The effective neutrino mass of neutrinoless double-beta decays: how possible to fall into a well Zhi-zhong Xing and Zhen-hua Zhao Eur. Phys. J. C, 77 3 (2017) 192 Published online: 27 March 2017 DOI: 10.1140/epjc/s10052-017-4777-x