Splitting the Dirac equation: the case of longitudinal potentials
  Recently, we have demonstrated that some subsolutions of the free Duffin-Kemmer-Petiau and the Dirac equations obey the same Dirac equation with some built-in projection operators. In the present paper we study the Dirac equation in the interacting case. It is demonstrated that the Dirac equation in longitudinal external fields can be also splitted into two covariant subequations. 
Andrzej Okninski
math-ph math.MP