Ground ring of two-dimensional string theory

String theories with two-dimensional space-time target spaces are characterized by the existence of a "ground ring" of operators of spin (0, 0). By understanding this ring, one can understand the symmetries of the theory and illuminate the relation of the critical string theory to matrix models. The symmetry groups that arise are, roughly, the area-preserving diffeomorphisms of a two-dimensional phase space that preserve the Fermi surface (of the matrix model) and the volume-preserving diffeomorphisms of a three-dimensional cone. The three dimensions in question are the matrix eigenvalue, its canonical momentum, and the time of the matrix model.