On the critical points of solutions of PDE in a non-convex settings: the case of concentrating solutions

In this paper we are concerned with the number of critical points of solutions of nonlinear elliptic equations. We will deal with the case of non-convex, contractile and non-contractile planar domains. We will prove results on the estimate of their number as well as their index. In some cases we will provide the exact calculation. The toy problem concerns the multi-peak solutions of the Gel’fand problem, namely $$\{\begin{array}{lll}-\mathrm{\Delta }u=\lambda e^u& in\mathrm{\Omega }u=0& on\mathrm{\partial }\mathrm{\Omega },\end{array}$$ where $\mathrm{\Omega }\subset {\mathbb{R}}^2$ is a bounded smooth domain and $\lambda >0$ is a small parameter.