Simple criterion for the occurrence of Bose-Einstein condensation

We examine the occurrence of Bose-Einstein condensation in both nonrelativistic and relativistic systems with no self-interactions in a general setting. A simple condition for the occurrence of Bose-Einstein condensation can be given if we adopt generalized $\zeta$-functions to define the quantum theory. We show that the crucial feature governing Bose-Einstein condensation is the dimension $q$ associated with the continuous part of the eigenvalue spectrum of the Hamiltonian for nonrelativistic systems or the spatial part of the Klein-Gordon operator for relativistic systems. In either case Bose-Einstein condensation can only occur if $q\ge3$.