Optimal estimate of the life span of solutions to the heat equation with a nonlinear boundary condition

Consider the heat equation with a nonlinear boundary condition $$ \partial_t u=\Delta u,\quad x\in{\bf R}^N_+,\,\,\,t>0,\qquad \partial_\nu u=u^p, \quad x\in\partial{\bf R}^N_+,\,\,\,t>0,\qquad u(x,0)=\kappa\psi(x),\quad x\in D:=\overline{{\bf R}^N_+}, $$ where $N\ge 1$, $p>1$, $\kappa>0$ and $\psi$ is a nonnegative measurable function in ${\bf R}^N_+ :=\{y\in{\bf R}^N:y_N>0 \}$. Let us denote by $T(\kappa\psi)$ the life span of solutions to this problem. We investigate the relationship between the singularity of $\psi$ at the origin and $T(\kappa\psi)$ for sufficiently large $\kappa>0$ and the relationship between the behavior of $\psi$ at the space infinity and $T(\kappa\psi)$ for sufficiently small $\kappa>0$. Moreover, we give an optimal estimate to $T(\kappa\psi)$, as $\kappa\to\infty$ or $\kappa\to+0$.

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