Two-mixing texture for three active neutrinos
  The conjecture that among three massive neutrinos $\nu_1, \nu_2, \nu_3$ there is no direct mixing between $\nu_1$ and $\nu_3$ leads to a two-mixing texture for three active neutrinos $\nu_e, \nu_\mu, \nu_\tau$. This texture, much discussed previously, is neatly consistent with the observed deficits of solar $\nu_e$'s and atmospheric $\nu_\mu$'s, but (without extra mixing with at least one sterile neutrino $\nu_s$) predicts no LSND effect for accelerator $\nu_\mu$'s. In this option, the masses $m^2_1\stackrel{<}{\sim} m^2_2 << m^2_3$ are readily estimated. The characteristic feature of the two-mixing neutrino texture that {\it only the close neighbours in the hierarchy of massive neutrinos $\nu_1, \nu_2, \nu_3$ mix significantly} may be physically meaningful. Going out from the notion of mixing matrix we construct an intrinsic occupation-number operator whose {\it eigenvalues} 0, 1, 2 {\it numerate the three generations of massive neutrinos}. Analogical constructions work also for charged leptons as well as for up and down quarks. 
W. Krolikowski (Warsaw Univ.)