Note on Bose-Einstein condensation of photons

This paper provides, firstly, a succinct mathematical derivation of Bose-Einstein condensation (BEC) of photons elaborating on previous results [M\"uller, E.E., Annals of Phys. 184, 219-230 (1988); M\"uller, E.E. Physica 139A, 165-174 (1986)] including new results on the condensate function, and, secondly, applies this framework to consistently explain experimental findings reported in Klaers J., Schmitt, J., Vewinger, F & Weitz, M., Nature 468, 545-548 (2010). The theoretical approach presented here invites to significantly widen the experimental framework for BEC of photons including three-dimensional photon resonators and thermalization mechanisms different from a dye medium in the cavity.