Neutrino mixing and the exponential form of the Pontecorvo–Maki–Nakagawa–Sakata matrix G. Dattoli and K.V. Zhukovsky Eur. Phys. J. C, 55 4 (2008) 547-552 Published online: 27 May 2008 DOI: 10.1140/epjc/s10052-008-0620-8