When a copula is archimax

We discuss necessary and sufficient conditions for a copula to be archimax. We give a differential equation (depending on a function parameter) whose solution gives an additive generator of an Archimedean copula. We present two important applications of this differential equation. The first concerns the surprising fact that every Archimedean copula induces an uncountable family of such copulas. The other application is to a construction of Archimedean copulas from functions having the properties of opposite diagonal sections of Archimedean copulas.