Generalized Hirota Equations in Models of 2D Quantum Gravity
  We derive a set of bilinear functional equations of Hirota type for the partition functions of the $sl(2)$ related integrable statistical models defined on a random lattice. These equations are obtained as deformations of the Hirota equations for the KP integrable hierarchy, which are satisfied by the partition function of the ensemble of planar graphs. 
Jorge Alfaro and Ivan Kostov