Canad. Math. Bull. 53(2010), 77-86
Printed: Mar 2010
An example is given of a UFD which has an infinitely generated Derksen invariant. The ring is "almost rigid" 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.
14R20 - Group actions on affine varieties [See also 13A50, 14L30]
13A50 - Actions of groups on commutative rings; invariant theory [See also 14L24]
13N15 - Derivations