Are gravitational wave ringdown echoes always equal-interval? Yu-Tong Wang, Zhi-Peng Li, Jun Zhang, Shuang-Yong Zhou and Yun-Song Piao Eur. Phys. J. C, 78 6 (2018) 482 Published online: 11 June 2018 DOI: 10.1140/epjc/s10052-018-5974-y