Neutrino oscillations in the early universe: how can large lepton asymmetry be generated?

The lepton asymmetry that could be generated in the early universe through oscillations of active to sterile neutrinos is calculated (almost) analytically for small mixing angles, sin 20 < 10^-2. It is shown that for a mass squared differencc. 5M^2 = -I eV^2 it rnay rise at most by six orders of magnitude from the initial "normal" value ~10^-10 , as the backreaction from the refraction index terminates this rise while the asymmetry is still small. Only for very large mass differences, \(delta)m^2 ~ 10^9 eV^2, the lepton asymmetry could reach a significant magnitude exceeding 0.1.