“…At cross-section level, the basic lowest-order factorisation properties have of course been well-known since the early days of perturbative QCD [238, 248-250, 287, 289, 506, 507], and they are directly relevant for the cancellation of infrared divergences at NLO. Collinear splitting kernels for hadronic cross sections are also necessary ingredients for all applications of collinear factorisation in QCD, and they have by now been determined to three loops [354,356,[508][509][510], with remarkable partial results at four and five loops [395,397,[511][512][513]. Detailed studies at amplitude level are somewhat more recent: the general form of the factorisation of tree-level gauge amplitudes in soft and collinear limits was established in the late eighties, using recursion relations [514,515] or general expression for the amplitudes derived from string theory [516,517]; a combined factorisation including both soft and collinear limits was later proposed in [518,519].…”