Wednesdays at 3:30 pm in Avery 351

Commutative Algebra Seminar

University of Nebraska ‐ Lincoln



The seminar will meet from 3:30 to 5 pm. Talks should aim to be 60 minutes long with ample time after for questions or informal discussion.


2022/2023 academic year

  • Wednesday, January 25, 2023, 3:30 pm

    Alexandra Seceleanu (UNL)

    Principal symmetric ideals

    Abstract: Consider a homogeneous polynomial \(f\) in variables \(x_1, \ldots, x_n\). The set of polynomials obtained from \(f\) by permuting the variables in all possible ways generates an ideal, which we call a principal symmetric ideal. What can we say about the Betti numbers of a principal symmetric ideal? I will give a general answer in this talk.


  • Wednesday, February 1, 2023, 3:30 pm

    Mark Walker (UNL) TBA

    Abstract: TBA



  • Wednesday, February 15, 2022, 3:30 pm

    Roger Wiegand (UNL)

    Decompositions of pure projective modules over local rings

    Abstract: An \(R\)-module is termed pure projective if it is projective relative to pure-exact sequences. There is a useful structure theorem: A module is pure projective if and only if it is a direct summand of a direct sum of finitely presented modules. Assume now that \(R\) is a Noetherian local ring. Is every pure-projective module actually a direct sum of finitely generated modules? The answer is "no", even when the module is a direct summand of \(M^{\omega}\) for some finitely generated module \(M\). We discuss this and the related question of which modules over the completion of \(R\) are extended from \(R\)-modules. This is joint work with Dolors Herbera (Barcelona) and Pavel Prihoda (Prague).


  • Wednesday, February 22, 2022, 3:30 pm

    Keller VandeBogert (Notre Dame)

    TBA

    Abstract: TBA


  • Past talks from the 2022/2023 academic year

  • Thursday, December 8, 2022, 12:30 pm

    Nawaj KC (UNL)

    The length conjecture

    Abstract: Suppose \( (R, \mathfrak{m}) \) is a noetherian local ring. The length conjecture of Iyengar-Ma-Walker contends that if \(I \subseteq R\) is an \(\mathfrak{m}\)-primary ideal of finite projective dimension, \(\ell_R(R/I) \geqslant e(R)\), where \(e(R) := e(\mathfrak{m}, R)\) is the Hilbert Samuel multiplicity of the maximal ideal. In this talk, we will introduce this innocuous open problem. We will also present our solution to this problem, joint with Andrew Soto Levins, in the case \(\textrm{dim} (R) =2\). Our new idea is to use a result of Skalit in local intersection theory and a notion of lifting modules.


  • Wednesday, November 16, 2022, 3:30 pm

    Jake Kettinger (UNL)

    New Perspectives on Geproci-ness

    Abstract: The geproci property is a recent development in the world of geometry. We define the notion of the geproci set and discuss new developments in the characterization of the geproci property. These developments come from analyzing configurations of points in new settings, such as in positive characteristic and in the non-reduced setting. These developments come with exciting new connections to combinatorics and group theory. Finally, we will discuss natural next questions these developments can lead to, especially with regard to investigating the combinatorial connection.


  • Wednesday, November 9, 2022, 3:30 pm

    Kriti Goel (University of Utah)

    Hilbert-Kunz multiplicity of powers of an ideal

    Abstract: P. Monsky proved the existence of Hilbert-Kunz multiplicity in 1983. Since then, it has been extensively studied, partly because of its connections with the theory of tight closure and its unpredictable behaviour. Unlike the Hilbert-Samuel multiplicity, the Hilbert-Kunz multiplicity need not be an integer. In this talk, we consider Hilbert-Kunz multiplicity of powers of an ideal, in an attempt to write it as a function of the power of the ideal. This involves a surprising connection with the Hilbert-Samuel coefficients of Frobenius powers of an ideal.


  • Wednesday, November 2, 2022, 3:30 pm

    Eloísa Grifo (UNL)

    Bounding the dimension of cohomological support varieties

    Abstract: Given a complex of R-modules M, we can associate to M a projective variety containing homological information about M; this is the cohomological support variety of M. When R is a complete intersection, any projective variety can be realized as the cohomological support of some M. However, when R is Cohen-Macaulay but not a complete intersection, this realizability question has a negative answer; to show this, we give bounds on the dimension of any cohomological support variety over R. We will also discuss a connection between cohomological support varieties and the homotopy Lie algebra of R. This is joint work with Ben Briggs and Josh Pollitz.


  • Wednesday, October 26, 2022, 3:30 pm

    Tom Marley (UNL)

    Using contracting endomorphisms to detect ring singularities

    Abstract: Let \(R\) be a local ring with maximal ideal \(m\). A endomorphism \(\phi:R\to R\) is called contracting if \(\phi^i(m)\subseteq m^2\) for some \(i>0\). An important example of a contracting endomorphism is the Frobenius endomorphism in the case \(R\) is a ring of prime characteristic. We discuss ways in which actions of such an endomorphism detect when the ring is regular, a complete intersection, or Gorenstein. We review classical results of Kunz and Rodicio, as well as more recent results by Avramov, Hochster, Iyengar, Yao, myself and Brittney Falahola (a recent PhD student), and others. We focus in particular on a recent result of Pinches Dirnfeld which characterizes Gorenstein rings by the action of Frobenius on a dualizing complex for \(R\), answering a question by Falahola and myself.


  • Wednesday, October 19, 2022, 3:30 pm

    Levi Heath (UNL)

    Enumerative geometry in physics and quantum Serre duality for quasimaps

    Abstract: Let X be a smooth variety and let Z be a complete intersection in X defined by a section of a vector bundle E over X. Gromov-Witten and quasimap invariants of X are integrals over Kontsevich's moduli space of stable maps to X and Ciocan-Fontanine and Kim's moduli space of stable quasimaps respectively. Originally proposed by Givental, quantum Serre duality refers to a precise relationship between the Gromov-Witten invariants of Z and those of the dual vector bundle \(E^\vee\). In this talk, we motivate the study of Gromov-Witten and quasimap invariants, which have applications in theoretical physics, and present a quantum Serre duality statement for quasimap invariants. We describe how working with quasimaps allows us to obtain a comparison that is simpler, and that also holds in greater generality than previous quantum Serre duality results in Gromov-Witten theory. This is joint work with Mark Shoemaker.


  • Wednesday, October 12, 2022, 3:30 pm

    Paolo Mantero (University of Arkansas)

    Formulas for symbolic powers of ideals.

    Abstract: In this talk we provide a couple of formulas to compute symbolic powers of unmixed, generically complete intersection ideals in Cohen-Macaulay rings. For instance, we give a multiplicity-based characterization for the \(m\)th symbolic power \( I^{(m)} \) of an ideal I, a formula to obtain \(I^{(m)}\) as the saturation of \(I^{m}\) with respect to an explicit ideal only depending on \(I\) (and not \(m\)), and effective bounds for the exponent achieving the saturation. We plan to discuss the connection with a conjecture by Eisenbud and Mazur on \(\textrm{ann}(I^{(m)}/I^m)\), which we prove in a generalized form, and a couple of applications of the formulas.


  • Friday, October 7, 2022, 2:30 pm (note the unusual date and time)

    Craig Huneke (University of Virginia)

    Title: A monomial licci problem

    Abstract: This talk is based on ongoing work with Claudia Polini and Bernd Ulrich. An ideal is licci if it is in the linkage class of a complete intersection. We began with a basic question: if I is a licci and square-free monomial ideal in a polynomial ring in n-variables, must the number of generators of I be at most n? This simple question has strong consequences if it has a positive answer. Several positive answers will be given, some of which unexpectedly use Boij-Soderberg theory.


  • Wednesday, September 28, 2022, 3:30 pm

    Joshua Rice (Iowa State University)

    Numerics of Koszul Algebras and Generic Collections of Lines in Projective Space

    Abstract: We briefly introduce Koszul algebras and discuss some of their interesting homological and numerical properties. We then prove that if M is a generic collection of lines in projective space, then, depending on the size of M, we can guarantee that the coordinate ring is Koszul. This theorem partially generalizes a result of Conca, Trung, and Valla. Furthermore, if the collection M is too large, the coordinate ring is not Koszul; this is achieved using the numerical properties of Koszul algebras.


  • Wednesday, September 21, 2022, 3:30 pm

    Javier González Anaya (University of California, Riverside)

    Finite generation of symbolic Rees algebras from a geometric perspective

    Abstract: The problem of finite generation of symbolic Rees algebras of space monomial curves has received renewed interest in the past few years because of the essential role it plays in Castravet and Tevelev's proof that the compactified moduli space of genus zero curves with N marked points is not a Mori Dream Space as soon as N is larger than 133. Subsequent work of Karu-Gonzalez and Hausen-Keicher-Laface improved the result to N larger than 9 using toric geometry. In this talk I'll explain how this geometric perspective offers a powerful tool to study the problem of finite generation, how it recovers and expands classical results in the area, and present the main results by Gonzalez, Karu and myself.


  • Wednesday, September 7, 2022, 3:30 pm

    Mark Walker (UNL)

    BGG from a topological point of view

    Abstract: The Berstein-Gel'fand-Gel'fand (BGG) correspondence refers to an equivalence of categories that relates modules over a polynomial ring and modules over an exterior algebra in the same number of variables. In this (mostly expository) talk, I will channel my inner topologist and explain the BGG correspondence from the point of view of topological spaces equipped with continuous torus actions. I'll also discuss the To*** Rank Conjectures, were *** = ral or *** = tot.


  • Wednesday, August 31, 2022, 4 pm (note the unusual time)

    Brian Harbourne (UNL)

    Algebraic Geometric Concepts (weakly) Motivated by Inverse Scattering

    Abstract: Studying inverse scattering problems has led to remarkable advances in science and technology, from computed tomography to the determination of the double helix structure of DNA. Here I will discuss current research which applies this idea to classification problems in algebraic geometry. In inverse scattering problems (ISP), one tries to discern an object's structure from structure in projected or reflected data. We carry this idea over to algebraic geometry and commutative algebra by asking to classify objects based on the structure of projected images. As a main focus, I will discuss work to classify sets of points in projective space whose projection from a general point is a complete intersection in a hyperplane.


  • 2021/2022 academic year

  • Thursday, May 5, 2022, 11 am

    Srikanth Iyengar (University of Utah)

    Congruence modules and Wiles defect for local rings over discrete valuation rings

    Abstract: Andrew Wiles, as part of his proof of Fermat’s theorem, discovered a criterion for a map between local rings over a fixed discrete valuation ring, and of relative dimension zero, to be an isomorphism of complete intersections. This result has been subsequently generalized by Lenstra, Diamond, Fakhruddin, Khare, and others, for it has applications in number theory, around the problem of modularity lifting. I will present some results from an ongoing joint collaboration with Khare and Manning that extends these results to higher relative dimension. The focus of the talk will be on the commutative algebra aspects but I will try to indicate why these results are of interest to researchers in number theory.




  • Past talks

  • Wednesday, April 20, 2021, 3:30 pm

    Daniel Duarte (Universidad Autónoma de Zacatecas, Mexico)

    Nash blowups in positive characteristic

    Abstract: Given a normal variety over a field of positive characteristic, we show that its Nash blowup is an isomorphism if and only if the variety is non-singular. This result is obtained by combining general properties of a suitable Grassmanian together with recent developments on derivations and differential operators. Joint work with Luis Núñez Betancourt.



  • Wednesday, April 13, 2021, 3:30 pm (canceled)

    Jack Jeffries (UNL)

    Are determinantal rings direct summands of polynomial rings?

    Abstract: Over any infinite field, the generic determinantal rings are known to be fixed subrings of the action of the general linear group on a polynomial ring. Since the general linear group is linearly reductive in characteristic zero, these generic determinantal rings are direct summands of polynomial rings, which explains many of their good properties in this case. In positive characteristic, these determinantal rings have many of the same good properties even though the general linear group is no longer linearly reductive. In this talk we investigate if these determinantal rings continue to be direct summands of polynomial rings in characteristic p>0. We will also encounter some interesting varieties related to linear algebra along the way.

    This is joint work with Mel Hochster, Vaibhav Pandey, and Anurag Singh.



  • Wednesday, April 6, 2021, 3:30 pm

    Ben Briggs (MSRI)

    The higher cotangent modules

    Abstract: The cotangent complex is an important but difficult to understand object associated to a ring homomorphism. It connects closely with some more familiar commutative algebra invariants: you can see the module of differentials, the conormal module, and the first Koszul homology as the first few syzygies inside the cotangent complex. In general, these syzygies are known as the cotangent modules.

    Quillen conjectured (for maps of finite flat dimension) that the cotangent complex can only be bounded for complete intersection homomorphisms. This was proven by Avramov in 1999. I will explain how to get a new proof (of a stronger result) by paying attention to the cotangent modules. This is all joint work with Srikanth Iyengar.



  • Wednesday, March 23, 2021, 3:30 pm

    Sylvia Wiegand (UNL)

    Ideals in a local ring under small perturbations

    Abstract: Consider the following properties for a commutative Noetherian local ring \( (R,\mathfrak m,k) \):
  • (AR) Every finitely generated \(R\)-module \(M\) such that, for every \(i>0\), \(\textrm{Ext}^i_R(M,M \oplus R)=0 \) is a free module.
  • (HW) Every torsion-free finitely generated \(R\)-module \(M\) with rank such that \(M \otimes_R M^*\) is MCM is a free module.
  • Two conjectures related to these properties are: the Auslander-Reiten Conjecture (ARC), that every local ring \( R \) satisfies (AR); and the Huneke-R Wiegand Conjecture (HWC), that every Gorenstein local ring satisfies (HW). Our focus is on quasi-fiber product rings — rings for which there exists a regular sequence \(\underline x\) in \(\mathfrak m\) such that \(\mathfrak m/(\underline x) \) decomposes into a nontrivial direct sum of ideals of \( R/(\underline x) \). We show that quasi-fiber product rings satisfy a sharpened form of (ARC) and we make some observations related to (HWC).


  • Wednesday, March 9, 2021, 3:30 pm

    Luís Duarte (University of Genoa)

    Ideals in a local ring under small perturbations

    Abstract: Let I be an ideal of a Noetherian local ring R. We study how properties of I change for small perturbations, that is, for ideals J that are the same as I modulo a large power of the maximal ideal. In particular, assuming that J has the same Hilbert function as I, we show that the Betti numbers of J coincide with those of I. We also compare the local cohomology modules of R/J with those of R/I.



  • Wednesday, March 2, 2021, 3:30 pm

    Shah Roshan Zamir (UNL)

    Nagata's theorem on the ring of invariants

    Abstract: In 1900 David Hilbert published a list of 23 open problems. The 14th problem concerns the finite generation of the ring of invariants of a group acting on a finitely generated K-algebra. Nagata's theorem gives a positive answer for geometrically reductive groups, a large class of groups, acting on finitely generated K-algebras. In this talk, we will prove the graded case of Nagata's theorem. It is notable that in 1959 Nagata also provided a negative answer to Hilbert's 14th problem for arbitrary groups.



  • Wednesday, February 23, 2021, 3:30 pm

    David Lieberman (UNL)

    The Dimension of D-modules and Bernstein's Inequality

    Abstract: Given a module M over an A-algebra R, one can construct the ring of differential operators on M over R. This ring is a subset of the A-linear maps from M to M, and is a rich source of study for commutative algebra. In particular much work has been done in studying modules over the ring of differential operators, which we call D-modules. In this talk, we will investigate the notion of dimension for D-modules and prove a hallmark property on the lower bound of dimension. This somewhat surprising result is known as Bernstein's inequality, which we will prove for D-modules over the ring of differential operators on a polynomial ring in characteristic zero.



  • Wednesday, February 9, 2021, 3:30 pm

    Uli Walther (Purdue University)

    Lyubeznik and Cech-de Rham numbers

    Abstract: If Y is an affine variety inside \(\mathbb{C}^n\) cut out by the ideal \(I\) inside, and \(m\) a distinguished maximal ideal of, \(\mathbb{C}[x_1,...,x_n]\), one can attach two sets of numbers to them, either by applying the de Rham functor the D-module \(H^t_I(R)\), or the D-module restriction functor for the inclusion \(\textrm{Spec}(R/m) \hookrightarrow \textrm{Spec} (R)\). It turns out that these numbers are in fact functions of Y and not of the embedding into an affine space. In the talk we discuss known facts as well as some recent insights on these double arrays of numbers. This will include some general vanishing results, as well as a discussion on when the associated spectral sequence for which these arrays are the \(E_2\)-page, collapses.



  • Wednesday, February 16, 2021, 3:30 pm

    Andrew Soto-Levins (UNL)

    A Rigidity Theorem for Ext

    Abstract: The goal of this talk is to present the following theorem: if R is an unramified hypersurface, if M and N are finitely generated R modules, and if \( \operatorname{Ext}_{R}^{n}(M, N) = 0 \) for some \( n \leqslant \operatorname{grade} M \), then \( \operatorname{Ext}_{R}^{i}(M, N) = 0 \) for \( i \leqslant n\). A corollary of this says that \( \operatorname{Ext}_{R}^{i}(M, M) \neq 0\) for \(i \leqslant \operatorname{grade} M \). This gives a partial answer to a question of Jorgensen: if \( (R, m, k) \) is a complete intersection and if M is a nonzero finitely generated module of finite projective dimension, then must \( \operatorname{Ext}_{R}^{n}(M, M) \) be nonzero for \( 0 \leqslant n \leqslant \operatorname{pd}_{R}(M) \)?



  • Wednesday, February 2, 2021, 3:30 pm

    Claudia Miller (Syracuse University)

    Torsion in exterior powers of differentials over complete intersection rings

    Abstract: In this talk, after a short review of the definition of and facts about Kaehler differentials, I will give some history behind the classic Lipman-Zariski Conjecture and the generalized Lipman-Zariski questions of Graf. Then I’ll give some results on the torsion and cotorsion of exterior powers of the module of Kaehler differentials over complete intersection rings, and say how these are used to prove a generalized Lipman-Zariski result under certain conditions. This is joint work with Sophia Vassiliadou.



  • Wednesday, January 26, 2021, 3:30 pm

    Alexandra Seceleanu (UNL)

    Axial constants of homogeneous ideals

    Abstract: In commutative algebra, generic initial ideals are monomial ideals used for estimating important homological features of arbitrary ideals of a polynomial ring. In this talk we introduce new invariants termed axial constants, which can be read off the generic initial ideal, and we explain how the axial constants relate to better known algebraic and homological invariants. This is joint work with Michael DeBellevue, Shah Roshan Zamir, and the members of the Polymath Jr group on computational algebra.



  • Wednesday, December 2nd, 2021, 3:30 pm, on zoom

    Brian Harbourne (UNL)

    The concept of geproci subsets of P3: a timeline

    Abstract: Interest in geproci sets grew out of work on unexpected hypersurfaces. We define the notion of a geproci set, we give examples and we discuss some recent results, all in the context of a timeline of relevant events.


  • Wednesday, November 17, 2021, 3:30 pm

    Eloísa Grifo (UNL)

    Macaulay2

    Abstract: We will talk about Macaulay2: what is it, what one can do with it, and how one can learn how to use it.


  • Wednesday, November 3rd, 2021, 3:30 pm

    Jack Jeffries (UNL)

    A Jacobian Criterion in Mixed Characterstic

    Abstract: The classical Jacobian criterion is an important tool for finding singular points on a variety over a (perfect) field. How can we find the singular locus over the p-adics or over the integers? In this talk, I'll discuss a new analogue of the Jacobian criterion that gives a simple description of the singular locus in this setting. This criterion uses a curious notion of differentiation by a prime number called p-derivations. If time permits, we will also discuss an extension of the theory of Kahler differentials to this mixed characteristic setting. This is based on joint work with Melvin Hochster.


  • Wednesday, October 20th, 2021, 3:30 pm

    Michael K. Brown (Auburn University)

    Minimal free resolutions of differential modules

    Abstract: A differential module is a pair \( (M, d) \), where \( M \) is a module, and d is an endomorphism of \( M \) that squares to \(0\). This is a generalization of a familiar notion: when \(M\) is graded and d has degree \(-1\), \( (M, d) \) is a chain complex. In this talk, I'll discuss a theory of minimal free resolutions of differential modules over local rings. This is joint work with Daniel Erman.


  • Wednesday, October 13th, 2021, 3:30 pm

    Josh Pollitz (University of Utah)

    Cohomological support in local algebra

    Abstract: Cohomological supports have been integral in revealing structural information in commutative algebra. They were imported from modular representation theory to local algebra by Avramov, and were put on centerstage by Avramov and Buchweitz in 2000 during their investigations of cohomology modules over complete intersection rings. In the past twenty years this theory has been further developed, extended and applied by Avramov-Iyengar, Burke-Walker, Jorgensen, P-, and many others. In this talk, classical and recent applications of cohomological support will be surveyed and I will discuss two new support theories that have been developed in two separate collaborations: one joint with Briggs and Grifo, and the other joint with Briggs and McCormick.


  • Wednesday, October 6, 2021, 3:30 pm

    Roger Wiegand (UNL)

    Vanishing of Tor over quasi-fiber product rings

    Abstract: A fiber product ring is a local ring \( (R,\mathfrak{m},k) \) whose maximal ideal \(\mathfrak{m} \) decomposes as a direct sum of two non-zero ideals. (Local rings are always assumed to be commutative and Noetherian.) Equivalently, \(R\) is isomorphic to a fiber product \(S\times_k T\), where \( S \) and \( R \) are local rings, both different from \(k\). More generally, a quasi-fiber product ring is a local ring \(R\) such that \(R/(x_1, \ldots, x_r)\) is a fiber product ring for some regular sequence \( (x_1, ..., x_r) \) and some \(r \geqslant 0\). We study vanishing of \(\textrm{Tor}\) and \(\textrm{Ext}\) over these rings, with particular focus on the Auslander-Reiten and Huneke-Wiegand Conjectures. This is joint work with Thiago Freitas, Victor Jorge Pérez, and Sylvia Wiegand.


  • Wednesday, September 29, 2021, 3:30 pm

    Michael DeBellevue (UNL)

    Graded Deviations and the Koszul Property

    Abstract: The graded deviations \(\varepsilon_{ij}(R)\) of a graded ring \(R\) record the vector space dimensions of the graded pieces of a certain Lie algebra attached to the minimal resolution of the quotient of \(R\) by its homogeneous maximal ideal. Vanishing of deviations encodes properties of the ring: for example, \(\varepsilon_{ij}(R)= 0\) for \(i \geqslant 3\) if and only if \(R\) is complete intersection and, provided \(R\) is standard graded, \(\varepsilon_{ij}(R)\) whenever \(I\) is not equal to \(j\) implies R is Koszul. We extend this fact by showing that if \( \varepsilon_{ij}(R)=0 \) whenever \(j\) and \(i \geqslant 3\), then \( R \) is a quotient of a Koszul algebra by a regular sequence. This answers a conjecture by Ferraro.


  • Wednesday, September 22, 2021, 3:30 pm

    Vaibhav Pandey (University of Utah)

    Are determinantal rings direct summands of polynomial rings?

    Abstract: Over an infinite field, the generic determinantal rings are known to be fixed subrings of the action of the general linear group on a polynomial ring. Since the general linear group is linearly reductive in characteristic zero, it follows from a theorem of Hochster and Roberts that these generic determinantal rings are direct summands of polynomial rings (in characteristic zero). In this talk we investigate if these determinantal rings continue to be direct summands of the polynomial rings in which they naturally embed into by the above group action in characteristic p>0. Note that the general linear group is not linearly reductive in characteristic p>0! This is joint work with Mel Hochster, Jack Jeffries, and Anurag Singh.


  • Wednesday, September 15, 2021, 3:30 pm

    Tom Marley (UNL)

    Gorenstein projective dimensions and levels of complexes

    Abstract: We review (or introduce for some/many) the concept of the Gorenstein projective dimension of a module, and how it can be used to characterize Gorenstein local rings. We then show how this dimension can be generalized for complexes. We then define a related concept, called the Gorenstein level or G-level of a complex, and show how that is related to the Gorenstein projective dimension of a complex.


  • Wednesday, September 8, 2021, 3:30 pm

    Eloísa Grifo (UNL)

    A survey of Harbourne's Conjecture

    Abstract: Harbourne's conjecture on the containment problem for symbolic and ordinary powers of ideals is not true in its original form, but it has sparked a lot of different research avenues. We will discuss some of the known counterexamples but mostly focus on the different variations of the conjecture that are true or still open.


  • Wednesday, September 1, 2021, 3:30 pm

    Mark Walker (UNL)

    On the cone of Betti tables for a singular ring

    Abstract: This is joint work with Srikanth Iyengar and Linquan Ma. Let k be a field and R a standard graded k-algebra. When R is a polynomial ring, "Boij-Soderberg Theory" (developed by Boij, Eisenbud, Erman, Schreyer, Soderberg, and others) gives a description of the rational cone spanned by the Betti tables of finitely generated graded R-modules. We give extensions of these results to other graded rings. For instance, we prove the following: When the characteristic of k is prime and R is any Cohen-Macaulay standard graded k-algebra, then the cone of Betti tables of graded R-modules of finite length and finite projective dimension coincides with that for a polynomial ring of the same dimension. We also have results for complexes of graded modules, which include the case when R is not Cohen-Macaulay. Eisenbud and Erman have previously established results such as these under the assumption that the associated projective scheme Proj(R) admits an Ulrich sheaf. The central technique we use is the notion of a lim Ulrich sequence of graded R-modules. In prime characteristic, such sequences exist by a theorem of Ma.



  • Organizers

  • Eloísa Grifo
  • Mark Walker