“…They involve fewer variables than discrete models so they can reveal the leading physics structure and are easier for numerical simulation. Many interesting continuum models can be found in the literature on surface morphological evolution; see [22,25,7,29,30,24,20,4,10] for one dimensional models and [19,31] for two dimensional models. The study of relation between the discrete ODE models and the corresponding continuum PDE has raised lots of interest.…”