The Effect of Weak Interactions on the Ultra-Relativistic Bose-Einstein   Condensation Temperature
  We calculate the ultra-relativistic Bose-Einstein condensation temperature of a complex scalar field with weak lambda Phi^4 interaction. We show that at high temperature and finite density we can use dimensional reduction to produce an effective three-dimensional theory which then requires non-perturbative analysis. For simplicity and ease of implementation we illustrate this process with the linear delta expansion. 
D. J. Bedingham and T. S. Evans