  In the Quantum theory the wave functions form a full and orthonormalized
functional space. For this reason we can use the equation on eigenfunctions and
eigenstates and find the eigenenergies $E_n$ and eigenfunctions $\Psi_n$ to
determine the physical characteristics of the considered systems (or models).
In the Quantum theory the observed values are the average value of operators.
Since the wave functions create a full and orthonormalized space, the average
values of operators coincide with eigenvalues of operators. This situation
takes place in the case of strong and electromagnetic interactions. However,
the average values of the weak interaction operators are equal to zero since
only the left-handed components of spinors participate in the weak
interactions. It means that in these interactions the connected states cannot
exist. Then the weak interaction of the particles (scatterings, decays) can be
considered only in the framework of the standard perturbative approach. It is
shown that the weak interactions cannot generate masses and the equation for
Green's function of the weak interacting fermions (neutrinos) in the matter
coincides with the equation for Green's function of fermions in vacuum. And in
the result we come to a conclusion: the mechanism of resonance enhancement of
neutrino oscillations in matter (i.e. MSW effect) cannot exist.
