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Ngo Viet Trung

Publications and source records attributed to Ngo Viet Trung.

50 records · Page 3Linked to original sources
arXiv2004

Asymptotic behaviour of arithmetically Cohen-Macaulay blow-ups

This paper addresses problems related to the existence of arithmetic Macaulayfications of projective schemes. Let Y be the blow-up of a projective scheme X = Proj R along the ideal sheaf of I \subset R. It is known that there are embeddings Y \cong Proj k[(I^e)_c] for c \ge d(I)e + 1, where d(I) denotes the maximal generating degree of I, and that there exists a Cohen-Macaulay ring of the form k[(I^e)_c] if and only if H^0(Y,O_Y) = k, H^i(Y,O_Y) = 0 for i = 1,...,dim Y-1, Y is equidimensional and Cohen-Macaulay. Cutkosky and Herzog asked when there is a linear bound on c and e ensuring that k[(I^e)_c] is a Cohen-Macaulay ring. We obtain a surprising compelte answer to this question, namely, that under the above conditions, there are well determined invariants a and b such that k[(I^e)_c] is Cohen-Macaulay for all c > d(I)e + a and e > b. Our approach is based on recent results on the asymptotic linearity of the Castelnuovo-Mumford regularity of ideal powers. We also investigate the existence of a Cohen-Macaulay Rees algebra of the form R[(I^e)_ct] (which provides an arithmetic Macaulayfication for X). If R has negative a*-invariant, we prove that such a Cohen-Macaulay Rees algebra exists if and only if f_*O_Y = O_X, R^i f*O_Y = 0 for i > 0, Y is equidimensional and Cohen-Macaulay. Especially, these conditions imply the Cohen-Macaulayness of R[(I^e)_ct] for all c > d(I)e + a and e > b. The above results can be applied to obtain several new classes of Cohen-Macaulay algebras.

math.AC↗
arXiv2003

Borel-fixed ideals and reduction number

The aim of this paper is to study the relationship between reduction numbers and Borel-fixed ideals in all characteristics. By definition, Borel-fixed ideals are closed under certain specializations which is similar to the strong stability. We will estimate the number of monomials which can be specialized to a given monomial. As a consequence, we obtain a combinatorial version of the well-known Eakin-Sathaye's theorem which bounds the reduction number in terms of the Hilbert function. Furthermore, we show that the bound of Eakin-Sathaye's theorem is attained by the reduction number of a lex-segment monomial ideal. This result answers a question of Conca in the affirmative. We will also show that the reduction number of the lex-segment ideal is bounded exponentially by the reduction number of the given ideal.

math.AC↗
arXiv2002

On the asymptotic linearity of Castelnuovo-Mumford regularity

Let R be a standard graded ring over a commutative Noetherian ring with unity and I a graded ideal of R. Let M be a finitely generated graded R-module. We prove that there exist integers e and ρ_M(I) such that for all large n, reg(I^nM)= ρ_M(I)n+e.

math.AC↗
arXiv2002

Castelnuovo-Mumford regularity and extended degree

The main result of this paper shows that the Castelnuovo-Mumford regularity of the tangent cone of a local ring is effectively bounded by the dimension and any extended degree. From this it follows that there are only a finite number of Hilbert-Samuel functions of local rings with given dimension and extended degree.

math.AC↗
arXiv2002

Positivity of mixed multiplicities

This paper studies mixed multiplicities of an arbitrary standard bigraded algebra and mixed multiplicities of two ideals I, J in a local ring (A,m), where I is an m-primary ideal and J an arbitrary ideal. The main results are criteria for their positivity which can be used to compute them effectively. We also show that the range of positive mixed multiplicities of a bigraded algebra is rigid if the algebra satisfies the first chain condition and is connected in codimension one and that this range is always rigid for mixed multiplicities of ideals. These results can be used to study the mu-invariants of analytic hypersurfaces, the degree of rational varieties obtained by blowing-up projective spaces, and the degree of the Stuckrad-Vogel cycles in intersection theory.

math.AC↗
arXiv2002

Hilbert polynomials of non-standard bigraded algebras

This paper is a systematic study of the Hilbert polynomial of a bigraded algebra R which are generated by elements of bidegrees (1,0), (d_1,1),...,(d_r,1), where d_1,...,d_r are non-negative integers. The obtained results can be applied to study Rees algebras of homogeneous ideals and their diagonal subalgebras. The Hilbert polynomial is computed explicitly in the following cases: (1) R is a bigraded polynomial ring of the above type; (2) R is the Rees algebra of an ideal generated by a homogeneous regular sequence; (3) R is the Rees algebra of the ideal generated by the maximal minors of a generic (r-1) by r matrix.

math.AC↗
arXiv2002

Specialization of modules

Let R be a polynomial ring over k(u), where k is a field k and u is a finite family or inderterminates. The paper introduces the specialization of an arbitrary finitely generated R-module by the substitution of u to elements of k. This definition does not depend on the presentation of the given module and preserves basic properties and operations on modules (e.g. annihilator, dimension, Ext and Tor) for almost all specializations.

math.AC↗
arXiv2002

Specialization of modules over a local ring

This paper is a continuation of an earlier paper of the authors on the probem of specializations of modules. The aim here is to define specialisations of finitely generated modules over local rings of the form k(u)[X]_P, where u is a family of indeterminates over a field k. The definition depends on the associated primes of the specialisation of the prime ideal P by the substitution of u to elements of k. As in the non-local case, the introduced specializations preserve basic properties and operations on modules.In particular, annihilators of local cohomology modules commute with specializations. As a consequence, Cohen-Macaulayness and Buchsbaumness are preserved by almost al substitutions. The paper also studies the non-Cohen-Macaulay locus and the singular locus of specializations of factor rings of k(u)[X]_P.

math.AC↗
arXiv2002

Grobner bases, local cohomology and reduction number

D. Bayer and M. Stillman showed that Grobner bases can be used to compute the Castelnuovo-Mumford regularity, which is a measure for the vanishing of graded local cohomology modules. The aim of this paper is to show that the same method can be applied to study other cohomological invariants as well as the reduction number.

math.AC↗
arXiv2002

Constructive characterization of the reduction numbers

We present a constructive description of minimal reductions with a given reduction number. This description has interesting consequences on the minimal reduction number, the big reduction number, and the core of an ideal. In particular, it helps solve a conjecture of Vasconcelos on the relationship between reduction numbers and initial ideals.

math.AC↗
arXiv2001

Problems and algorithms for affine semigroups

In this article we overview those aspects of the theory of affine semigroups and their algebras that have been relevant for our own research, and pose several open problems. Answers to these problems would contribute substantially to the further development of the theory. The paper treats two main topics: (1) affine semigroups and several covering properties for them and (2) algebraic properties for the corresponding rings (Koszul, Cohen-Macaulay, different ``sizes'' of the defining binomial ideals). We emphasize the special case when the initial data are encoded into lattice polytopes. The related objects -- polytopal semigroups and algebras -- provide a link with the classical theme of triangulations into unimodular simplices. We have also included an algorithm for checking the semigroup covering property in the most general setting. Our counterexample to certain covering conjectures was found by the application of a small part of this algorithm. The general algorithm could be used for a deeper study of affine semigroups.

math.AC↗
arXiv2001

Groebner bases and regularity of Rees algebras

In this paper we study homological properties of the Rees ring R of the graded maximal ideal of a standard graded k-algebra A. In particular we are interested the comparison of the depth and regularity of A and R.

math.AC↗