Variational theory of the Ricci curvature tensor dynamicsClaudio Cremaschini, Jiří Kovář, Zdeněk Stuchlík and Massimo TessarottoEur. Phys. J. C, 81 11 (2021) 1030DOI: https://doi.org/10.1140/epjc/s10052-021-09847-6