Reliability of the Optimized Perturbation Theory for scalar fields at   finite temperature
  The thermodynamics of a massless scalar field with a quartic interaction is studied up to third order in the Optimized Perturbation Theory (OPT) method. A comparison with other nonperturbative approaches is performed such that the reliability of OPT is accessed. 
R. L. S. Farias, D. L. Teixeira Jr and R. O. Ramos
hep-ph cond-mat.stat-mech hep-th nucl-th