Seminars from prior years are listed below; for the current seminar page, visit

November 22, 2013,

Robert Easton (Cal Poly, San Luis Obispo), An alternate Tropical Nullstellensatz

November 15, 2013

Anurag Singh, Local cohomology over the integers

October 25, 2013

Jack Jeffries, Separating invariants, II

September 27, 2013

Jack Jeffries, Separating invariants, I

September 20, 2013

Emily Witt (University of Minnesota), Local cohomology with support in ideals of maximal minors

This is joint work with Claudiu Raicu and Jerzy Weyman. Consider the polynomial ring over a field of characteristic zero whose entries come from a matrix of indeterminates. The ideal generated by the maximal minors of the matrix has provided useful examples in many areas of mathematics; however, the structure of the local cohomology modules of the ring with support in this ideal has remained mysterious until recently. We give an explicit (GL-equivariant) description of these local cohomology modules. Note that the study of local cohomology with support in more general determinantal ideals (i.e., ideals of non-maximal minors) is much better understood from recent work of Lyubeznik, Singh, and Walther, and of Raicu and Weyman.

September 13, 2013

Pedro Teixeira (Knox College), A friendly introduction to p-fractals and Hilbert-Kunz functions, II

September 6, 2013

Pedro Teixeira (Knox College), A friendly introduction to p-fractals and Hilbert-Kunz functions, I

August 30, 2013

Organizational meeting

January 25, 2013

Srikanth Iyengar (University of Nebraska-Lincoln), Torsion in tensor powers of modules over local rings

In 1961 Auslander proved that over any regular local ring R, if the n-fold tensor power of a finitely generated R-module M is torsion-free for some n larger than or equal to the dimension of R, then M is free. In my lectures, I will discuss some recent developments in this line of research, including work of C. Huneke and R. Wiegand, and L. Avramov and myself.

November 3, 2011

Christopher Hacon, Singularities of pluri-theta divisors in positive characteristic

We show that if (X, Theta ) is a PPAV over an F-finite field of characteristic p and D in |m*Theta|, then (X,(1/m)D) is a limit of strongly F-regular pairs and in particular mult_x(D) is at most m*dim X for any x in X.

October 13, 2011

Uli Walther (Purdue University), Freeness and the geometry of the singular locus

Motivated by hyperplane arrangements, I'll talk about freeness of divisors. In particular, I'll discuss a recent theorem that describes conditions that imply freeness, based on the Jacobian ideal. Lots of the talk should be easily understandable.

September 29, 2011

Jack Jeffries, Graded Betti numbers of modules over polynomial rings and the Boij-Söderberg conjectures

We discuss the Boij-Söderberg conjectures, which were proved by Eisenbud-Schreyer in the Cohen-Macaulay case, and subsequently by Boij in the non-CM case. These classify which Betti diagrams arise, up to scalar multiple, as the Betti table of a module over the standard graded polynomial ring S=k[x_1, ..., x_n]. Moreover, the (proven) conjectures guarantee a unique decomposition of each Betti diagram into a positive sum of "pure" diagrams. We will discuss the main applications of this theory, and if time permits, we will also discuss decompositions of the Betti diagrams of complete intersections.

September 22, 2011

Lance Miller, Singularities in charecteristic 0 vs. characteristic p>0

September 15, 2011

Anurag Singh, F-pure thresholds of hypersurfaces

September 8, 2011

Kevin Tucker (Princeton University), Which ideals are test ideals?

June 3, 2011

Kazuhiko Kurano (Meiji University), Nagata conjecture and symbolic Rees rings of space monomial primes

April 29, 2011

Lance Miller, Finiteness properties of local cohomology modules and F-modules

This is a continuation in a series of expository talks by various speakers.

April 22, 2011

Paul Roberts, Finiteness properties of local cohomology modules in mixed characteristic

This is a continuation in a series of expository talks by various speakers.

April 13, 2011

Bhargav Bhatt (University of Michigan), F-thresholds of elliptic curves

F-pure thresholds are certain invariants of singularities in characteristic p that are supposed to mimic the behaviour of log canonical thresholds in characteristic 0. In particular, there exists theorems and deep conjectures relating the two numbers when one specialises from characteristic 0 to positive characteristic, provided the prime p is allowed to suitably large. In this talk, we will introduce these numbers, and discuss some calculations in a fixed characteristic p. Contrary to the conjectural situation where the prime p is allowed to vary, we will show that when p is fixed, the behaviour of these numbers is closely related to the arithmetic of the singularities. The main example discussed will be the cone on an elliptic curve, where the invariant is completely determined with the aid of some geometry (especially, the deformation theory) of elliptic curves; this answers a question raised by M. Mustata. Time permitting, we will discuss some higher dimensional calculations.

April 8, 2011

Anurag Singh, Finiteness properties of local cohomology modules

This is a continuation in a series of expository talks by various speakers.

April 1, 2011

Max Nitsche (Max Planck Institute Leipzig), Castelnuovo-Mumford regularity of simplicial affine semigroup rings

The Eisenbud-Goto conjecture has been open for over 25 years and even for simplicial affine semigroup rings the conjecture is widely open. We will discuss the conjecture and the already known results, before we turn our attention to the simplicial case. I will explain why the simplicial case is special and why I care about monomial ideals. Moreover, I will provide examples where the conjecture is far away to be sharp. Finally, we will discuss the latest results on the conjecture in the seminormal simplicial case. Time permitting, I will talk a little about monomial curves.

March 4, 2011

Anurag Singh, Finiteness properties of local cohomology modules

This is a continuation in a series of expository talks by various speakers.

February 25, 2011

Kazuma Shimomoto (Meiji University), Local Bertini theorem and the Iwasawa main conjecture

I will talk about the Bertini-type theorem for normal local rings in mixed characteristic, which improves Flenner's version in a strong form. As applications, we discuss characteristic ideals and the Iwasawa main conjecture. This is a report on joint work with T. Ochiai.

February 18, 2011

Anurag Singh, Finiteness properties of local cohomology modules

This will be the first in a series of expository talks by various speakers to go through Lyubeznik's Finiteness properties of local cohomology modules (an application of D-modules to Commutative Algebra).

February 11, 2011

Kevin Tucker, F-signature of arbitrary finite quotient singularities

The F-signature of a positive characteristic local ring is a numerical measure of singularities defined by asymptotically counting the number of splittings of the iterates of Frobenius. In this talk, we shall briefly review a proof of the existence of F-signature and subsequently consider how this invariant behaves after a module-finite ring extension. As an application, we give an explicit computation of the F-signature of an arbitrary finite quotient singularity.

February 7, 2011

Daniel Erman (Stanford University), Tensor complexes

The most fundamental complexes of free modules over a commutative ring are the Koszul complex, which is constructed from a vector (i.e., a 1-tensor), and the Eagon--Northcott and the Buchsbaum--Rim complexes, which are constructed from a matrix (i.e., a 2-tensor). I will discuss a multilinear generalization of these complexes, which we construct from an arbitrary higher tensor. Our construction provides detailed new examples of minimal free resolutions, as well as a unifying view on several well-known examples. This is joint work with Berkesch, Kummini, and Sam.

February 4, 2011

Lance Miller, F-purity in non-normal rings

Let R be a reduced ring of prime characteristic. For each positive integer e, set F_*^e R to be the e-th iterate Frobenius pushfoward of R, that is R as a set with module structure twisted by the eth iterate of the Frobenius map. Given an R-linear map from F_*^e R back to R, such a map always extends to a corresponding map on the normalization R^N. In this talk, we show under some tameness assumptions on R, surjectivity of the extended map on the normalization implies surjectivity of the original map. This is joint work with Karl Schwede.

January 21, 2011

Julian Chan, Embedding groups into cohomology modules

December 3, 2010

Julian Chan, Torsion of local cohomology modules

Let R be a commutative ring and I an ideal of R. Huneke asked if the local cohomology modules H_I^k(R) have only finitely many associated prime ideals. Singh constructed a counterexample with R a hypersurface. Singh showed that the local cohomology module H^3_(x,y,z)(R) has p-torsion. Equivalently, it contains an isomorphic copy of the abelian group Z/pZ for every prime integer p. Under certain conditions several authors have shown that the set of associated primes is finite. I show how to embed any finitely generated abelian group into a graded component of the local cohomology module.

November 12, 2010

Florian Enescu (Georgia State University), A finiteness condition on local cohomology modules in positive characteristic

The talk will raise a question on the lattice of Frobenius invariant submodules of the highest local cohomology module of a local ring with support in its maximal ideal and answer it in some special cases.

October 27, 2010

Gennady Lyubeznik (University of Minnesota), An upper bound on the number of F-jumping coefficients of a principal ideal

F-jumping coefficients are characteristic p analogues of jumping coefficients defined via multiplier ideals in characteristic zero. For a polynomial f with an isolated singularity in characteristic zero there is an upper bound on the number of its jumping coefficients in terms of the Jacobian of f. We describe a characteristic p analog of this result. An interesting feature of the proof is the use of differential operators (in characteristic p!) even though the statement of the result has nothing to do with differential operators. This is joint work with Mordechai Katzman and Wenliang Zhang.

October 22, 2010

Jesse Elliott (Cal State Channel Islands), t-closure and integer-valued polynomial rings

The operation of t-closure on the ideals of an integral domain is a useful tool for studying certain classes of integral domains, such as the UFD's, Krull domains, and PVMD's. In this talk I will introduce the t-closure operation and give some applications to the theory of integral domains and in particular to the study of integer-valued polynomial rings.

October 8, 2010

Kevin Tucker, Generalized F-Signature

This talk will build upon that of the previous week, where F-signature of a local ring in positive characteristic was introduced. Using a well-known duality argument, we present a generalization of F-signature to divisor pairs. As before, we show this generalization may be used to test for (strong) F-regularity. Furthermore, these techniques allow us to verify an open question of I. Aberbach and F. Enescu: the splitting-dimension (or s-dimension) of a positive characteristic local ring is the same as the dimension of the splitting prime. The work presented herein is joint with M. Blickle and K. Schwede.

October 1, 2010

Kevin Tucker, F-Signature

In positive characteristic, the existence of splittings of the Frobenius map and its iterates has strong algebraic and geometric consequences. In this talk, we will sketch a proof of the existence of a numerical invariant, first formally defined by C. Huneke and G. Leuschke, which gives an asymptotic measure of the number of Frobenius splittings. Explicitly, suppose R is a d-dimensional complete Noetherian local ring with prime characteristic p > 0 and perfect residue field. Let q = p^e for e in N, and R^{1/q} the ring of q-th roots of elements of R. Denote by a_e the maximal rank of a free R-module appearing in a direct sum decomposition of R^{1/q}. The F-signature - which can be extended to arbitrary local rings in positive characteristic - is by definition the limit s(R) := lim_e a_e/(q^d) and has previously been shown to exist only in special cases. The proof of the main result is based on certain uniform Hilbert-Kunz estimates of independent interest.

September 10, 2010

Olivier Serman (Universite Lille 1), Local factoriality through products and quotients

September 3, 2010

Lance Miller, Witt-Burnside rings attached to pro-p groups

The classical Witt vectors are a functorial construction which takes perfect fields of characteristic p to p-adically complete domains of characteristic 0. This functor was generalized by Dress and Siebeneicher to a family of functors parameterized by profinite groups. Witt's original functor corresponds to the p-adic integers as an additive pro-p group. In this talk, I will explore some examples of these functors corresponding to other pro-p groups taken over fields of characterisitc p. We will see some properties that are surprising when compared to the classical case.

August 27, 2010

Lance Miller, The p-typical Witt Vectors

The finite unramified extensions of Q_p (in a fixed algebraic closure) are in one-to-one correspondence with the finite fields of characteristic p. In this talk I will describe a functorial construction, due to Witt, which turns finite fields of characteristic p into unramified extensions of Q_p. This talk will be an expository preparation for the next seminar and will be accessible for graduate students who have taken abstract algebra.

August 18, 2010

Mordechai Katzman (University of Sheffield), Effective computation of ideals compatible with a Frobenius near splitting

A Frobenius near splitting of a commutative ring R of prime characteristic p is an additive map f : R -> R with the property that f(r^p a) = r f(a). Given a near splitting f, we call an ideal I f-compatible if f(I) is contained in I. In this talk I show a method for producing all prime compatible ideal recently discovered by Karl Schwede and myself. If time permits, I will further discuss a generalization of this method and its applications.

April 16, 2010

Jean Chan (Central Michigan University), Stabilities of Hilbert-Kunz functions

March 19, 2010

Akiyoshi Sannai (University of Tokyo), F-signature for graded Gorenstein rings

February 12, 2010

Hailong Dao (University of Kansas), Non-commutative desingularizations and cluster-tilting objects over Cohen-Macaulay rings

Let M be a reflexive module over a Cohen-Macaulay local ring R and A = Hom(M,M). If A has finite global dimension, then it has been proposed to serve as a non-commutative analogue of desingularizations of Spec(R). In this talk, we will survey what is known about when such module M exists, and discuss the connections with birational geometry and representation theory of commutative rings. Part of the new results are joint work with Craig Huneke.

March 13, 2009

Erin Chamberlin (Brigham Young University), Infinite Cohen-Macaulay posets and non-Noetherian Stanley-Reisner rings

February 6, 2009

Anurag K. Singh, Bockstein homomorphisms in local cohomology

January 16, 2009

Lars W. Christensen (Texas Tech University), Injective modules and co-base change

November 21, 2008

Maurice Rojas (Texas A & M University), Hilbert's Tenth Problem in low dimensions

November 14, 2008

Anurag K. Singh, An algorithm for computing the integral closure

November 7, 2008

Karl Schwede (University of Michigan), Refinements of test elements and F-singularities of pairs

October 31, 2008

Paul Roberts, Fontaine rings and local cohomology

September 26, 2008

Oana Veliche, Comparing complexities of pairs of modules