Exorcising the Ostrogradsky ghost in coupled systems
  The Ostrogradsky theorem implies that higher-derivative terms of a single
mechanical variable are either trivial or lead to additional, ghost-like
degrees of freedom. In this letter we systematically investigate how the
introduction of additional variables can remedy this situation. Employing a
Lagrangian analysis, we identify conditions on the Lagrangian to ensure the
existence of primary and secondary constraints that together imply the absence
of Ostrogradsky ghosts. We also show the implications of these conditions for
the structure of the equations of motion as well as possible redefinitions of
the variables. We discuss applications to analogous higher-derivative field
theories such as multi-Galileons and beyond Horndeski.
