The method of generating functions in exact scalar field inflationary cosmology Sergey V. Chervon, Igor V. Fomin and Aroonkumar Beesham Eur. Phys. J. C, 78 4 (2018) 301 Published online: 17 April 2018 DOI: 10.1140/epjc/s10052-018-5795-z