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A {\em complex} canonical transformation is found that takes the fourth orderderivative Pais-Uhlenbeck oscillator into two independent harmonic oscillatorsthus showing that this model has energy bounded from below, unitarytime-evolution and no negative norm states, or ghosts. Such transformationyields a positive definite inner product consistent with reality conditions inthe Hilbert space. The method is illustrated by eliminating the negative normstates in a complex oscillator. Extensions to other higher order mechanicalmodels and field theory are discussed.

Coping with the Pais-Uhlenbeck oscillator's ghosts in a canonical approach