  In this work we develop a mathematical formalism which allows obtaining
oscillation formula for neutrino of any energy. We demonstrate that in the
ultra-relativistic limit the results obtained in this new approach agree with
the previously used phenomenological theory which is only applicable to
ultra-relativistic neutrinos. To this end we do the following. The Hilbert
spaces of particle states are constructed in such a way that the neutrinos are
combined in a multiplet with its components being considered as different
quantum states of a single particle. The same is done for the charged leptons
and the down- and up-type quarks. In the theory based on the Lagrangian of the
fermion sector of the Standard Model modified in accordance with this
construction, the phenomenon of neutrino oscillations arises as a direct
consequence of the general principles of quantum field theory. Using the
example of the pion decay, when the resulting neutrino has to be
ultra-relativistic, it is shown that the neutrino states produced in the decay
process can be described by a superposition of states with different masses and
identical canonical momenta with very high accuracy.
