On higher-order flavour-singlet splitting and coefficient functions at large x

We discuss the large- x behaviour of the splitting functions P and P and of flavour-singlet coefficient functions, such as the gluon contributions C and C to the structure functions F, in massless perturbative QCD. These quantities are suppressed by one or two powers of (1-x) with respect to the (1 terms which are the subject of the well-known threshold exponentiation. We show that the double-logarithmic contributions to P, P and C at order αs4 can be predicted from known third-order results and present, as a first step towards a full all-order generalization, the leading-logarithmic large- x behaviour of P, P and C at all orders in α.