On quasinormal frequencies of black hole perturbations with an external source Wei-Liang Qian, Kai Lin, Jian-Pin Wu, Bin Wang and Rui-Hong Yue Eur. Phys. J. C, 80 10 (2020) 959 Published online: 17 October 2020 DOI: 10.1140/epjc/s10052-020-08539-x