On Coron's problem for weakly coupled elliptic systems
  We consider the following critical weakly coupled elliptic system \[ \begin{cases} -\Delta u_i = \mu_i |u_i|^{2^*-2}u_i + \sum_{j \neq i} \beta_{ij} |u_j|^{\frac{2^*}{2}} |u_i|^{\frac{2^*-4}{2}} u_i & \text{in $\Omega_\varepsilon$} u_i >0 & \text{in $\Omega_\varepsilon$} u_i = 0 & \text{on $\partial \Omega_\varepsilon$},\end{cases} \qquad i =1,\dots,m, \] in a domain $\Omega_\varepsilon \subset \mathbb{R}^N$, $N=3,4$, with small shrinking holes as the parameter $\varepsilon \to 0$. We prove the existence of positive solutions of two different types: either each density concentrates around a different hole, or we have groups of components such that all the components within a single group concentrate around the same point, and different groups concentrate around different points. 
Angela Pistoia and Nicola Soave
math.AP