BI-HAMILTONIAN STRUCTURES OF D-BOUSSINESQ AND BENNEY-LAX EQUATIONS

The dispersionless-Boussinesq and Benney-Lax equations are equations of hydrodynamic type which can be obtained as reductions of the dispersionless Kadomtsev-Petviashvili equation. We find that for the three-component reduction, the dispersionless Boussinesq and Benney-Lax equations are the same up to a diffeomorphism. This equivalence becomes manifest when the equations of motion are cast into the form of a triplet of conservation laws. Furthermore, in this form we are able to recognize a non-trivial scaling symmetry of these equations which plays an important role in the construction of their bi-Hamiltonian structure. We exhibit a pair of compatible Hamiltonian operators which belong to a restricted class of Dubrovin and Novikov operators appropriate to a system of conservation laws. The recursion operator for this system generates three infinite sequences of conserved Hamiltonians.