# Syracuse Algebra Seminar

Fridays 3:30-4:25 PM, Carnegie 115

Organizers: Steven Diaz and Josh Pollitz

Schedule of Talks Spring 2024:

Upcoming talks:

April 12th: Ritvik Ramkumar (Cornell University), Cartwright-Sturmfels ideals and their moduli

Abstract: Cartwright-Sturmfels ideals, CS-ideals for short, are multigraded ideals whose generic initial ideals are radical. First studied by Cartwright and Sturmfels, some examples include the ideals of maximal minors of a matrix of linear forms, binomial edge ideals, closure of linear spaces, and multiview ideals. In this talk, I will discuss the geometry of CS-ideals inside the multigraded Hilbert scheme, with a particular focus on bigraded CS-ideals. This is joint work with Alessio Sammartano.

April 26th: Hamid Rahmati (Syracuse University)

Past talks:

February 2nd: Steven Diaz (Syracuse University), Eigenvalues and Eigenvectors of Tensors

Abstract: This is joint work with Adam Lutoborski. We present a possible extension of the concepts of eigenvalues and eigenvectors from matrices to tensors of higher order. Our goal is to find an extension that preserves as much as possible the concept of diagonalization. We present what can be considered to be an extension of the theorem that a real symmetric matrix can be orthogonally diagonalized.

March 15th: Luigi Ferraro (University of Texas Rio Grande Valley), Trimming five generated Gorenstein ideals

Abstract: Let R be a regular local ring of dimension 3 with maximal ideal m. Let I be a Gorenstein ideal of R of grade 3. Buchsbaum and Eisenbud proved that there is a skew-symmetric matrix of odd size such that I is generated by the sub-maximal pfaffians of this matrix. Let I be the ideal obtained by multiplying some of the pfaffian generators of I by m; we say that J is a trimming of I. In a previous work, A. Hardesty and I constructed an explicit free resolution of R/J and computed a DG algebra structure on this resolution. We used the products on this resolution to study the Tor algebra of such trimmed ideals. Missing from our result was the case where I is five generated. In this talk, which is based on a joint work with F. Moore, we will address this case after covering the necessary background information.

March 29th: Janina Letz (Bielefeld University/UCLA), Generation time for biexact functors and Koszul objects in triangulated categories

Abstract: The derived category of modules over a commutative ring captures many properties of the ring. One approach is to study its triangulated structure through finite building. An object X finitely builds an object Y, if Y can be obtained from X by taking cones, suspensions and retracts. The X-level measures the number of cones required in this process. I will explain the behavior of level with respect to tensor products and other biexact functors. This results makes use of the fact that the derived category is an enhanced triangulated category. I will further present applications to Koszul objects, which generalize Koszul complexes. This is joint work with Marc Stephan.

April 5th: Ben Briggs (University of Copenhagen/SLMath), Koszul duality tricks for (seemingly) non-Koszul rings

Abstract: This is a talk about a relative version of Koszul duality in commutative algebra, and how it can tell us about the asymptotic homological algebra of (seemingly) non-Koszul local rings. It’s all joint work with James Cameron Janina Letz and Josh Pollitz.

A homomorphism f of commutative local rings, say S to R, has a derived fibre F (a differential graded algebra over the residue field k of R) and we say that f is Koszul if F is formal and its homology H(F) (the Tor algebra of R and k over S) is a Koszul algebra in the classical sense. I’ll explain why this is a very good definition and how it is satisfied by many many examples. The main application is the construction of explicit free resolutions over R in the presence of a Koszul homomorphism. This construction simultaneously generalises the resolutions of Priddy over a Koszul algebra, the resolutions of Shamash and Eisenbud over a complete intersection ring, and the bar resolutions of Iyengar and Burke over a Golod ring.

If you think you’ve seen similar talks before - don’t be fooled. I’ll try to give a different perspective this time.

Schedule of Talks Fall 2023:

September 8th: Josh Pollitz (Syracuse University), Frobenius pushforwards and generators for the derived category

Abstract: By now it is quite classical that one can understand singularities in prime characteristic commutative algebra and algebraic geometry through properties of the Frobenius endomorphism. The foundational result illustrating this is a celebrated theorem of Kunz characterizing the regularity of a noetherian ring (in prime characteristic) in terms of whether a Frobenius push forward is flat. In this talk, I'll discuss a structural explanation of the theorem of Kunz, that also recovers it, and other theorems of this ilk. Namely, I’ll discuss recent joint work with Ballard, Iyengar, Lank, and Mukhopadhyay where we show that over an F-finite noetherian ring of prime characteristic high enough Frobenius push forwards generate the bounded derived category.

September 15th: Michael DeBellevue (Syracuse University), k-Summands of Syzygies of Burch Rings via the Bar Resolution

Abstract: In recent work, Dao and Eisenbud have defined the notion of a Burch ring. They have shown that for any module over a Burch ring of depth zero, the n'th syzygy contains direct summands of the residue field for all n>6. In joint work with Claudia Miller, we investigated how this behavior is explained by the bar resolution. I will define Burch rings and the bar resolution, and show how it can be used to describe some cycles which generate k-summands of syzygies of an R-module M. When M is a Golod module, we show that the number of these elements grows exponentially as the homological degree increases.

September 29th: Richard Bartels (Syracuse University), Generalized Loewy length of Cohen-Macaulay local and graded rings

Abstract: In his 1994 paper, Ding proved that for a Gorenstein local ring (R,m,k) with Cohen-Macaulay associated graded ring and infinite residue field, the generalized Loewy length gll(R) and index of R are equal. However, if k is finite, equality may not hold. In this talk, I will prove a relation between gll(R) and index(R) that generalizes Ding’s theorem for one-dimensional Cohen-Macaulay local rings with finite index. In particular, this generalization applies when the residue field is finite or infinite. I will then show that for certain one-dimensional hypersurfaces, this relation gives a formula for the generalized Loewy length. Finally, I will introduce a graded version of the generalized Loewy length, and determine its value for numerical semigroup rings.

October 6th: No seminar-Fall Break Weekend

October 13th: James Cameron (University of Utah), Koszul ring maps and transferring resolutions in local algebra

Abstract: I will discuss what it means for a map of commutative local rings to be Koszul, and how a Koszul ring map allows homological information to be transferred from the source to the target. This gives, in many cases, constructions of “universal resolutions” of modules over a non-regular local ring in terms of a finite amount of data. I will discuss these universal resolutions, and how the construction of these resolutions generalizes other transfer-of-resolutions results, such as the Eisenbud-Shamash construction. A key ingredient in these results are A-infinity algebra techniques, and I will give a brief introduction to A-infinity algebras and their role as a helpful generalization of dg-algebras. All this work is joint with Benjamin Briggs, Janina Letz, and Josh Pollitz.

October 27th: Gordana Todorov (Northeastern University), Higher Auslander Algebras

Abstract: A celebrated theorem of Maurice Auslander about artin algebras describes the correspondence between {algebras A of representation finite} and {algebras B, with gl.dim.B ≤ 2 ≤ dom.dim.B} which are now called Auslander algebras. Higher Auslander algebras were introduced by O. Iyama as algebras C, with {gl.dim.C ≤ k ≤ dom.dim.C} and it was also shown that there is correspondence between {higher representation finite algebras} and {higher Auslander algebras} (up to certain equivalences). A large part of my talk is on recent work of Emre Sen. In addition, Emre Sen, Shijie Zhu and I obtained another method of constructing higher Auslander algebras in the family of Nakayama algebras. Using this method we get a complete characterization of higher Auslander Nakayama algebras.

November 3rd: Dave Jorgensen (University of Texas-Arlington), Variations on a classical dimension inequality

Abstract: For a pair of finitely generated modules M and N over a codimension c complete intersection ring R with M⊗N having finite length over R, we pay special attention to the inequality dim(M)+dim(N) ≤ dim(R) +c. In particular, we develop an extension of Hochster's theta invariant whose nonvanishing detects equality. In addition, we consider a parallel theory where dimension and codimension are replaced by depth and complexity, respectively. This is joint work with Petter Bergh and Peder Thompson.

November 10th: Des Martin (Syracuse University), On the splitting of the 2nd exterior power of the conormal module from Tor

Abstract: Let R be a local Noetherian ring and I an ideal of R. In “The Syzygies of the Conormal Module,” Simis and Vasconcelos give an embedding map, and a somewhat cryptic remark that this map causes a splitting of the 2ndexterior power of the conormal module from Tor_1(I,R/I). In this talk we utilize differential graded algebra resolutions to define an explicit splitting map and show that if I/I^2 is free (or has enough free summands), then the 2nd exterior power of the conormal module (or its free summands) will split from Tor_1(I,R/I).

November 17th: Torkil Stai (NTNU), Perhaps Nakayama Algebra

Abstract: After semisimple ones, Nakayama algebras are the best understood algebras. But while their representation theory is under control, basic questions regarding their homological nature remain unanswered. We will look at both these features, insisting on accessibility for non-experts. Time permitting, we will also mention recent work with Fosse and Oppermann.

November 24th: No seminar-Thanksgiving Break

December 1st: Alexandra Seceleanu (University of Nebraska-Lincoln), Principal symmetric ideals

Abstract: A ubiquitous theme in mathematics is that general members in a family of mathematical objects have nice properties. We focus on the family of principal symmetric ideals, that is, ideals generated by the orbit of a homogeneous polynomial under the action of the symmetric group. In joint work with Megumi Harada and Liana Sega we determine the minimal free resolutions for a general member of this family, as well as other notable properties it satisfies.

December 8th: David Lieberman (University of Nebraska-Lincoln), Making Sandwiches of Functional Equations: Bernstein's inequality, Bernstein-Sato polynomials, and Sandwich Bernstein Sato Polynomials

Abstract: For a polynomial ring over a field K, the ring of differential operators is known as the Weyl algebra: the non-commutative K-algebra whose generators are the "multiply by a variable" maps and the partial derivatives with respect to each variable. In this setting a powerful result is Bernstein's Inequality, which puts bounds on the dimension of modules over the Weyl Algebra. Another closely related object of study is the Bernstein-Sato polynomial for an element of the polynomial ring. In this talk, I will share some recent developments in proving Bernstein's Inequality for the ring of differential operators over some singular rings, as well as a "Sandwich" version of Bernstein-Sato polynomials that gives a new method for showing the inequality in novel settings. This work is a collaboration with my advisor Jack Jeffries.