Some remarks on the fifth-order KdV equations

In this paper, we present the differential operators for the generalized fifth-order KdV equation. We give formal proofs on the Hamiltonian properties including the skew-adjointness and Jacobi identity by using the prolongation method. Our results show that there are three third-order Hamiltonian operators which can be used to construct the Hamiltonians. However, no fifth-order operators are shown to pass the Hamiltonian test, although there are an infinite number of them, and they are skew-adjoint.