On the invasion of an unstable structureless state by a stable hexagonal pattern

We investigate the invasion of a hexagonal pattern at the expense of an ini tially structureless state. We show that even in the vicinity of threshold higher-order contributions in the amplitude equations play a decisive role: the invasion velocity of the hexagonal state as evaluated from marginal st ability increases. We find that the roll belt that forms around the hexagon s does not widen, contrary to previous studies (L. M. Pismer et al., Europh ys. Lett., 27 (1994) 433). We confirm this result by numerical simulation a nd also present results on the fact that even though far from the threshold hexagons exhibit complex temporal oscillations their invasion velocity is still given by the linear marginal stability criterion. Furthermore we prov ide a heuristic argument on the existence and the width of the roll belt.