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Published online by Cambridge University Press: 20 January 2009
In the quotient ring of differential polynomials modulo cubic terms the usual odd order hierarchy of Korteweg–de Vries equations can be supplemented by an even order hierarchy. The full (even and odd) sequence is generated by an Olver recursion operator of order one and any pair has zero bracket in the quotient ring. The even order equations do not possess a Hamiltonian structure and thus their related Rosencrans densities are considered.