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When Is an Associated Graded Ring Cohen-Macaulay If It Is a Domain? Youngsu Kim Purdue University

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When Is an Associated Graded Ring Cohen-Macaulay If It Is a Domain? Youngsu Kim Purdue University When is an associated graded ring Cohen-Macaulay if it is a domain? Youngsu Kim Purdue University What is an associated graded ring? [HKU, Question 4.11] Theorem Let (R, m) be a Noetherian local ring with maximal ideal m. Let I Let (R, m) be a Gorenstein local ring and I an m-primary ideal. Is the Let (R, m) be a Cohen-Macaulay local ring with ecodim(R) = 2, be an R-ideal. There are graded algebras associated to the ideal I. extended Rees algebra R[It, t−1] Gorenstein if it is quasi-Gorenstein? i.e. Rˆ can be written as S/I where (S, n) is a regular local ring Here we review the blowup algebras of I and their relationships, Definition: A ring is called quasi-Gorenstein if it is isomorphic to its and I is a height two Cohen-Macaulay ideal. Assume that I is R ⊂ R[It] ⊂ R[It, t−1] ⊂ R[t, t−1]. canonical module (but not necessarily Cohen-Macaulay). 3-generated and ordn(I) is at most 4. If grm(R) is a domain then it is Cohen-Macaulay. The rings R[It] and R[It, t−1] are called Rees algebra and extended Remarks on the question i Rees algebra, respectively. The associated graded ring with respect Definition: ordn(I) = max{i|I ⊂ n } to I, grI(R), is (R, m) 2 2 3 ∼ −1 −1 ∼ • [HKU, Corollary 4.12] Let be a 2-dimensional regular local ring gr (R) := R/I⊕I/I ⊕I /I ··· = R[It, t ]/(t ) = R[It]/IR[It]. −1 Sketch of the proof I and let I be an m-primary ideal. If R[It, t ] is quasi-Gorenstein, then −1 −1 Since t is a homogeneous non zerodivisor on R[It, t ], it follows, R[It, t−1] is Gorenstein. √ • By [HB], one can write I = I2(M) where gr (R) for instance, that I is Cohen-Macaulay (Gorenstein) if and only I = m • [HH, Example 4.7] shows that the assumption cannot be x x x 11 12 13 −1 M . = if R[It, t ] is Cohen-Macaulay (Gorenstein), respectively. removed. x21 x22 x23 Remark • Modify M to obtain Special case of [HKU, Question 4.11] ∗ ∗ ∗ x x x ∗ 11 12 13 M = ∗ ∗ ∗ x x x Most ring-theoretic properties (e.g. reduced, irreducible, integrally Let (R, m) be a Gorenstein local ring. Is grm(R) Gorenstein if grm(R) 21 22 23 ∗ ∗ closed, Cohen-Macaulay, etc) of the associated graded ring pass to is a domain? with grade J = 2 for J := I2(M ). Here x denotes the leading the ambient ring R. −1 x ∈ S gr S Remark: If grm(R) is a domain then R[It, t ] is quasi-Gorenstein. form of an element in n( ). When is an associated ring Cohen-Macaulay? • Then we can lift the relations among the generators of J to R, which is equivalent to say J = I∗ where I∗ denotes the leading A great deal of research has been done on this question. One ap- • More generally, one can ask. ideal of I, i.e. the ideal generated by the leading forms of all proach uses numerical criteria based on analytic spread, reduction elements of I. Question 2 ∼ ∗ number, or Hilbert-Samuel multiplicity. [Ulrich] summarizes such • This shows that grm(R) = grn(S)/I is Cohen-Macaulay by results. Another approach is assuming the associated graded ring is Let (R, m) be a Cohen-Macaulay local ring. Is grm(R) Cohen- [HB]. a domain. More results are known when an associated graded ring Macaulay if it is a domain? becomes Gorenstein. We list one here. Remark: the condition "domain" is necessary Positive answer when ecodim is small [Hochster, HSV] Let S = C[[x, y, u, v, w]], n = (x, y, u, v, w) and I = I2(M) where 2 3 3 x + u 0 xv + u The embedding codimension measures how far a ring is from a regular (R, m) I M = . Let be a Gorenstein local ring, and let be an ideal ∼ 3 3 local ring. That is, ecodim(R) = h means that Rˆ = S/I where (S, n) xy + u xw + x 0 of R whose associated graded ring grI(R) is a Cohen-Macaulay 2 is a regular local ring and I ⊂ n is a height h ideal in S. Set R = S/I. Then grm(R) is not Cohen-Macaulay and it is not a domain. Then grI(R) is also Gorenstein. The question has a positive answer if domain, although the other assumptions of the theorem are satisfied. • ecodim(R) = 1 References Question 1 • ecodim(R) = 2 and R is a complete intersection ring. How much can we relax the condition "complete intersection" if [HB] L. Burch, A note on ideals of homological dimension one in local domains, Proc. Cambridge Philos. Soc. 63 (1967), 661–662. [HKU] W. Heinzer, M. Kim and B. Ulrich, The Gorenstein and complete intersection properties of associated graded rings, J. Pure Appl. Can we remove the assumption "the associated graded ring is ecodim(R) = 2? To this end we ask the following question. Algebra 201 (2005), no. 1, 264–283. Cohen-Macaulay" in the statement above? To this end Heinzer, [HSV] J. Herzog, A. Simis and W. V. Vasconcelos, On the canonical module of the Rees algebra and the associated graded ring of an ideal, J. Algebra 105 (1987), no. 2, 285–302. Kim and Ulrich raised the following question. Question 3 [Hochster] M. Hochster, Criteria for Equality of Ordinary and Symbolic Powers of Primes, Math. Z. 133 (1973), 53–65. [HH] S. Huckaba and C. Huneke, Powers of ideals having small analytic deviation, American J. Math. 114 (1992), 367–403. [HS] C. Huneke and I. Swanson, Integral Closure of Ideals, Rings, and Modules, London Mathematical Lecture Note Series 336, Let (R, m) be a Cohen-Macaulay local ring. Assume that Cambridge University Press (2006). ecodim(R) = 2 and R is an almost complete intersection. Is grm(R) [Ulrich] B. Ulrich, Cohen-Macaulayness of Associated Graded Rings and Reduction Numbers of Ideals, ICTP lecture notes, (1994). Cohen-Macaulay if it is a domain?.
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