  We show that additional solutions must be ignored (in differences of the
Schrodinger and Klein-Gordon equations) in the Dirac equation, where usually
passed the second order radial equation, called the reduced equation, instead
of a system. Analogously to the Schrodinger equation, in this process the Dirac
delta function appears, which was unnoted during the full history of quantum
mechanics. This unphysical term we remove by a boundary condition at the
origin. However, the distribution theory imposes on the radial function strong
restriction and by this reason practically for all potentials, even regular,
use of these reduced equations is not permissible. At the end we include
consideration in the framework of two-dimensional Dirac equation. We show that
even here the additional solution does not survives as a result of usual
physical requirements.
