Neutrino mixing and the exponential form of the Pontecorvo–Maki–Nakagawa–Sakata matrixG. Dattoli and K.V. ZhukovskyEur. Phys. J. C, 55 4 (2008) 547-552DOI: https://doi.org/10.1140/epjc/s10052-008-0620-8