  The frequency of a classical periodic system and the energy levels of the
corresponding quantum system can both be obtained using action variables. We
demonstrate the construction of two forms of the action variable for a one
dimensional harmonic oscillator in classical, relativistic and quantum regimes.
The relativistic effects are considered as perturbative, within the context of
a non-relativistic quantum formalism. The transition of the relativistic
quantum system to both classical relativistic and classical non-relativistic
regimes is illustrated in a unified framework. Formulas for the frequency of a
classical relativistic oscillator and the energy eigenvalues of the
corresponding quantum oscillator for the weak relativistic case are derived.
Also studied are the non-relativistic and classical limits of these formulas
which provide valuable insights on the parallels between relativistic and
non-relativistic systems on the one hand and between classical and quantum
systems on the other.
