A COVARIANT GAUGE-INVARIANT 3-DIMENSIONAL DESCRIPTION OF RELATIVISTICBOUND-STATES

A formalism is presented that allows covariant three-dimensional bound -state equations to be derived systematically from four-dimensional on es without the use of delta functions. The amplitude for the interacti on of a bound state described by these equations with an electromagnet ic probe is constructed. This amplitude is shown to be gauge-invariant if the formalism is truncated at the same coupling-constant order in both the interaction kernel of the integral equation and the electroma gnetic current operator.