The nonlinear heat equation involving highly singular initial values and new blowup and life span results

Abstract

In this paper we prove local existence of solutions to the nonlinear heat equation $u_t = \Delta u +a |u|^\alpha u, \; t\in (0,T),\; x=(x_1,\ldots , x_N)\in {\mathbb {R}}^N,\; a = \pm 1,\; \alpha >0;$ with initial value $u(0)\in L^1_{\mathrm{{loc}}}({\mathbb {R}}^N{\setminus }\{0\}),$ anti-symmetric with respect to $x_1,\; x_2,\ldots , x_m$ and $|u(0)|\le C(-1)^m\partial _{1}\partial _{2}\cdots \partial _{m}(|x|^{-\gamma })$ for $x_1>0,\ldots , x_m>0,$ where $C>0$ is a constant, $m\in \{1, 2,\ldots , N\},$ $0<\gamma <N$ and $0<\alpha <2/(\gamma +m).$ This gives a local existence result with highly singular initial values. As an application, for $a=1,$ we establish new blowup criteria for $0<\alpha \le 2/(\gamma +m),$ including the case $m=0.$ Moreover, if $(N-4)\alpha <2,$ we prove the existence of initial values $u_0 = \lambda f,$ for which the resulting solution blows up in finite time $T_{\max }(\lambda f),$ if $\lambda >0$ is sufficiently small. We also construct blowing up solutions with initial data $\lambda _n f$ such that $\lambda _n^{[({1\over \alpha }-{\gamma +m\over 2})^{-1}]}T_{\max }(\lambda _n f)$ has different finite limits along different sequences $\lambda _n\rightarrow 0.$ Our result extends the known “small lambda” blow up results for new values of $\alpha$ and a new class of initial data.