2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/165720A {\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.5 pagesQuantum PhysicsGeneral Relativity and Quantum CosmologyHigh Energy Physics - Theorytext