Abelian Sandpile — Self-Organized Criticality

BTW Sandpile (Bak-Tang-Wiesenfeld 1987): when a cell has ≥4 grains, it topples — distributing one grain to each neighbor. Boundary cells lose grains. The system self-organizes to a critical state where avalanche sizes follow a power law P(s) ∝ s^(−τ). No tuning required — criticality emerges spontaneously.