Higher derivative theories with constraints : Exorcising Ostrogradski's   Ghost
  We prove that the linear instability in a non-degenerate higher derivative theory, the Ostrogradski instability, can only be removed by the addition of constraints if the original theory's phase space is reduced. 
Tai-jun Chen, Matteo Fasiello, Eugene A. Lim, Andrew J. Tolley
hep-th astro-ph.CO gr-qc math-ph math.MP