['In', 'order', 'to', 'obtain', 'the', 'atmospheric', 'neutrino', 'mass', 'splitting', 'while', 'keeping', 'the', 'maximal', 'mixing', 'between', 'νμ', 'and', 'ντ', ',', 'we', 'introduce', 'a', 'small', 'perturbation', 'to', 'the', 'neutrino', 'mass', 'matrix', 'without', 'breaking', 'Z3', 'symmetry', '.']