Coping with the Pais-Uhlenbeck oscillator's ghosts in a canonical   approach
  A {\em complex} canonical transformation is found that takes the fourth order derivative Pais-Uhlenbeck oscillator into two independent harmonic oscillators thus showing that this model has energy bounded from below, unitary time-evolution and no negative norm states, or ghosts. Such transformation yields a positive definite inner product consistent with reality conditions in the Hilbert space. The method is illustrated by eliminating the negative norm states in a complex oscillator. Extensions to other higher order mechanical models and field theory are discussed. 
Aldo D\'ector, Hugo A. Morales-T\'ecotl, Luis F. Urrutia, J. David   Vergara