoint operator \(A\in\mathcal{L}(E,E)\) which commutes with \(P_{o}\) and \(P_{1}\) such that
Thus we have
as \(\|Y\|_{E} \to0\). Since \(\lambda_{h+1} I\le A\), it follows that
Therefore we have
as \(\|Y\|_{E} \to0\). Similarly we can choose a bounded self-adjoint operator \(B\in\mathcal {L}(E,E)\) which commutes with \(P_{o}\) and \(P_{2}\) such that
This leads to
as \(\|Y\|_{E} \to0\). Thus \(Y=\theta\), \(\theta=(0,\ldots,0)\) is neither minimum nor degenerate. □
Lemma 3.3
Assume that F satisfies conditions (F1)-(F4). Then
Proof
The proof can be found in the proof of Lemma 3.1. □
Proof of Theorem 1.1
By Lemma 2.2, \(\tilde{I}(Y)\) is continuous and Fréchet differentiable in V. By Lemma 3.1, \(-\tilde{I}(Y)\) satisfies the (PS) condition. By Lemma 3.2, \(Y=\theta\) is neither minimum nor degenerate. By Lemma 3.3, \(\tilde{I}(Y)\to-\infty\) as \(\|Y\|_{E}\to \infty\). We note that \(\max_{Y\in V}\tilde{I}(Y)>0\) is another critical value of \(\tilde{I}\). Thus there exists the third critical point of \(\tilde{I}(Y)\). Thus (1.1) has at least three nontrivial solutions. □
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Acknowledgements
This work (Tacksun Jung) was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (KRF-2010-0023985).
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Jung, T., Choi, QH. Some elliptic system and reduction method. J Inequal Appl 2015, 96 (2015). https://doi.org/10.1186/s13660-015-0614-7
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DOI: https://doi.org/10.1186/s13660-015-0614-7