Formal recursion operators of integrable nonevolutionary equations and Lagrangian systems

Agustín Caparrós Quintero, Rafael Hernández Heredero

We derive the general structure of the space of formal recursion operators of nonevolutionary equations~$q_{tt}=f(q,q_x,q_t,q_{xx},q_{xt},q_{xxx},q_{xxxx})$. This allows us to cl…