Relativistic anisotropic fluid spheres satisfying a non-linear equation of stateFrancisco Tello-Ortiz, M. Malaver, Ángel Rincón and Y. Gomez-LeytonEur. Phys. J. C, 80 5 (2020) 371DOI: https://doi.org/10.1140/epjc/s10052-020-7956-0