On covariant perturbations with scalar field in modified Gauss–Bonnet gravity Albert Munyeshyaka, Joseph Ntahompagaze, Tom Mutabazi and Manasse Mbonye Eur. Phys. J. C, 84 1 (2024) 51 Published online: 18 January 2024 DOI: 10.1140/epjc/s10052-023-12381-2