On the Stanley ring of cubical complex

We investigate the properties of the Stanley ring of a cubical complex, a cubical analogue of the Stanley-Reisner ring of a simplicial complex. We compute its Hilbert series in terms of thef-vector, and prove that by taking the initial ideal of the defining relations, with respect to the reverse lexicographic order, we obtain the defining relations of the Stanley-Reisner ring of the triangulation via “pulling the vertices” of the cubical complex. Applying an old idea of Hochster we see that this ring is Cohen-Macaulay when the complex is shellable, and we show that its defining ideal is generated by quadrics when the complex is also a subcomplex of the boundary complex of a convex cubical polytope. We present a cubical analogue of balanced Cohen-Macaulay simplicial complexes: the class of edge-orientable shellable cubical complexes. Using Stanley's results about balanced Cohen-Macaulay simplicial complexes and the degree two homogeneous generating system of the defining ideal, we obtain an infinite set of examples for a conjecture of Eisenbud, Green, and Harris. This conjecture says that theh-vector of a polynomial ring inn variables modulo an ideal which has ann-element homogeneous system of parameters of degree two, is thef-vector of a simplicial complex.     

On the computation of a-invariants

Thea-invariant of a graded Cohen-Macaulay ring is the least degree of a generator of its graded canonical module. We compute thea-invariants of (i) graded algebras with straightening laws on upper semi-modular lattices and (ii) the Stanley-Reisner rings of shellable weighted simplicial complexes. The formulas obtained are applied to rings defined by determinantal and pfaffian ideals.     

Deficiently extremal Cohen-Macaulay algebras

The aim of this paper is to study homological properties of deficiently extremal Cohen-Macaulay algebras. Eagon-Reiner showed that the Stanley-Reisner ring of a simplicial complex has a linear resolution if and only if the Alexander dual of the simplicial complex is Cohen-Macaulay. An extension of a special case of Eagon-Reiner theorem is obtained for deficiently extremal Cohen-Macaulay Stanley-Reisner rings.     

An alternative proof of a characterization of Cohen-Macaulay bipartite graphs

Recently, Herzog and Hibi explicitly described all Cohen-Macaulay bipartite graphs by using the Stanley-Reisner ideal of the Alexander dual of the simplicial complex Δ _P associated to a finite poset P. In this paper, we will present a short proof that does not use the Stanley-Reisner ideal of the Alexander dual of Δ _P .     

Combinatorial methods in the theory of Cohen-Macaulay rings

Characterizations of Cohen-Macaulay posets are given in terms of the nonsingularity of certain incidence matrices. These results are applied to derive a purely combinatorial construction of basic systems for Stanley-Reisner rings of shellable posets. A procedure for transferring identities from one polynomial ring to another is then used to obtain basic systems for partition rings.     

On toric face rings

Following a construction of Stanley we consider toric face rings associated to rational pointed fans. This class of rings is a common generalization of the concepts of Stanley-Reisner and affine monoid algebras. The main goal of this article is to unify parts of the theories of Stanley-Reisner and affine monoid algebras. We consider (non-pure) shellable fan’s and the Cohen-Macaulay property. Moreover, we study the local cohomology, the canonical module and the Gorenstein property of a toric face ring.     

Depth of modular invariant rings

It is well-known that the ring of invariants associated to a non-modular representation of a finite group is Cohen-Macaulay and hence has depth equal to the dimension of the representation. For modular representations the ring of invariants usually fails to be Cohen-Macaulay and computing the depth is often very difficult. In this paper¹ we obtain a simple formula for the depth of the ring of invariants for a family of modular representations. This family includes all modular representations of cyclic groups. In particular, we obtain an elementary proof of the celebrated theorem of Ellingsrud and Skjelbred [6].     

The facet ideal of a simplicial complex

 To a simplicial complex, we associate a square-free monomial ideal in the polynomial ring generated by its vertex set over a field. We study algebraic properties of this ideal via combinatorial properties of the simplicial complex. By generalizing the notion of a tree from graphs to simplicial complexes, we show that ideals associated to trees satisfy sliding depth condition, and therefore have normal and Cohen-Macaulay Rees rings. We also discuss connections with the theory of Stanley-Reisner rings. Received: 7 January 2002 / Revised version: 6 May 2002     

Generalized Cohen-Macaulay modules over rings with approximation property

Let (A, m) be an excellent Henselian ring with isolated singularity and letR be its completion. Then every indecomposable maximal Buchsbaum (resp. generalized Cohen-Macaulay)R-module is isomorphic with the completion of an indecomposable maximal Buchsbaum (resp. generalized Cohen-Macaulay)A-module. Hence one gets examples of non-complete, non-regular rings having finite Buchsbaum representation type.     

Shellable graphs and sequentially Cohen-Macaulay bipartite graphs

Associated to a simple undirected graph G is a simplicial complex ΔG whose faces correspond to the independent sets of G. We call a graph G shellable if ΔG is a shellable simplicial complex in the non-pure sense of Björner-Wachs. We are then interested in determining what families of graphs have the property that G is shellable. We show that all chordal graphs are shellable. Furthermore, we classify all the shellable bipartite graphs; they are precisely the sequentially Cohen-Macaulay bipartite graphs. We also give a recursive procedure to verify if a bipartite graph is shellable. Because shellable implies that the associated Stanley-Reisner ring is sequentially Cohen-Macaulay, our results complement and extend recent work on the problem of determining when the edge ideal of a graph is (sequentially) Cohen-Macaulay. We also give a new proof for a result of Faridi on the sequentially Cohen-Macaulayness of simplicial forests.     

Cohen-Macaulay Section Rings

In this paper, we study the section rings of sheaves of Cohen-Macaulay algebras (over a field ) on a ranked poset. A necessary and sufficient condition for these rings to be Cohen-Macaulay will be given. This is a further generalization of a result of Yuzvinsky, which generalizes Reisner's theorem concerning Stanley-Reisner rings.

     

Initial complex associated to a jet scheme of a determinantal variety

We show in this paper that the principal component of the first-order jet scheme over the classical determinantal variety of m×n matrices of rank at most 1 is arithmetically Cohen-Macaulay, by showing that an associated Stanley-Reisner simplicial complex is shellable.     

Dimension and automorphism groups of lattices

Every group is the automorphism group of a lattice of order dimension at most 4. We conjecture that the automorphism groups of finite modular lattices of bounded dimension do not represent every finite group. It is shown that ifp is a large prime dividing the order of the automorphism group of a finite modular latticeL then eitherL has high order dimension orM _p, the lattice of height 2 and orderp+2, has a cover-preserving embedding inL. We mention a number of open problems. Presented by C. R. Platt.     

Higher commutators in the loop space homology of K-products

We consider a problem of calculating the loop space homology for so-called polyhedral products defined by an arbitrary simplicial complex K. A presentation of this homology algebra is obtained from the homology of the complements of diagonal subspace arrangements, which, in turn, is calculated using an infinite resolution of the exterior Stanley-Reisner algebra. We get an explicit presentation of the loop homology algebra for polyhedral products for classes of simplicial complexes such as flag complexes and the duals of sequentially Cohen-Macaulay complexes in terms of higher commutator products. We give a construction for the iteration of higher products and discuss the relationship between this problem and problems in commutative algebra.     

The Upper Bound Conjecture and Cohen-Macaulay Rings

Let Δ be a triangulation of a (d ? 1)-dimensional sphere with n vertices. The Upper Bound Conjecture states that the number of i-dimensional faces of Δ is less than or equal to a certain explicit number c_i(n, d). A proof is given of a more general result. The proof uses the result, proved by G. Reisner, that a certain commutative ring associated with Δ is a Cohen-Macaulay ring.     

Multi-symbolic Rees algebras and strong F-regularity

Let I be a divisorial ideal of a strongly F-regular ring A. K.-i. Watanabe raised the question whether the symbolic Rees algebra is Cohen-Macaulay whenever it is Noetherian. We develop the notion of multi-symbolic Rees algebras and use this to show that is indeed Cohen-Macaulay whenever a certain auxiliary ring is finitely generated over A. Received August 10, 1998 / in final form October 18, 1999 / Published online July 20, 2000     

Good filtrations of symmetric algebras and strong F-regularity of invariant subrings

In this paper, we prove that a linear action of a reductive group on a polynomial ring with good filtrations over a field of characteristic p>0 yields a strongly F-regular (in particular, Cohen-Macaulay) invariant subring. The strongly F-regular property of some known examples of invariant subrings, such as the coordinate rings of Schubert varieties in Grassmannians, are recovered. A similar result over a field of characteristic zero is also proved. An erratum to this article is available at .     

Minimal lifts of dihedral 2-dimensional Galois representations

In this paper we study the connection between odd dihedral 2-dimensional modulo p Galois representations and modular forms with complex multiplication. More precisely, we prove that, for every such representation satisfying some explicit conditions, there exists a modular form giving rise to it of the type given by Serre’s conjecture (in its strong version) with the additional property of having complex multiplication.     

Cohen-Macaulayness of Special Fiber Rings

Abstract

Let (R, 𝔪) be a Noetherian local ring and let Ibe an R-ideal. Inspired by the work of Hübl and Huneke, we look for conditions that guarantee the Cohen-Macaulayness of the special fiber ring ? = ?/𝔪? of I, where ? denotes the Rees algebra of I. Our key idea is to require ‘good’ intersection properties as well as ‘few’ homogeneous generating relations in low degrees. In particular, if Iis a strongly Cohen-Macaulay R-ideal with G _?and the expected reduction number, we conclude that ? is always Cohen-Macaulay. We also obtain a characterization of the Cohen-Macaulayness of ?/K? for any 𝔪-primary ideal K. This result recovers a well-known criterion of Valabrega and Valla whenever K = I. Furthermore, we study the relationship between the Cohen-Macaulay property of the special fiber ring ? and the Cohen-Macaulay property of the Rees algebra ? and the associated graded ring 𝒢 of I. Finally, we focus on the integral closedness of 𝔪I. The latter question is motivated by the theory of evolutions.     

Resolutions of Stanley-Reisner rings and Alexander duality

Associated to any simplicial complex Δ on n vertices is a square-free monomial ideal I_Δ in the polynomial ring A = k[x₁, …, x_n], and its quotient k[Δ] = A/I_Δ known as the Stanley-Reisner ring. This note considers a simplicial complex Δ^* which is in a sense a canonical Alexander dual to Δ, previously considered in [1, 5]. Using Alexander duality and a result of Hochster computing the Betti numbers dim_k Tor_i^A (k[Δ],k), it is shown (Proposition 1) that these Betti numbers are computable from the homology of links of faces in Δ^*. As corollaries, we prove that I_Δ has a linear resolution as A-module if and only if Δ^* is Cohen-Macaulay over k, and show how to compute the Betti numbers dim_k Tor_i^A (k[Δ],k) in some cases where Δ^* is wellbehaved (shellable, Cohen-Macaulay, or Buchsbaum). Some other applications of the notion of shellability are also discussed.