Classical fields method for a relativistic interacting Bose gas
  We formulate a classical fields method for description of relativistic interacting bosonic particles at nonzero temperatures. The method relays on the assumption that at low temperatures the Bose field can be described by a c-number function. We discuss a very important role of the cut-off momentum which divides the field into dominant classical part and a small quantum correction. We illustrate the method by studying a thermodynamics of relativistic Bose field which dynamics is governed by the Klein-Gordon equation with $\lambda \psi^4$ term responsible for interactions. 
Emilia Witkowska, Pawe{\l} Zi\'n and Mariusz Gajda