ψ ψ ( x ) = ∫ ∫ d 4 p ( 2 π π ) 4 δ δ ( p 0 − − E ( p ) ) 2 E ( p ) ( A ( p ) e − − i p ⋅ ⋅ x + B ( p ) e + i p ⋅ ⋅ x ) θ θ ( p 0 ) = ∫ ∫ d 3 p ( 2 π π ) 3 1 2 E ( p ) ( A ( p ) e − − i p ⋅ ⋅ x + B ( p ) e + i p ⋅ ⋅ x ) | p 0 = + E ( p ) .