Integrable structure of box–ball systems: crystal, Bethe ansatz, ultradiscretization and tropical geometry

The box–ball system is an integrable cellular automaton on a one-dimensional lattice. It arises from either quantum or classical integrable systems by procedures called crystallization and ultradiscretization, respectively. The double origin of the integrability has endowed the box–ball system with a variety of aspects related to Yang–Baxter integrable models in statistical mechanics, crystal base theory in quantum groups, combinatorial Bethe ansatz, geometric crystals, classical theory of solitons, tau functions, inverse scattering method, action-angle variables and invariant tori in completely integrable systems, spectral curves, tropical geometry and so forth. In this review, we demonstrate these integrable structures of the box–ball system and its generalizations based on the developments in the last two decades. Dedicated to the memory of Professor Miki Wadati (topical review)