Multilinear Operators: The Natural Extension Of Hirota's Bilinear   Formalism
  We introduce multilinear operators, that generalize Hirota's bilinear $D$ operator, based on the principle of gauge invariance of the $\tau$ functions. We show that these operators can be constructed systematically using the bilinear $D$'s as building blocks. We concentrate in particular on the trilinear case and study the possible integrability of equations with one dependent variable. The 5th order equation of the Lax-hierarchy as well as Satsuma's lowest-order gauge invariant equation are shown to have simple trilinear expressions. The formalism can be extended to an arbitrary degree of multilinearity. 
B. Grammaticos (LPN, Universite Paris VII, Tour 24-14, 5eme etage,   75251 Paris, France), A. Ramani and J. Hietarinta (CPT,...