Model-independent approach to effective sound speed in multi-field inflation Antonio Enea Romano, Krzysztof Turzyński and Sergio Andrés Vallejo-Peña Eur. Phys. J. C, 82 8 (2022) 767 Published online: 30 August 2022 DOI: 10.1140/epjc/s10052-022-10669-3