where I 4 is the unit matrix in four dimensions, and employing the Feynman slash notation. This is the Dirac equation for a delta function source in spacetime. Using the momentum representation, S F ( x ′ , x ) = ∫ ∫ d 4 p ( 2 π π ) 4 exp ⁡ ⁡ [ − − i p ⋅ ⋅ ( x ′ − − x ) ] S ~ ~ F ( p ) , the equation becomes