On-axis tidal forces in Kerr spacetime Haroldo C. D. Lima Junior, Luís C. B. Crispino and Atsushi Higuchi Eur. Phys. J. Plus, 135 3 (2020) 334 Published online: 30 March 2020 DOI: 10.1140/epjp/s13360-020-00342-7