Periods of relativistic oscillators with even polynomial potentials
  We modify a non-perturbative approach based on the Principle of Minimal Sensitivity to calculate the periods of relativistic oscillators with even polynomial potentials of general form. The optimization of the variational parameter is adapted by using the ultrarelativistic limit of the period as a boundary condition, which determines the values of additional free parameters introduced. Resulting general approximations prove to be accurate over the whole solution domain and for the whole class of potentials. 
Mikhail P. Solon, J.P. Esguerra