Perturbative approach to f(R)-gravitation in FLRW cosmology Pham Van Ky, Nguyen Thi Hong Van and Nguyen Anh Ky Eur. Phys. J. C, 83 4 (2023) 330 Published online: 26 April 2023 DOI: 10.1140/epjc/s10052-023-11491-1