“…We prove also (Proposition 9.3) that if d : k(x, y, z) → k(x, y, z) is a derivation such that d(x) = y p , d(y) = z q , d(z) = x r , where p, q, r ∈ N, then k(x, y, z) d = k if and only if pqr 2. Some similar questions are studied in the interesting paper [18]. (The authors wish to thank the referee for pointing out this paper.…”