Single-Time Formalisms from Covariant Equations

The main purpose of this paper is to provide a concise general deduction of a result which has previously been reported and applied. The result is a single-time wave equation for a two-nucleon system obtained from the covariant two-body equation by means of an iteration procedure which employs the solution of the latter for an instantaneous interaction. The single-time wave function contains only positive energies. Possible generalizations to include negative-energy components and higher amplitudes are discussed, but none of these is carried out in detail.