Sharp Thresholds for the Porous Medium Equation with a Combustion Reaction in Higher Dimensions

Abstract

We study the porous medium equation with a combustion-type reaction, \[ u_t=\Delta u^m+f(u),\qquad x\in\mathbb R^N,\ t>0, \] for radial, nonnegative, compactly supported initial data. A complete classification of the long-time behaviour of bounded solutions is established. In dimension $N=2$, every such solution converges locally uniformly to one of the constants $0$, $\theta$, or $1$; for ordered families of initial data there exists a unique threshold parameter separating vanishing from spreading, and the critical solution converges to the ignition temperature $\theta$. In dimensions $N\ge3$, under a natural total-disconnectedness condition on the set of central values of ground states, every bounded solution converges locally uniformly to either $0$, $\theta$, $1$, or a radial ground state $U\in\mathcal S$. Moreover, the ignition state $\theta$ is excluded as a transition limit in $N\ge3$ via a novel normalized annular perturbation argument. For transition solutions, we provide estimates for the propagation speed of the free boundary: in dimension two, \[ b(t)\asymp \frac{\sqrt t}{(\log t)^{\frac{m-1}{2m}}}, \] and in dimensions $N\ge3$, whenever the limit is a ground state, \[ b(t)\asymp t^{\frac{m}{N(m-1)+2}}. \] These results reveal a sharp dimensional dichotomy in the degenerate combustion dynamics.