We consider the energy-critical semilinear focusing wave equation in dimension $N=3,4,5$. An explicit solution $W$ of this equation is known. By the work of C. Kenig and F. Merle, any solution of initial condition $(u_0,u_1)$ such that $E(u_0,u_1)<E(W,0)$ and $\|\nabla u_0\|_{ L^2}<\|\nabla W\|_{ L^2}$ is defined globally and has finite $L^{ \frac{ 2(N+1)}{ N-2}}_{ t,x}$-norm, which implies that it scatters. In this note, we show that the supremum of the $L^{ \frac{ 2(N+1)}{ N-2}}_{ t,x}$-norm taken on all scattering solutions at a certain level of energy below $E(W,0)$ blows-up logarithmically as this level approaches the critical value $E(W,0)$. We also give a similar result in the case of the radial energy-critical focusing semilinear Schr\"odinger equation. The proofs rely on the compactness argument of C. Kenig and F. Merle, on a classification result, due to the authors, at the energy level $E(W,0)$, and on the analysis of the linearized equation around $W$.
Categories: math.AP