The term "Green function" as used here coincides with the standard definition of a fundamental solution. For the model equation (1.1), a Green function is any solution to LG(p, p ) = −4πIδ(p, p ), where I denotes an appropriate identity operator (needed for non-scalar fields) and δ(p, p ) the Dirac distribution. By contrast, some authors define Green functions somewhat more restrictively. See, e.g., [1].2 Distributions like θ(p ≥ p )δ(σ) appearing here are not a priori, well-defined. They are to be interpreted as, e.g., lim →0 + θ(p ≥ p )δ(σ + ). See Sect. 4.1 of [2].