We consider the Cauchy problem of the nonlinear heat equation $u_t -\Delta u= u^{ b},\ u(0,x)=u_0$, with $b\geq 2$ 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$\dot B^{ -2/b}_{ n(b-1) b/2,q}(\mathbb{ R}^{ n})$, 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.
Categories: math.AP