1 + 3 covariant perturbations in power-law f(R) gravity Beatrice Murorunkwere, Joseph Ntahompagaze and Edward Jurua Eur. Phys. J. C, 81 4 (2021) 353 Published online: 23 April 2021 DOI: 10.1140/epjc/s10052-021-09134-4