Equation Icon  ∫ + -dnk-- ikx 2 2 0 Δ n(x ) = − 2πi (2π)n e δ(k + m )θ (k ), ∫ n ikx (88 ) Δ (x ) = − d--k------e-------- for ε → 0+, Fn (2 π)n k2 + m2 − iε