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Register a new userA Gorenstein simplicial complex for symmetric minors
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We show that the ideal generated by the $(n-2)$ minors of a general symmetric $n$ by $n$ matrix has an initial ideal that is the Stanley-Reisner ideal of the boundary complex of a simplicial polytope and has the same Betti numbers.
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We study the symmetric subquotient decomposition of the associated graded algebras $A^*$ of a non-homogeneous commutative Artinian Gorenstein (AG) algebra $A$. This decomposition arises from the stratification of $A^*$ by a sequence of ideals $A^*=C_A(0)supset C_A(1)supsetcdots$ whose successive quotients $Q(a)=C(a)/C(a+1)$ are reflexive $A^*$ modules. These were introduced by the first author, and have been used more recently by several groups, especially those interested in short Gorenstein algebras, and in the scheme length (cactus rank) of forms. For us a Gorenstein sequence is an integer sequence $H$ occurring as the Hilbert function for an AG algebra $A$, that is not necessarily homogeneous. Such a Hilbert function $H(A)$ is the sum of symmetric non-negative sequences $H_A(a)=H(Q_A(a))$, each having center of symmetry $(j-a)/2$ where $j$ is the socle degree of $A$: we call these the symmetry conditions, and the decomposition $mathcal{D}(A)=(H_A(0),H_A(1),ldots)$ the symmetric decomposition of $H(A)$. We here study which sequences may occur as the summands $H_A(a)$: in particular we construct in a systematic way examples of AG algebras $A$ for which $H_A(a)$ can have interior zeroes, as $H_A(a)=(0,s,0,ldots,0,s,0)$. We also study the symmetric decomposition sets $mathcal{D}(A)$, and in particular determine which sequences $H_A(a)$ can be non-zero when the dual generator is linear in a subset of the variables. Several groups have studied exotic summands of the Macaulay dual generator $F$. Studying these, we recall a normal form for the Macaulay dual generator of an AG algebra that has no exotic summands. We apply this to Gorenstein algebras that are connected sums. We give throughout many examples and counterexamples, and conclude with some open questions about symmetric decomposition.
We answer a question of Celikbas, Dao, and Takahashi by establishing the following characterization of Gorenstein rings: a commutative noetherian local ring $(R,mathfrak m)$ is Gorenstein if and only if it admits an integrally closed $mathfrak m$-primary ideal of finite Gorenstein dimension. This is accomplished through a detailed study of certain test complexes. Along the way we construct such a test complex that detect finiteness of Gorenstein dimension, but not that of projective dimension.
A set of polynomials G in a polynomial ring S over a field is said to be a universal Gru007foebner basis, if G is a Gru007foebner basis with respect to every term order on S. Twenty years ago Bernstein, Sturmfels, and Zelevinsky proved that the set of the maximal minors of a matrix X of variables is a universal Gru007foebner basis. Boocher recently proved that any initial ideal of the ideal of maximal minors of X has a linear resolution. In this paper we give a quick proof of the results mentioned above. Our proof is based on a specialization argument. Then we show that similar statements hold in a more general setting, for matrices of linear forms satisfying certain homogeneity conditions. More precisely, we show that the set of maximal minors of a matrix L of linear forms is a universal Gru007foebner basis for the ideal I that it generates, provided that L is column-graded. Under the same assumption we show that every initial ideal of I has a linear resolution. Furthermore, the projective dimension of I and of its initial ideals is n-m, unless I=0 or a column of L is identically 0. Here L is a matrix of size m times n, and m is smaller than or equal to n. If instead L is row-graded, then we prove that I has a universal Gru007foebner basis of elements of degree m and that every initial ideal of I has a linear resolution, provided that I has the expected codimension. The proofs are based on a rigidity property of radical Borel fixed ideals in a multigraded setting: We prove that if two Borel fixed ideals I and J have the same multigraded Hilbert series and I is radical, then I = J. We also discuss some of the consequences of this rigidity property.
Let $k$ be an arbitrary field. In this note, we show that if a sequence of relatively prime positive integers ${bf a}=(a_1,a_2,a_3,a_4)$ defines a Gorenstein non complete intersection monomial curve ${mathcal C}({bf a})$ in ${mathbb A}_k^4$, then there exist two vectors ${bf u}$ and ${bf v}$ such that ${mathcal C}({bf a}+t{bf u})$ and ${mathcal C}({bf a}+t{bf v})$ are also Gorenstein non complete intersection affine monomial curves for almost all $tgeq 0$.
Let $R$ be the face ring of a simplicial complex of dimension $d-1$ and ${mathcal R}(mathfrak{n})$ be the Rees algebra of the maximal homogeneous ideal $mathfrak{n}$ of $R.$ We show that the generalized Hilbert-Kunz function $HK(s)=ell({mathcal R}(mathfrak n)/(mathfrak n, mathfrak n t)^{[s]})$ is given by a polynomial for all large $s.$ We calculate it in many examples and also provide a Macaulay2 code for computing $HK(s).$
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