Electromagnetic properties of non-Dirac particles with rest spin $1/2$

We resolve a number of questions related to an analytic description of electromagnetic form factors of non-Dirac particles with the rest spin $1/2$. We find the general structure of a matrix antisymmetric tensor operator. We obtain two recurrence relations for matrix elements of finite transformations of the proper Lorentz group and explicit formulas for a certain set of such elements. Within the theory of fields with double symmetry, we discuss writing the components of wave vectors of particles in the form of infinite continued fractions. We show that for $Q^2\le0.5$ {(GeV/c)}$^2$, where $Q^2$ is the transferred momentum squared, electromagnetic form factors that decrease as $Q^2$ increases and are close to those experimentally observed in the proton can be obtained without explicitly introducing an internal particle structure.