  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 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 dynamical invariants for this model, investigating the influence of
potential interactions.
