Classification of extinction profiles for a one-dimensional diffusive Hamilton–Jacobi equation with critical absorption

Abstract

A classification of the behaviour of the solutions f(·, a) to the ordinary differential equation (|f′|^p-2f′)′ + f - |f′|^p-1 = 0 in (0,∞) with initial condition f(0, a) = a and f′(0, a) = 0 is provided, according to the value of the parameter a > 0 when the exponent p takes values in (1, 2). There is a threshold value a_* that separates different behaviours of f(·, a): if a > a_*, then f(·, a) vanishes at least once in (0,∞) and takes negative values, while f(·, a) is positive in (0,∞) and decays algebraically to zero as r→∞ if a ∊ (0, a_*). At the threshold value, f(·, a_*) is also positive in (0,∞) but decays exponentially fast to zero as r→∞. The proof of these results relies on a transformation to a first-order ordinary differential equation and a monotonicity property with respect to a > 0. This classification is one step in the description of the dynamics near the extinction time of a diffusive Hamilton–Jacobi equation with critical gradient absorption and fast diffusion.