The Rotor-Router Model
  Building on earlier work of Diaconis and Fulton (1991) and Lawler, Bramson, and Griffeath (1992), Propp in 2001 defined a deterministic analogue of internal diffusion-limited aggregation. This growth model is a "convergent game" of the sort studied by Eriksson (1996). In one dimension, we show that the model is equivalent to a simple dynamical system with three integer-valued parameters; an invariant of this dynamical system yields links between the behavior of the system and diophantine approximation of quadratic irrationals. In two dimensions, we give constraints on the shape of the occupied region, as well as evidence for the proposition that the asymptotic shape of the boundary, suitably rescaled as time goes to infinity, is a circle. While the rotor-router model displays many of the same intriguing features as the Bak-Tang-Weisenfeld abelian sandpile model, we suggest that the new model is likely to be easier to analyze rigorously. 
Lionel Levine