Blowup for the nonlinear heat equation with small initial data in scale-invariant Besov norms

Abstract

We consider the Cauchy problem of the nonlinear heat equation $u_t-\Delta u=u^b,u(0,x)=u_0$, with $b\ge 2$ and $b\in \mathbb{N}$. We prove that initial data $u_0\in \mathcal{S}({\mathbb{R}}^n)$ (the Schwartz class)arbitrarily small in the scale invariant Besov-norm${\dot{B}}_{n(b-1)b/2,q}^{-2/b}({\mathbb{R}}^n)$, can produce solutions that blow up in finite time. The case $b=3$ answers a question raised by Yves Meyer.Our result also proves that the smallness assumption put in an earlier work by C. Miao, B.~Yuan and B. Zhang, for the global-in-time solvability, is essentially optimal.

To appear on J. Funct. Anal