  The second-order equation in the (1/2,0)+(0,1/2) representation of the
Lorentz group has been proposed by A. Barut in the beginning of the 70s. It
permits to explain the mass splitting of leptons e,mu,tau. Recently, the
interest has grown to this model (see, for instance, the papers by S. Kruglov
and J. P. Vigier et al.). We continue the research deriving the equation from
the first principles, finding the dynamical invariants for this model,
investigating the influence of the potential interactions.
