Constructing (almost) rigid rings and a UFD having infinitely generated Derksen and Makar-Limanov invariant

An example is given of a UFD which has infinitely generated Derksen invariant. The ring is \textquotedblleft almost rigid\textquotedblright
meaning that the Derksen invariant is equal to the Makar-Limanov invariant. Techniques to show that a ring is (almost) rigid are discussed, among which is a generalization of Mason’s abc-theorem.