# Commutative Algebra Seminar

### Fall 2022, Friday 2:00–3:00 pm, LCB 222

 Date Speaker Title — click for abstract August 31, Wednesday Matt Baker Georgia Tech Non-archimedean and tropical geometry, algebraic groups, moduli spaces of matroids, and the field with one element I will give an introduction to Oliver Lorscheid's theory of ordered blueprints - one of the more successful approaches to 'the field of one element' - and sketch its relationship to Berkovich spaces, tropical geometry, Tits models for algebraic groups, and moduli spaces of matroids. The basic idea for the latter two applications is quite simple: given a scheme over Z defined by equations with coefficients in {0,1,-1}, there is a corresponding 'blue model' whose K-points (where K is the Krasner hyperfield) sometimes correspond to interesting combinatorial structures. For example, taking K-points of a suitable blue model for a split reductive group scheme G over Z gives the Weyl group of G, and taking K-points of a suitable blue model for the Grassmannian G(r,n) gives the set of matroids of rank r on {1,...,n}. Similarly, the Berkovich analytification of a scheme X over a valued field k coincides, as a topological space, with the set of T-points of X, considered as an ordered blue scheme over k. Here T is the tropical hyperfield, and T-points are defined using the observation that a (height 1) valuation on k is nothing other than a homomorphism to T. September 2 September 9 Manuel Blickle Universität Mainz Using the Riemann-Hilbert correspondence We will indicate how, using the Riemann-Hilbert correspondence, one can recover several results on local cohomology modules of polynomial rings. September 16 Seungsu Lee University of Utah On the behavior of F-signature on the Nef cone F-signature plays a crucial role when measuring singularities of varieties in positive characteristics. For example, if R is a local ring, s(R) = 1 implies R is regular, and 0 < s(R) < 1 implies R is strongly F-regular which is a char p analog of klt singularities. For a globally F-regular variety X, the F-signature of an ample invertible sheaf is defined as the F-signature of the section along the invertible sheaf over X. In this talk, we will discuss the F-signature is well-defined and is (locally Lipschitz) continuous on the ample cone, and how the signature extends to the boundary of the cone. This is joint work with Swaraj Pande. September 23 Keiichi Watanabe Nihon University and Meiji University Elliptic ideals in 2 dimensional normal local rings This is a joint work in progress with Tomohiro Okuma and Ken-ichi Yoshida. Let (A, m) be a 2-dimensional excellent normal local ring. Let I be an integrally closed m-primary ideal, and let f : X -> Spec(A) be a resolution of singularity such that I O_X = O_X(-Z) is invertible. Also let Q be a minimal reduction of I. We define \bar{r}(I) = min {r | \bar{I^{n+1}} = Q bar{I^n} for all n \ge r}, where \bar{I^n} is the integral closure of I^n. If A is a rational singularity, then H^1(X, O_X(-Z)) = 0 and \bar{I^2} = QI for every integrally closed ideal I, \bar{r}(I) = 1. We call I an elliptic ideal if \bar{r}(I) = 2. This naming comes from the fact that if A is an elliptic singularity, then \bar{r}(I) \le 2 for every integrally closed ideal. Today we discuss about the following topics: 1. Let \bar{G}(I) denote the associated graded ring of the filtration {\bar{I^n}}. For what I, is \bar{G}(I) Gorenstein? If I is an elliptic ideal, then \bar{G}(I) is Cohen-Macaulay but Gorenstein in very limited cases. We discuss about the condition for \bar{G}(I) to be Gorenstein. 2. We give a formula for Core(I) for elliptic ideals. September 30 Shiji Lyu Princeton University Some properties of splinters and birational derived splinters We will discuss the notions of splinters and birational derived splinters. They are related to various interesting notions of singularities, but less are known about them. We will discuss some basic properties of those notions including behavior under limit and etale extensions. Then we will discuss some more advanced properties, one of which involves ultrapower to prove. October 3rd 3:30pmLCB 215 (Special time/date/location) Luis Núñez-Betancourt Centro de Investigación en Matemáticas (CIMAT) Rees algebras of determinantal ideals In this talk we focus on Rees algebras and symbolic Rees algebras of determinantal ideals. In particular, we will show that they have mild singularities in prime characteristic. We will also discuss consequences for numerical invariants and initial ideals of the symbolic powers of these ideals. This is joint work with Alessandro De Stefani and Jonathan Montaño. October 7 David Eisenbud UC Berkeley Summands in high syzygies Work on infinite resolutions beyond the cases of complete intersections and Golod rings has tended to focus on the sequence of Betti numbers. Hai Long Dao and I have recently begun to study a question of a different kind, and I will report on this joint work: Let R = S/I be an Artinian quotient of a regular local ring S, with residue field k. When does it happen that k is a direct summand of a syzygy module in the R-free resolution of k, or indeed in the R-free resolution of every module? We were surprised by what we found experimentally, and were able to prove a little of what we observed. October 14 Fall break October 21 Jennifer Kenkel University of Michigan Lengths of local cohomology using some surprising Hilbert-Kunz functions We investigate the lengths of certain local cohomology modules over polynomial rings. By fixing the degree component, and using the fact that the length of an Artinian ring is the same as that of its injective hull, we transform this into a question about rings of the form $k[x_1,\ldots,x_n]/(x_1^k,\ldots,x_n^k)$, and the annihilator of $x_1 + \cdots + x_n$ therein. We in particular use refinements of functions introduced by Han and Monsky. This was motivated by questions about behavior of the length of local cohomology with support in the maximal ideal of thickenings, that is, $R/I^t$ as $t$ grows. This project is joint work with Mel Hochster. October 28 Sarasij Maitra University of Utah Discussions on Berger's Conjecture In this talk, we will discuss a conjecture of R. Berger dating back to around 1963. He conjectured that in a reduced one dimensional local complete algebra over a perfect field k, the torsion submodule of the module of differentials should capture information about the regularity of the algebra. We will talk about some recent partial developments in this direction. These recent results were obtained in joint work with Craig Huneke and Vivek Mukundan. November 4 Trung Chau University of Utah Barile-Macchia resolutions Resolving a monomial ideal over the polynomial ring is an easy task, but resolving it minimally isn't. In this talk we discuss Barile-Macchia resolutions, which is a new construction of free resolutions for all monomial ideals using discrete Morse theory, and point out important classes of ideals for which these resolutions are minimal. This project is joint work with Selvi Kara. November 11 Homological properties of the relative Frobenius ma University of Utah Homological properties of the relative Frobenius map Given a noetherian ring of positive characteristic, Kunz proved that the Frobenius endomorphism is flat if and only if the ring is regular. Radu and André established an analogue for homomorphisms of noetherian rings of positive characteristic, proving that such a map regular if and only if the relative Frobenius map is flat. In this talk, we present more general results of this ilk, relating the homological properties of the fibers of a ring homomorphism to those of the relative Frobenius map. November 18 December 2 Jack Jeffries University of Nebraska Lincoln TBA TBD

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