1. Yu. Ignat’ev and A. Agathonov, Russ. Phys. J. 61 (19) (2018) (to be publised); Yu. Ignat’ev, A. Agathonov and I. Kokh, arXiv: 1808. 04570.
2. D. S. Gorbunov and V. A. Rubakov, Introduction to the Theory of the Early Universe: Cosmological Perturbations and Inflationary Theory (World Scientific, Singapore, 2011).
3. V. A. Belinskii, L. P. Grishchuk, Ya. B. Zel’dovich, and I. M. Khalatnikov, Sov. Phys. JETP62, 195–203 (1985).
4. V. M. Zhuravlev, JETP 120, 1042 (2001).
5. L. A. Urena-Lopez and M. J. Reyes-Ibarra, arXiv: 0709. 3996.