2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/132605An 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.Accepted to Canad. Math. BullAlgebraic GeometryCommutative Algebra14R20, 13A50, 13N15.text