Db. Fairlie, EQUATIONS WITH AN INFINITE NUMBER OF EXPLICIT CONSERVATION-LAWS, Proceedings of the Royal Society of Edinburgh. Section A. Mathematics, 127, 1997, pp. 11-19
A large class of first-order partial nonlinear differential equations
in two independent variables which possess an infinite set of polynomi
al conservation laws derived from an explicit generating function is c
onstructed. The conserved charge densities are all homogeneous polynom
ials in the unknown functions which satisfy the differential equations
in question. The simplest member of the class of equations is related
to the Born-Infeld Equation in two dimensions. It is observed that so
me members of this class possess identical charge densities. This enab
les the construction of a set of multivariable equations with an infin
ite number of conservation laws.