Critical points of the Moser-Trudinger functional

Abstract

On a smooth bounded 2-dimensional domain $\Omega$ we study the heat flow $u_t=\Delta u +\lambda (t)ue^{u^2}$ ($\lambda(t)$ is such that $d/dt ||u(t,\cdot)||_{H^1_0}=0$) introduced by T. Lamm, F. Robert and M. Struwe to investigate the Moser-Trudinger functional $E(v)=\int_{\Omega} (e^{v^2}-1)dx, v\in H^1_0(\Omega).$ We prove that if $u$ blows-up as $t\to\infty$ and if $E(u(t,\cdot))$ remains bounded, then for a sequence $t_k\to\infty$ we have $u(t_k,\cdot)\rightharpoonup 0$ in $H^1_0$ and $\|u(t_k,\cdot)\|_{H^1_0}^2\to 4\pi L$ for an integer $L\ge 1$. We couple these results with a topological technique to prove that if $\Omega$ is not contractible, then for every $0<\Lambda\in \mathbb{R} \setminus 4 \pi \mathbb{N}$ the functional $E$ constrained to $M_\Lambda=\{v\in H^1_0(\Omega):||v||_{H^1_0}^2=\Lambda \}$ has a positive critical point. We prove that when $\Omega$ is the unit ball and $\Lambda$ is large enough, then $E|_{M_\Lambda}$ has no positive critical points, hence showing that the topological assumption on $\Omega$ is natural.