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Canonical modules of Rees algebras. (English) Zbl 1115.13007

Let \(R\) be a regular domain essentially of finite type over a field, and \(I\) an ideal of \(R\). Assume that the Rees algebra \(R(I)=\bigoplus_{n\geq 0} I^n\) is Cohen-Macaulay and normal. Let \(P_1,\dots, P_t\) be the divisorial prime ideals of \(IR(I)\) with \(IR(I)= \bigcap_{i=1} P^{(d_i)}_i\), and suppose that \(v_{P_i}\mid R= v_{p_i}\) with \(p_i= P_i\cap R\) (\(v_{P_i}\)’s are the Rees valuations of \(I\)). Then the main theorem states that a canonical module \(\omega_{R(I)}\) of \(R(I)\) has the class \[ \sum^t_{i=1} (d_i+ 1- \text{ht}(p_i)[P_i]= [IR(I)]+ \sum^t_{i=1} (1- \text{ht}(p_i)[P_i]. \] Moreover, \(R(I)\) is Gorenstein if and only if \(d_i= \text{ht}(p_i)- 1\) for all \(i= 1,\dots, t\). The proof uses the fact that the canonical module commutes with subintersections under certain conditions. The formula is applied to compute the canonical classes of certain determinantal rings.

MSC:

13A30 Associated graded rings of ideals (Rees ring, form ring), analytic spread and related topics
13B22 Integral closure of commutative rings and ideals
13C20 Class groups
13C40 Linkage, complete intersections and determinantal ideals

Software:

Normaliz

References:

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