Bose-Einstein condensation beyond perturbation theory: Goldstone singularities and instanton solution Juha Honkonen, Marina V. Komarova and Mikhail Yu. Nalimov Eur. Phys. J. B, 87 3 (2014) 75 Published online: 26 March 2014 DOI: 10.1140/epjb/e2014-40910-2