Null gravitational redshift by a Reissner–Nordström black hole in the strong field limitGuansheng He, Chaohong Pan, Xia Zhou, Weijun Li and Lin LiEur. Phys. J. C, 80 6 (2020) 512DOI: https://doi.org/10.1140/epjc/s10052-020-8085-5