“…The characteristic approach to numerical relativity is based on null coordinates x α = (u, r, x A ), with u the retarded time, r a luminosity distance and x A coordinates on the sphere [1, 2,3,4,5,6,7,8,12,13]. In its 3-dimensional implementation, the angular coordinates chosen are stereographic coordinates x A = (ζ,ζ) on the sphere, which is covered with two stereographic coordinate patches, as first presented in Ref.…”