Production of gravitational waves during preheating in the Starobinsky inflationary model Guoqiang Jin, Chengjie Fu, Puxun Wu and Hongwei Yu Eur. Phys. J. C, 80 6 (2020) 491 Published online: 01 June 2020 DOI: 10.1140/epjc/s10052-020-8061-0