Instabilities in a two-dimensional combustion model with free boundary

We prove instability of the planar travelling wave solution in a two-dimensional free boundary problem modelling the propagation of near-equidiffusional premixed flames in the whole plane. We reduce the problem to a fixed boundary fully nonlinear parabolic system. The spectrum of the linearized operator contains an interval [0, omega(c)], omega(c) > 0, so we cannot construct backward solutions. We use an argument about stability of dynamical systems in Banach spaces in order to prove pointwise instability of the moving front.