This equation can be inverted in the sense of distributions, noting that the equation xf (x) = 1 has the solution (see Sokhotski–Plemelj theorem) f ( x ) = 1 x ± ± i ε ε = 1 x ∓ ∓ i π π δ δ ( x ) , with ε implying the limit to zero. Below, we discuss the right choice of the sign arising from causality requirements.