Abstract:
We prove the existence of an unbounded sequence of sign-changing and non-radially symmetric solutions to the problem $-Delta u = |u|^{p-1}u$ in $Omega$, $u = 0$ on $partialOmega$, $u(gx)= u(x$), $ xin Omega$, $gin G$, where $Omega$ is an annulus of $mathbb{R}^N$ ($Ngeq 3$), $1

Abstract:
We study the following nonlinear Schr\"{o}dinger system which is related to Bose-Einstein condensate: {displaymath} {cases}-\Delta u +\la_1 u = \mu_1 u^{2^\ast-1}+\beta u^{\frac{2^\ast}{2}-1}v^{\frac{2^\ast}{2}}, \quad x\in \Omega, -\Delta v +\la_2 v =\mu_2 v^{2^\ast-1}+\beta v^{\frac{2^\ast}{2}-1} u^{\frac{2^\ast}{2}}, \quad x\in \om, u\ge 0, v\ge 0 \,\,\hbox{in $\om$},\quad u=v=0 \,\,\hbox{on $\partial\om$}.{cases}{displaymath} Here $\om\subset \R^N$ is a smooth bounded domain, $2^\ast:=\frac{2N}{N-2}$ is the Sobolev critical exponent, $-\la_1(\om)<\la_1,\la_2<0$, $\mu_1,\mu_2>0$ and $\beta\neq 0$, where $\lambda_1(\om)$ is the first eigenvalue of $-\Delta$ with the Dirichlet boundary condition. When $\bb=0$, this is just the well-known Brezis-Nirenberg problem. The special case N=4 was studied by the authors in (Arch. Ration. Mech. Anal. 205: 515-551, 2012). In this paper we consider {\it the higher dimensional case $N\ge 5$}. It is interesting that we can prove the existence of a positive least energy solution $(u_\bb, v_\bb)$ {\it for any $\beta\neq 0$} (which can not hold in the special case N=4). We also study the limit behavior of $(u_\bb, v_\bb)$ as $\beta\to -\infty$ and phase separation is expected. In particular, $u_\bb-v_\bb$ will converge to {\it sign-changing solutions} of the Brezis-Nirenberg problem, provided $N\ge 6$. In case $\la_1=\la_2$, the classification of the least energy solutions is also studied. It turns out that some quite different phenomena appear comparing to the special case N=4.

Abstract:
We study the following doubly critical Schr\"{o}dinger system $$-\Delta u -\frac{\la_1}{|x|^2}u=u^{2^\ast-1}+ \nu \al u^{\al-1}v^\bb, \quad x\in \RN, -\Delta v -\frac{\la_2}{|x|^2}v=v^{2^\ast-1} + \nu \bb u^{\al}v^{\bb-1}, \quad x\in \RN, u, v\in D^{1, 2}(\RN),\quad u, v>0 in $\RN\setminus{0}$},$$ where $N\ge 3$, $\la_1, \la_2\in (0, \frac{(N-2)^2}{4})$, $2^\ast=\frac{2N}{N-2}$ and $\al>1, \bb>1$ satisfying $\al+\bb=2^\ast$. This problem is related to coupled nonlinear Schr\"{o}dinger equations with critical exponent for Bose-Einstein condensate. For different ranges of $N$, $\al$, $\bb$ and $\nu>0$, we obtain positive ground state solutions via some quite different methods, which are all radially symmetric. It turns out that the least energy level depends heavily on the relations among $\al, \bb$ and 2. Besides, for sufficiently small $\nu>0$, positive solutions are also obtained via a variational perturbation approach. Note that the Palais-Smale condition can not hold for any positive energy level, which makes the study via variational methods rather complicated.

Abstract:
We study the following singularly perturbed problem for a coupled nonlinear Schr\"{o}dinger system: {displaymath} {cases}-\e^2\Delta u +a(x) u = \mu_1 u^3+\beta uv^2, \quad x\in \R^3, -\e^2\Delta v +b(x) v =\mu_2 v^3+\beta vu^2, \quad x\in \R^3, u> 0, v> 0 \,\,\hbox{in $\R^3$}, u(x), v(x)\to 0 \,\,\hbox{as $|x|\to \iy$}.{cases}{displaymath} Here, $a, b$ are nonnegative continuous potentials, and $\mu_1,\mu_2>0$. We consider the case where the coupling constant $\beta>0$ is relatively large. Then for sufficiently small $\e>0$, we obtain positive solutions of this system which concentrate around local minima of the potentials as $\e\to 0$. The novelty is that the potentials $a$ and $b$ may vanish at someplace and decay to 0 at infinity.

Abstract:
Let $\Omega\subset \R^N$ ($N\geq 3$) be an open domain (may be unbounded) with $0\in \partial\Omega$ and $\partial\Omega$ be of $C^2$ at $0$ with the negative mean curvature $H(0)$. By using variational methods, we consider the following elliptic systems involving multiple Hardy-Sobolev critical exponents, $$\begin{cases} -\Delta u+\lambda^*\frac{u}{|x|^{\sigma_0}}-\lambda_1 \frac{|u|^{2^*(s_1)-2}u}{|x|^{s_1}}=\lambda \frac{1}{|x|^{s_2}}|u|^{\alpha-2}u|v|^\beta\quad &\hbox{in}\;\Omega,\\ -\Delta v+\mu^*\frac{v}{|x|^{\eta_0}}-\mu_1 \frac{|v|^{2^*(s_1)-2}v}{|x|^{s_1}}=\mu \frac{1}{|x|^{s_2}}|u|^{\alpha}|v|^{\beta-2}v\quad &\hbox{in}\;\Omega,\\ (u,v)\in D_{0}^{1,2}(\Omega)\times D_{0}^{1,2}(\Omega), \end{cases}$$ where $ 0\leq \sigma_0, \eta_0, s_2<2, s_1\in (0,2);$ the parameters $ \lambda^*\neq 0, \mu^*\neq 0, \lambda_1>0, \mu_1>0, \lambda\mu>0$; $\alpha,\beta>1$ satisfying $\alpha+\beta \leq 2^*(s_2)$. Here, $2^*(s):=\frac{2(N-s)}{N-2}$ is the critical Hardy-Sobolev exponent. We obtain the existence and nonexistence of ground state solution under different specific assumptions. As the by-product, we study \be\lab{zou=a1} \begin{cases} &\Delta u+\lambda \frac{u^p}{|x|^{s_1}}+\frac{u^{2^*(s_2)-1}}{|x|^{s_2}}=0\;\quad \hbox{in}\;\Omega,\\ &u(x)>0\;\hbox{in}\;\Omega,\\ & u(x)=0\;\hbox{on}\;\partial\Omega, \end{cases} \ee we also obtain the existence and nonexistence of solution under different hypotheses. In particular, we give a partial answers to a generalized open problem proposed by Y. Y. Li and C. S. Lin (ARMA, 2012). Around the above two types of equation or systems, we systematically study the elliptic equations which have multiple singular terms and are defined on any open domain. We establish some fundamental results. \vskip0.23in {\it Key words:} Elliptic system, Ground state, Hardy-Sobolev exponent.

Abstract:
Let $\Omega$ be a $C^1$ open bounded domain in $\R^N$ ($N\geq 3$) with $0\in \partial \Omega$. Suppose that $\partial\Omega$ is $C^2$ at $0$ and the mean curvature of $\partial\Omega$ at $0$ is negative. Consider the following perturbed PDE involving two Hardy-Sobolev critical exponents: $$ \begin{cases} &\Delta u+\lambda_1 \frac{u^{2^*(s_1)-1}}{|x|^{s_1}}+\lambda_2\frac{u^{2^*(s_2)-1}}{|x|^{s_2}}+\lambda_3\frac{u^p}{|x|^{s_3}}=0\;\quad \hbox{in}\;\Omega,\\ &u(x)>0\;\hbox{in}\;\Omega,\;\, u(x)=0\;\hbox{on}\;\partial\Omega, \end{cases} $$ where $00, 1< p\leq 2^*(s_3)-1$. The existence of ground state solution is studied under different assumptions via the concentration compactness principle and the Nehari manifold method. We also apply a perturbation method to study the existence of positive solution.

Abstract:
By using variational methods, we study the existence of mountain pass solution to the following doubly critical Schr\"{o}dinger system: $$ \begin{cases} -\Delta u-\mu_1\frac{u}{|x|^2}-|u|^{2^{*}-2}u &=h(x)\alpha|u|^{\alpha-2}|v|^\beta u\quad \rm{in}\; \R^N, -\Delta v-\mu_2\frac{v}{|x|^2}-|v|^{2^{*}-2}v &= h(x)\beta |u|^{\alpha}|v|^{\beta-2}v\quad \rm{in}\; \R^N, \end{cases} $$ where $\alpha\geq 2, \beta\geq 2, \alpha+\beta\leq 2^*$;\; $ \mu_1, \mu_2\in [0, \frac{(N-2)^2}{4})$. The weight function $h(x)$ is allowed to be sign-changing so that the nonlinearities include a large class of indefinite weights. We show that the $PS$ condition is satisfied at higher energy level when $\alpha+\beta=2^*$ and obtain the existence of mountain pass solution. Besides, a nonexistence result of the ground state is given.

Abstract:
Study the following two-component elliptic system% \begin{equation*} \left\{\aligned&\Delta u-(\lambda a(x)+a_0)u+u^3+\beta v^2u=0\quad&\text{in }\bbr^4,\\% &\Delta v-(\lambda b(x)+b_0)v+v^3+\beta u^2v=0\quad&\text{in }\bbr^4,\\% &(u,v)\in\h\times\h,\endaligned\right.% \end{equation*} where $a_0,b_0\in\bbr$ are constants; $\lambda>0$ and $\beta\in\bbr$ are parameters and $a(x), b(x)\geq0$ are potential wells which are not necessarily to be radial symmetric. By using the variational method, we investigate the existence of ground state solutions and general ground state solutions (i.e., possibly semi-trivial) to this system. Indeed, to the best of our knowledge, even the existence of semi-trivial solutions is also unknown in the literature. We observe some concentration behaviors of ground state solutions and general ground state solutions. The phenomenon of phase separations is also excepted. It seems that this is the first result definitely describing the phenomenon of phase separation for critical system in the whole space $\bbr^4$. Note that both the cubic nonlinearities and the coupled terms of the system are all of critical growth with respect to the Sobolev critical exponent.

Abstract:
Let $\Omega\subset \R^N$ ($N\geq 3$) be an open domain which is not necessarily bounded. By using variational methods, we consider the following elliptic systems involving multiple Hardy-Sobolev critical exponents: $$\begin{cases} -\Delta u-\lambda \frac{|u|^{2^*(s_1)-2}u}{|x|^{s_1}}=\kappa\alpha \frac{1}{|x|^{s_2}}|u|^{\alpha-2}u|v|^\beta\quad &\hbox{in}\;\Omega,\\ -\Delta v-\mu \frac{|v|^{2^*(s_1)-2}v}{|x|^{s_1}}=\kappa\beta \frac{1}{|x|^{s_2}}|u|^{\alpha}|v|^{\beta-2}v\quad &\hbox{in}\;\Omega,\\ (u,v)\in \mathscr{D}:=D_{0}^{1,2}(\Omega)\times D_{0}^{1,2}(\Omega), \end{cases}$$ where $s_1,s_2\in (0,2), \alpha>1,\beta>1, \lambda>0,\mu>0,\kappa\neq 0, \alpha+\beta\leq 2^*(s_2)$. Here, $2^*(s):=\frac{2(N-s)}{N-2}$ is the critical Hardy-Sobolev exponent. We mainly study the critical case (i.e., $\alpha+\beta=2^*(s_2)$) when $\Omega$ is a cone (in particular, $\Omega=\R_+^N$ or $\Omega=\R^N$). We will establish a sequence of fundamental results including regularity, symmetry, existence and multiplicity, uniqueness and nonexistence, {\it etc.} In particular, the sharp constant and extremal functions to the following kind of double-variable inequalities $$ S_{\alpha,\beta,\lambda,\mu}(\Omega) \Big(\int_\Omega \big(\lambda \frac{|u|^{2^*(s)}}{|x|^s}+\mu \frac{|v|^{2^*(s)}}{|x|^s}+2^*(s)\kappa \frac{|u|^\alpha |v|^\beta}{|x|^s}\big)dx\Big)^{\frac{2}{2^*(s)}}$$ $$\leq \int_\Omega \big(|\nabla u|^2+|\nabla v|^2\big)dx$$ for $(u,v)\in {\mathscr{D}} $ will be explored. Further results about the sharp constant $S_{\alpha,\beta,\lambda,\mu}(\Omega)$ with its extremal functions when $\Omega$ is a general open domain will be involved.

Abstract:
In this paper, we will study the following PDE in $\R^N$ involving multiple Hardy-Sobolev critical exponents: $$ \begin{cases} \Delta u+\sum_{i=1}^{l}\lambda_i \frac{u^{2^*(s_i)-1}}{|x|^{s_i}}+u^{2^*-1}=0\;\hbox{in}\;\R^N,\\ u\in D_{0}^{1,2}(\R^N), \end{cases} $$ where $00$ for $1\leq i\leq k$; $\lambda_i<0$ for $k+1\leq i\leq l$. We prove the existence and non-existence of the positive ground state solution. The symmetry and regularity of the least-energy solution are also investigated.