Wavelength Selection in Systems Far from Equilibrium

It is shown that for systems developing stationary periodic patterns there exists at most one stable wavelength state if a supercritical region is connected to a subcritical one by the imposition of a slow spatial variation of the external parameters. In nonpotential systems the selected wavelength depends on the particular combination of parameters that vary but not on the (slow) rate of spatial variation. Suitable parameter variations force the system into a dynamic state.