ON THE INITIAL GROWTH OF INTERFACES IN REACTION-DIFFUSION EQUATIONS WITH STRONG ABSORPTION

We study the initial growth of the interfaces of non-negative local solutions of the equation u(t) = (u(m))xx - lambdau(q) when m greater-than-or-equal-to 1 and 0 < q < 1. We show that if u(x, 0) greater-than-or-equal-to C(-x)+2/(m-q) with C > C0, for some explicit C0 = C0(lambda, m, q), then the free boundary zeta(t) = sup {x: u(x, t) > 0} is a ''heating front''. More precisely zeta(t) greater-than-or-equal-to at(m-q)/2(1-q) for any t small enough and for some a > 0. If on the contrary, u(x, 0) less-than-or-equal-to C(-x)+2/(m-q) with C < C0, then zeta(t) is a ''cooling front'' and in fact zeta(t) less-than-or-equal-to -at(m-q)/2(1-q) for any t small enough and for some a > 0. Applications to solutions of the associated Cauchy and Dirichlet problems are also given.