Cosmological axial gravitational waves in the f(R) Starobinsky model Aisha Siddiqa, Syeda Z. B. Mehwish and Márcio E. S. Alves Eur. Phys. J. Plus, 136 4 (2021) 415 Published online: 20 April 2021 DOI: 10.1140/epjp/s13360-021-01413-z