having a structure very similar to that of the ordinary three-dimensional nonrelativistic Schrödinger equation. It is a manifestly covariant equation, but at the same time its three-dimensional structure is evident. The four-vectors p ⊥ ⊥ μ μ and x ⊥ ⊥ μ μ have only three independent components since P ⋅ ⋅ p ⊥ ⊥ = P ⋅ ⋅ x ⊥ ⊥ = 0 . The similarity to the three-dimensional structure of the nonrelativistic Schrödinger equation can be made more explicit by writing the equation in the c.m. frame in which P = ( w , 0 → → ) , p ⊥ ⊥ = ( 0 , p → → ) , x ⊥ ⊥ = ( 0 , x → → ) .