“…127-128]. On the other hand, it has been proved in [2,Lemma 2.13], that if P is a graded ideal of a Z-graded ring R, then √ P is a graded ideal of R. So, Proposition 2.15 is a correction for [8,Theorem 4]. In fact, we do not know whether the general result in Proposition 2.15 is true for any group G, but there is a fault in the proof and the former proof works for Z-graded rings.…”