Note that if ∑ agg ∈ I, then ∑ ag = 0 so that ∑ agg = ∑ agg− ∑ ag1 = ∑ ag(g−1), so that every element in I is a Z-linear combination of the elements g− 1 (g ∈ G, g 6= 1). Also, if ∑ ag(g− 1) = 0, it follows that ag = 0 for all g so that the elements g − 1 (g ∈ G, g 6= 1) are linearly independent. (b) Notice that the ideal generated by the elements s− 1 (s ∈ S) is the same as the ideal generated...