Threshold phenomena for symmetric-decreasing radial solutions of reaction-diffusion equations

We study the long time behavior of positive solutions of the Cauchy problem for nonlinear reaction-diffusion equations in ℝ^N with bistable, ignition or monostable nonlinearities that exhibit threshold behavior. For L² initial data that are radial and non-increasing as a function of the distance to the origin, we characterize the ignition behavior in terms of the long time behavior of the energy associated with the solution. We then use this characterization to establish existence of a sharp threshold for monotone families of initial data in the considered class under various assumptions on the nonlinearities and spatial dimension. We also prove that for more general initial data that are sufficiently localized the solutions that exhibit ignition behavior propagate in all directions with the asymptotic speed equal to that of the unique one-dimensional variational traveling wave.