  A ring R is said to be rigid if the only locally nilpotent derivation of R is
the zero derivation. Let B = (direct sum of B_n for n in Z) be a Z-graded
commutative integral domain of characteristic 0. For each positive integer d,
consider the Veronese subring B(d) of B, defined by B(d) = (direct sum of the
B_n for n in dZ). We study the properties of the set of positive integers d
such that B(d) is non-rigid, and give some applications.
