Zeldovich bistable reaction-diffusion equation (combustion front model):
  ∂u/∂t − D·Δu = k·u·(1−u)·(u−α), 0 < α < 1.
Stationary points: u = 0 (stable), u = 1 (stable), u = α (unstable). The asymmetric reaction term makes the α parameter the threshold separating which initial condition leads to which stable state.

Vary α to see the threshold behaviour: for α < 0.5 the u=1 well wins (mostly red); for α > 0.5 the u=0 well wins (mostly blue). At α = 0.5 the equation is symmetric, but the asymmetric reaction term still produces sharper, faster-moving fronts than the bistable form — this is the canonical model of a propagating combustion front (Zeldovich, 1938).

Red intensity: u (1 ⟶ saturated red). Blue intensity: 1−u (1 ⟶ saturated blue at u = 0). Black polyline: cross-cut of u along j = N/2.