Multiderivations of Coxeter arrangements (Q1608585)

Let \(V\) be an \(\ell\)-dimensional Euclidean space, and \(S\) the algebra of polynomial functions on \(V\). Let \(\mathcal A\) be a Coxeter arrangement, that is, the set of reflecting hyperplanes of a finite irreducible subgroup \(G\) of the orthogonal group \(O(V)\). The paper studies the derivation modules \(D^{(m)}({\mathcal A})\), introduced by \textit{G. M. Ziegler} [Singularities, Proc. IMA Participating Inst. Conf., Iowa City/Iowa 1986, Contemp. Math. 90, 345-359 (1989; Zbl 0678.51010)]. The module \(D^{(m)}\) consists of derivations \(\theta\) on \(S\) such that \(\theta(\alpha_H)\in S\alpha_H^m\) for the defining linear form \(\alpha_H\) of each hyperplane \(H\in {\mathcal A}\). The author shows here that \(D^{(m)}\) is a free \(S\)-module of rank \(\ell\) for all \(m\). The \(m=1\) case was previously shown by \textit{K. Saito} [On the uniformization of complements of discriminant loci. In: Conference Notes. Amer. Math. Soc. Summer Institute, Williamstown (1975)]. The \(m=2\) case was shown by \textit{L. Solomon} and \textit{H. Terao} [Comment. Math. Helv. 73, 237-258 (1998; Zbl 0949.52009)]. The proof explicitly constructs a basis, and makes heavy use of the primitive derivation.