  Previously (A. Akhmeteli, J. Math. Phys., v. 52, p. 082303 (2011)), the Dirac
equation in an arbitrary electromagnetic field was shown to be generally
equivalent to a fourth-order equation for just one component of the
four-component Dirac spinor function. This was done for a specific (chiral)
representation of gamma-matrices and for a specific component. In the current
work, the result is generalized for a general representation of gamma-matrices
and a general component (satisfying some conditions). The resulting equivalent
of the Dirac equation is also manifestly relativistically covariant and should
be useful in applications of the Dirac equation.
