GROUND RINGS AND THEIR MODULES IN 2D-GRAVITY WITH C-LESS-THAN-OR-EQUAL-TO-1 MATTER

All solvable two-dimensional quantum gravity models have non-trivial BRST cohomology with vanishing ghost number. These states form a ring and all the other states in the theory fall into modules of this ring. The relations in the ring and in the module have a physical interpretation. The existence of these rings and modules leads to non-trivial constraints on the correlation functions and goes a long way toward solving these theories in the continuum approach.