# Syracuse University Algebra Seminar

2:00–3:00 Tuesdays
Physics 233

The old hand-rolled version of this page has been retired in favor of the Google calendar on the department web site. We keep this here as a historical archive of past seminars.

## Previous Seminars

### March 22, 2011

Olgur Celikbas, University of Kansas
Title: On the rigidity of Tor
Abstract:

I will survey some of the classical results on the rigidity of Tor for modules and discuss related questions over local rings.

### March 8

Mark Kleiner, SU
Title: Induced modules over triangular matrix algebras

### March 1

Mark Kleiner, SU
Title: Triangular matrix rings in the representation theory of algebras, part 2

### February 22

Mark Kleiner, SU
Title: Triangular matrix rings in the representation theory of algebras

### February 15

Kosmas Diveris, SU
Title: On conditions concerning the vanishing of cohomology, part 2
Abstract:

It is known that the Auslander-Reiten conjecture holds for rings having what is called the AC property. We define a (possibly) weaker condition which all AC rings of finite injective dimension satisfy. We show that the Auslander-Reiten conjecture follows from this condition.

### February 8

Kosmas Diveris, SU
Title: On conditions concerning the vanishing of cohomology

### February 1

Declan Quinn, SU
Title: Fundamental but not well-known results on matrix rings
Abstract:

This would cover the Amitsur-Levitzki and Pare-Schelter theorems and introduce central polynomials if there is time.

## Fall 2010 Seminars

### December 7

Tom Bleier, SU
Title: Rational Divisor Classes on $\bar{\mathcal{M}}_g$, (Part II)
Abstract: PDF

### November 30

Tom Bleier, SU
Title: Rational Divisor Classes on $\bar{\mathcal{M}}_g$
Abstract: PDF

### November 16

Declan Quinn, SU
Abstract:

The classical Skolem-Noether theorem gives conditions for an automorphism or a derivation of a central simple algebra to be inner. We will consider gradings of such algebras, what it means for them to be inner and an analogous result.

### November 9

Steven P Diaz, SU
Title: Schubert Calculus
Abstract:

We will define what a Schubert variety is and explain how they are used to do computations in the Chow ring of a Grassmannian.

### November 2

Claudia Miller, SU
Title: On a generalization of Kunz's theorem by Avramov et al., part 2
Abstract:

I will talk on a recently posted paper by Avramov, Hochster, Iyengar, and Yao on a generalization of Kunz's theorem from the 60s characterizing regular rings via the Frobenius map.

### October 26

Claudia Miller, SU
Title: On a generalization of Kunz's theorem by Avramov et al.

### October 19

Steven P. Diaz, SU
Title: How many lines meet four general lines in three space?
Abstract:

We will attempt to describe how Grassmannians which parameterize subspaces of a fixed dimension of a fixed vector space can be used to solve problems in enumerative geometry concentrating on the problem mentioned in the title. Given four general lines in projective three space, how many lines are there that meet all four lines?

### October 12

Hamid Rahmati, SU
Title: Brauer–Thrall for totally reflexive modules
Abstract: PDF, or:

Let $(R, \mathfrak m , k)$ be a commutative local ring. A finite $R$-module $M$ is called totally reflexive if it is reflexive and $\rm{Ext}^i_R(M,R) = \rm{Ext}^i_R(M^*,R) = 0$ for all $i > 0$. Assume that $R$ is not Gorenstein and there are elements $w, x \in \mathfrak m$ such that $\rm {Ann}_R(x) = (w)$ and $\rm {Ann}_R(w) = (x)$. For every $n \in \mathbb N$ there exists an indecomposable totally reflexive module that is minimally generated by $n$ elements. Moreover, if $k$ is infinite then for every $n \in \mathbb N$ there are $|k|$ pairwise non-isomorphic indecomposable totally reflexive modules that are minimally generated by $n$ elements.

### September 28

Marju Purin, SU
Title: Complexity of Trivial Extensions, Part 2
Abstract: PDF

### September 21

Marju Purin, SU
Title: Complexity of Trivial Extensions, Part 1

### September 14

Simon Smith, SU
Title: Permutation-ends of primitive groups
Abstract:

An end of an infinite graph can be thought of as a point of infinity. Now many of you will have heard, at least in passing, of ends of finitely generated groups. Beloved by combinatorial group theorists the world over, they are the ends of the Cayley graph of the group. Much has been written about them, and their relation to the structure of finitely generated groups. They are, as you say in America, a Big Deal.

Infinite primitive groups – the “atoms” of permutation group theory – are rarely finitely generated, even when one places quite strict finiteness conditions on them. Ends, one might think, are not useful when studying infinite permutation groups. But this is not the case! By imposing relatively mild finiteness conditions on these groups, it transpires that there is a very natural graph associated with each group, the ends of which I propose to call permutation-ends.

Assuming nothing more than basic undergraduate group theory, I will introduce you all to some of the research I’ve been involved in over the summer. In particular, I’ll show you how a little information about a permutation-end yields a lot of exciting information about the group.

### September 7

Mark Yerrington, SU
Title: The lambda dimension of a commutative ring
Abstract: PDF

## Spring 2010 Seminars

### May 4

Petter Andreas Bergh, Norwegian University of Science and Technology
Title: The dimension of a triangulated category
Abstract:

This is joint work with Srikanth Iyengar, Henning Krause and Steffen Oppermann. Given a triangulated category, its dimension measures how many steps are needed in order to generate the category from one of its objects. This concept was introduced quite recently by Rouquier, who used it to show that there exist finite dimensional algebras of arbitrarily large representation dimension. In this talk, we provide a lower bound for the dimension, and apply this to derived categories of local complete intersections and finite dimensional algebras.

### April 27

Graham Leuschke, SU
Title: Tame and Wild Cohen-Macaulay Type, part 2
Abstract:

In the representation theory of finite-dimensional algebras over a field, Drozd's trichotomy theorem says that an algebra has either tame module type or wild module type. Loosely, these two possibilities correspond to: hoping for a classification theorem, or throwing up our hands in dismay. We'd very much like a similar trichotomy result in other representation-theoretic contexts, specifically for maximal Cohen--Macaulay modules over a Cohen--Macaulay local ring. I'll give a little background on the problem, including definitions of tame and wild CM type, and talk about recent work with Andrew Crabbe (Syracuse) which shows that hypersurfaces of multiplicity four or more, in three or more variables, have wild CM type.

### April 20

Graham Leuschke, SU
Title: Tame and Wild Cohen-Macaulay Type

### April 15 (Note special Thursday seminar!)

Nicole Snashall, University of Leicester
Title: Koszul algebras and Hochschild cohomology

### April 6

Steven Diaz, SU
Title: Grassmannians for Mathphobics
Abstract:

A Grassmannian parameterizes subspaces of a fixed dimension of a fixed finite dimensional vector space. I will try to explain what a Grassmannian is and show that it is an example of a fine moduli space (recall the talk of Thomas Bleier). I will not succeed. Anyone with even a good undergraduate course in linear algebra should be able to understand at least some of this talk.

### March 30

Title: Reduced VS Non-Reduced Point Sets - A Score Sheet for Invariants
Abstract:

In this talk we will compare algebraic invariants of reduced and "fat point" sets. By trimming down fat points, we will obtain upper and lower bounds for the Hilbert function, graded Betti numbers, and initial degree of any fat point set in projective 2-space. Empowered with the Hilbert function bounds, we can verify a special case of a conjectured inequality of Harbourne and Huneke involving the initial degrees of a fat point set and its support.

### March 23

Title: T-Koszul algebras
Abstract:

Let $\Lambda =\oplus_{n \geq 0} \Lamba_n$ be a graded algebra over a field $k$. We assume $dim_k \Lambda_i < \infty$ for all $i \geq 0$, but we do not assume that $\Lambda_0$ is semi-simple. Suppose $gldim \Lambda_0 < \infty$. Let $T$ be a graded $\L$-module concentrated in degree zero.

In this talk I propose the following new definition of T-Koszul algebras: $\Lambda$ is a $T$-Koszul algebra if both (1) and (2) hold.
1. $T$ is a tilting $\Lambda_0$-module.
2. T is graded self-orthogonal as a $\Lambda$-module.
We prove that many quasi-hereditary Koszul algebras have a $T$-Koszul algebra structure coming form the standard (Verma) modules.

### March 9

Kosmas Diveris, SU
Title: The Auslander-Reiten conjecture for Gorenstein rings.
Abstract:

Tokuji Araya has shown that if the Auslander-Reiten conjecture holds in codimension one for a commutative Gorenstein ring R, then it holds for R. We will present Araya's proof from the paper "The Auslander-Reiten conjecture for Gorenstein rings".

### March 2

Andrew Crabbe, SU
Title: Ranks of indecomposable maximal Cohen-Macaulay modules, part 2
Abstract:

Let R be a one-dimensional reduced Noetherian local ring of infinite Cohen-Macaulay type, and let P_1,. . . , P_s be the minimal prime ideals of R. From work of Roger Wiegand and others, it is known that for every positive integer r there is an indecomposable maximal Cohen-Macaulay R-module M of constant rank r, i.e. M is free of the same rank r when localized at each P_i. In this talk we explore the following question: For which non-trivial s-tuples (r_1,...,r_s) is there an indecomposable maximal Cohen-Macaulay R-module M such that M is free of rank r_i when localized at P_i?

### February 23

Andrew Crabbe, SU
Title: Ranks of indecomposable maximal Cohen-Macaulay modules

### February 16

Claudia Miller, SU
Title: Smoke's result on reducing modules to cyclic ones within a certain Grothendieck group, part 2
Abstract:

We discuss a paper of W. Smoke on how to reduce modules to cyclic ones modulo certain modules with nice intersection properties.

### February 9

Claudia Miller, SU
Title: Smoke's result on reducing modules to cyclic ones within a certain Grothendieck group

### February 2

Title: Orbit closures in varieties parametrizing the objects in the derived category.
Abstract:

A famous theorem by Zwara and Riedtmann gives an algebraic characterization of orbit closure (degeneration) in module varieties. In joint work with Jensen and Su we have proved an analogous theorem for varieties of A1-modules. Our result should be seen as the derived category version of the Zwara-Riedtmann Theorem, and contains that theorem as a special case. In this talk I will describe the varieties and orbit closures involved and show how they gives us new insight into the derived category of a nite dimensional algebra. I will also explain why new techniques might be needed for obtaining a characterization of orbit closures in varieties of algebras.

### January 26

Thomas Bleier, SU
Title: Introduction to the Moduli Space of Curves (Part II)
Abstract:

In this talk I will state an existence theorem for the coarse moduli space for curves of a fixed genus greater than 1. We will then discuss hyperelliptic curves and calculate the dimension of the hyperelliptic locus in the moduli space. Lastly, we will look at a modular compactification of the moduli space.

### January 15

Thomas Bleier, SU
Title: Introduction to the Moduli Space of Curves (Part I)
Abstract:

In this talk we will look at what it means to have a family of curves over a base scheme, and describe a functor from schemes to sets that sends a scheme to the set of all such families of curves, of a given genus, over that scheme. We will also look at what it means for a such a functor to be representable and consequences of being representable. The talk will conclude by looking at the case of elliptic curves.

## Fall 2009 Seminars

### December 8

Title: Orbit closures in varieties parametrizing the objects in the derived category, Part 2.
Abstract:

A famous theorem by Zwara and Riedtmann gives an algebraic characterization of orbit closure (degeneration) in module varieties. In joint work with Jensen and Su we have proved an analogous theorem for varieties of A1-modules. Our result should be seen as the derived category version of the Zwara-Riedtmann Theorem, and contains that theorem as a special case. In this talk I will describe the varieties and orbit closures involved and show how they gives us new insight into the derived category of a nite dimensional algebra. I will also explain why new techniques might be needed for obtaining a characterization of orbit closures in varieties of algebras.

### December 1cancelled! watch for a rescheduling next semester

Steven P. Diaz, SU
Title: What is a Weierstrass point?

### November 24

No seminar this week: happy Thanksgiving!

### November 17

Claudia Miller, SU
Title: Limit Hilbert-Kunz Multiplicities

### November 10

Suanne Lu, Le Moyne
Title: Equivariant K-Theory of Toric Varieties

### November 3

Title: Orbit closures in varieties parametrizing the objects in the derived category.
Abstract:

A famous theorem by Zwara and Riedtmann gives an algebraic characterization of orbit closure (degeneration) in module varieties. In joint work with Jensen and Su we have proved an analogous theorem for varieties of A1-modules. Our result should be seen as the derived category version of the Zwara-Riedtmann Theorem, and contains that theorem as a special case. In this talk I will describe the varieties and orbit closures involved and show how they gives us new insight into the derived category of a nite dimensional algebra. I will also explain why new techniques might be needed for obtaining a characterization of orbit closures in varieties of algebras.

### October 27

Suanne Lu, Le Moyne
Title: Equivariant K-Theory of Affine Toric Varieties, part 2

### October 20

Suanne Lu, Le Moyne
Title: Equivariant K-Theory of Affine Toric Varieties

Dan Zacharia, SU

### October 6

Dan Zacharia, SU
Title: Subadditive functions, part 2 (the rescheduling)

### September 29

R. Inanc Baykur, Brandeis University
Title: Smooth four-manifolds, surfaces and exotica

In this talk, we will focus on a 4-dimensional phenomenon: Existence of infinitely many smooth 4-manifolds, which are all homeomorphic to a standard manifold but pairwise non-diffeomorphic; and in a parallel vein, existence of infinitely many embeddings of surfaces in a fixed standard 4-manifold, which are ambiently homeomorphic but non-diffeomorphic. We will discuss how to construct such examples.

Dan Zacharia, SU

Dan Zacharia, SU

### September 8

Steven P. Diaz, SU
Title: Complex Analysis

### September 1

Dieter Happel, Technical University of Chemnitz
Title: The representation dimension for iterated tilted algebras

## Spring 2009 Seminars

### April 28

Title: Commutative versions of the Brauer–Thrall Conjectures
Abstract:

The Brauer-Thrall conjectures concern the number of non-isomorphic indecomposable finitely generated modules over an Artinian ring. The first conjecture (now a theorem) says that if the ring has infinite representation type (that is, there are infinitely many non-isomorphic indecomposables), then there must be indecomposables of arbitrarily large finite length. The second says there are infinitely many $n$ for which there are infinitely many indecomposables of length $n$.

In this talk I will transplant these conjectures to commutative algebra and survey what is known. We will consider maximal Cohen-Macaulay modules over local Noetherian rings. "Length" is replaced by "rank" or "multiplicity". For one-dimensional local rings, maximal Cohen-Macaulay modules are just non-zero torsion-free modules (also called "lattices" in algebraic number theory).

### April 21

No seminar this week. Enjoy "Syracuse Showcase" instead.

### April 14

Mark Kleiner, SU
Title: Representations of quivers and root systems of Kac-Moody algebras and Coxeter groups, part 2

### April 7

Mark Kleiner, SU
Title: Representations of quivers and root systems of Kac-Moody algebras and Coxeter groups

### March 31

Mark Kleiner, SU
Title: Representations of quivers and root systems of Kac-Moody algebras

### March 24

Kosmas Diveris, SU
Title: Standard resolutions and CM approximations, part 2
Abstract:

We will summarize several results concerning resolutions over complete intersections, then define CM approximations and the related delta invariants of modules over Gorenstein rings.  We will then show that over a hypersuface there is a connection between the delta invariants of a module and its standard resolution.

### March 17

Kosmas Diveris, SU
Title: Standard resolutions and CM approximations, part 1

### March 3 (cancelled -- watch for the rescheduling)

Janet Striuli, Fairfield University
Title: Growth of Bass Numbers
Abstract:

Let R be a commutative Noetherian local ring with residue field k. Important aspects of the structure of the ring can be detected by the cohomology groups Ext^i_R(k,k) and Ext^i_R(k,R). The k-vector space dimensions of the cohomology groups  Ext^i_R(k,k) are called Betti numbers, while the dimensions of Ext^i_R(k,R) are known as the Bass numbers of R.  While the resulting sequence of Betti numbers has been studied very extensively, little is known about the sequence of Bass numbers.  We will present similarities and differences in these sequences and some recent developments on the study of Bass numbers.

### February 24

Graham Leuschke, SU
Title: Non-commutative desingularizations of determinantal varieties 3
Abstract:

I will define and give some background on what it should mean to be a "non-commutative resolution of singularities," with emphasis on examples. Then I'll explain joint work with Buchweitz and Van den Bergh showing existence of such desingularizations for a large class of rings defined by minors of generic matrices.

This third part will focus on connections with the representation theory of quivers.

### February 17

Bogdan Petrenko, SUNY Brockport
Title: On the probability of generating the ring of integer matrices
Abstract:

The main goal of my talk is to explain why the asymptotic probability that m random elements generate the ring of 2-by-2 integer matrices equals zeta(m)^{-1}zeta(m-1)^{-1}, where zeta is the Riemann zeta function. This formula is part of my joint work with Rostyslav Kravchenko (Texas A&M) and Marcin Mazur (Binghamton University).

### February 10

Frank Moore, Cornell University
Title: Cohomology of Products of Local Rings
Abstract:

The Ext algebra of a commutative noetherian local ring $R$ with residue field $k$ is a graded $k$-algebra that carries subtle information about $R$.  In this talk, I will discuss the structure of the Ext algebra of local rings that arise by combining local rings $S$ and $T$ with the same residue field in various ways.

Starting with $R = S \times_k T$, the fiber product of $S$ and $T$, I will describe the Ext algebra of $R$ as well as the structure of certain modules over it.  Next I will discuss joint work with Avramov in which we study the connected sum $S \# T$ of $S$ and $T$, which is defined only when $S$ and $T$ are artinian Gorenstein rings. Although we have yet to describe the Ext algebra of a connected sum completely, I will discuss some partial results of ours in this direction.

### February 3

Graham Leuschke, SU
Title: Non-commutative desingularizations of determinantal varieties ii
Abstract:

I will define and give some background on what it should mean to be a "non-commutative resolution of singularities," with emphasis on examples. Then I'll explain joint work with Buchweitz and Van den Bergh showing existence of such desingularizations for a large class of rings defined by minors of generic matrices.

### January 27

Graham Leuschke, SU
Title: Non-commutative desingularizations of determinantal varieties

### January 20

Title: AR-sequences in subcategories
Abstract:

This is sort of a continuation of my previous talk. I will say more about how to compute AR-sequences in subcategories.

### January 13

Steven P. Diaz, SU
Title: Fun with Riemann-Roch
Abstract:

We will show how many of the older versions of the Riemann-Roch theorem follow from the Grothendieck-Riemann-Roch theorem. If time permits we will also show some of the ways the Riemann-Roch theorem is used to study curves.

## Fall 2009 Seminars

### December 2

Steven P. Diaz, SU
Title: The Grothendieck-Riemann-Roch Theorem, pt. 3
Abstract:

This will be a brief introduction to Chern classes, the Grothendieck group and the Grothendieck-Riemann-Roch theorem.

### November 25

No seminar. Happy Thanksgiving!

### November 18

Title: AR-sequences for modules with a Verma flag
Abstract:

The category of modules orthogonal to a given tilting module has Auslander-Reiten sequences. In this talk I will illustrate a method for computing these sequences using the category of so(4)-modules with a Verma flag as an example.

### November 11

Alex Dugas, UC Santa Barbara
Title: Periodic Algebras

### November 4

Steven P. Diaz, SU
Title: The Grothendieck-Riemann-Roch Theorem, Part 2
Abstract:

This will be a brief introduction to Chern classes, the Grothendieck group and the Grothendieck-Riemann-Roch theorem.

### October 28

Steven P. Diaz, SU
Title: The Grothendieck-Riemann-Roch Theorem

### October 21

Marju Purin, SU
Title: Complexity and Periodicity, Part 2
Abstract:

We study modules over an artin k-algebra Λ where k is a commutative artin ring. We introduce the notions of complexity and periodicity of a module. We proceed to show that under a certain ﬁnite generation assumption bounded resolutions must become eventually periodic. This talk is based on the paper ”Complexity and Periodicity” by Petter Bergh.

### October 14

Marju Purin, SU
Title: Complexity and Periodicity

### October 7

Steven P. Diaz, SU
Title: Determinantal Varieties
Abstract:

The talk will cover some of the basic properties of determinantal varieties and also include a brief introduction to Grassmannians.

### September 23

Dieter Happel, TU Chemnitz
Title: On the coefficients of the Coxeter polynomial

### September 16

Andrew Crabbe, SU
Title: Building Indecomposable Modules
Abstract:

A common way to understand a ring is to study certain subcategories of its modules, in particular, the indecomposable objects in such categories. Over rings with dimension 0, it's reasonable to look at the entire category of modules, however, over larger rings, its beneficial to restrict to a more tractable collection. The subcategory of greatest interest has been that of all maximal Cohen-Macaulay modules. By knowing the number of indecomposable maximal Cohen-Macaulay modules or whether there is a bound on the "size" of these modules, one can gain important information about the ring (for instance, the dimension of its singular locus). But what happens if you go away from the subcategory of maximal Cohen-Macaulay modules?

One purpose of this talk is to show that over certain rings with dimension greater than 1, one can build indecomposable modules that are arbitrarily "large" (where "large" could refer to the multiplicity, or in our case, the rank on the punctured spectrum), even if the ring does not permit the construction of arbitrarily "large" indecomposable maximal Cohen-Macaulay modules. These constructions are achieved using results for the generalized Hilbert-Samuel polynomials.

### September 9

Andrew Crabbe, SU
Title: Generalized Hilbert-Samuel polynomials
Abstract:

The purpose of this talk is to introduce a generalization of the classical Hilbert-Samuel polynomial. These generalized polynomials have been used to build "big" indecomposable modules over certain rings (which will be discussed in a 2nd talk), as well as to provide information about the dimensions of large syzygies of modules. The talk should be accessible and many of the basic ideas will be explained.

### September 2

Title: Hochschild homology and global dimension
Abstract:

The aim of the talk is to give a gentle introduction to my recent paper with Petter A. Bergh (with the same title).

### August 26

Organizational Meeting

## Spring 2008 Seminars

### April 29

Dan I. Zacharia, SU
Title: Auslander-Reiten triangles in derived categories, the return

### April 22

Gary Kennedy, Ohio State Mansfield
Title: Monodromy of plane curves and quasi-ordinary surfaces
Abstract:

A quasi-ordinary surface f(x,y,z)=0 is one for which, at each singular point, there is a theory of Puiseux expansion as for plane curves. Taking a transverse slice with x=constant, one obtains a singular plane curve. Its Milnor fiber f(x,y)=e has two sorts of monodromy: (1) the Milnor monodromy (also called its horizontal monodromy), in which x is fixed while e varies around a small circle, (2) the vertical monodromy, in which e is fixed while x varies. In joint work with my colleague Lee McEwan, we have discovered simple recursive formulas for both monodromies.

### April 15

Dan I. Zacharia, SU
Title: Auslander-Reiten triangles in derived categories

### April 8

Kosmas Diveris, SU
Title: What makes a complex exact?

### April 1

Special Two-fer Seminar! (no fooling!)
Cornelia Yuen, SUNY Potsdam
Title: A minimal reduction of Ferrers Ideals
AND
Jason Howald, SUNY Potsdam
Title: Motivic integration and monomial ideals

### March 25

Liana Sega, UMKC
Title: Free resolutions over a class of local rings
Abstract:

I will consider a class of local Koszul rings with radical cube zero. Assuming the existence of a certain element, which we call Conca generator, we can describe the Poincare series of the residue field, and determine the structure of Ext(M,k) for a finite R-module M. The existence of the Conca generator guarantees that free resolutions over such rings have some remarkable properties: every finite module admits a syzygy which is Koszul and every module annihilated by the Conca generator is Koszul. We can describe completely the Koszul modules over Gorenstein rings with radical cube zero, and, in general, we have preliminary results towards understanding the Koszul property for modules with a fixed Hilbert series. This talk covers joint work with L. Avramov and S.Iyengar.

### March 18

Steven P. Diaz, SU
Title: There is more to resolutions than graded Betti numbers, Part 2
Abstract:

This is joint work of Diaz and Geramita. We find two finite sets of points in projective 3 space that differ geometrically, yet the graded minimal free resolutions of their ideals have the same graded Betti numbers. Inspired by this example we define a (hopefully new) discrete invariant one can associate to a graded minimal free resolution. This invariant distinguishes between these two sets of points.

### March 4

Claudia Miller, SU
Title: Rigidity of Frobenius, part deux

### February 26

Claudia Miller, SU
Title: Rigidity of Frobenius

### February 19

Steven P. Diaz, SU
Title: There is more to resolutions than graded Betti numbers
Abstract:

This is joint work of Diaz and Geramita. We find two finite sets of points in projective 3 space that differ geometrically, yet the graded minimal free resolutions of their ideals have the same graded Betti numbers. Inspired by this example we define a (hopefully new) discrete invariant one can associate to a graded minimal free resolution. This invariant distinguishes between these two sets of points.

### February 12

Declan Quinn, SU
Title: Kronecker products of representations of the symmetric group, part iv

### February 5

Declan Quinn, SU
Title: Kronecker products of representations of the symmetric group, part iii

### January 29

Declan Quinn, SU
Title: Kronecker products of representations of the symmetric group, part ii

### January 22

Declan Quinn, SU
Title: Kronecker products of representations of the symmetric group

## Fall 2007 Seminars

### December 4

Bogdan Petrenko, SUNY Brockport
Title: Some formulas for the smallest number of generators for finite direct sums of matrix algebras
Abstract:

Any finite direct sum of matrix algebras over an infinite field has 2 generators. This is no longer true in general for a finite direct sum of matrix algebras over other commutative rings. We obtain an asymptotic upper bound for the smallest number of generators for a finite direct sum of matrix algebras over a finite field. We also obtain an exact formula for the smallest number an(q) of generators for a direct sum of n copies of the 2-by-2 matrix algebra over a field with q elements. We show that a_n(2) is also the smallest number of generators for a direct sum of n copies of the 2-by-2 integer matrix ring. We remark that generators for a finite direct sum of integer matrix rings can be used as generators for a similar direct sum where the ring of integers is replaced with any ring with 1.

The talk will be based on my joint work with Said Sidki (Journal of Algebra, 310 (2007), no. 1, 15--40) and Rostyslav Kravchenko(arXiv:math/0611674).

### November 27

Mark Kleiner, SU
Title: Finite and Infinite Coxeter Groups

### November 13

Mark Kleiner, SU
Title: More on Coxeter groups

### November 6

Declan Quinn, SU
Title: An introduction to group representations and the symmetric group

### October 30

Steven Diaz, SU
Title: Schur Functors, Part 3

### October 23

Steven Diaz, SU
Title: Schur Functors, Part 2

### October 16

Steven Diaz, SU
Title: Schur Functors

### October 9

Jinjia Li, SU
Title: The Game of Tor and Frobenius, part 2
Abstract:

The Frobenius endomorphism is an important tool in studying homological questions in commutative algebra. I will first give a brief survey on topics related to homological properties of Frobenius, assuming only minimum background in commutative algebra and homological algebra. Then I will discuss some recent progress made in joint work with I. Aberbach of University of Missouri and, respectively, with C. Miller.

### October 2

Jinjia Li, SU
Title: The Game of Tor and Frobenius, part 1

### September 25

Mark Kleiner, SU
Title: Coxeter groups and representations of algebras, part 4

### September 18

Mark Kleiner, SU
Title: Coxeter groups and representations of algebras, part 3

### September 11

Mark Kleiner, SU
Title: Coxeter groups and representations of algebras, part 2

### September 4

Mark Kleiner, SU
Title: Coxeter groups and representations of algebras, part 1

### August 28

Organizational Meeting

## Spring 2007 Seminars

### April 30

Steven P. Diaz, SU
Title: The Gale Transform and Multi-Graded Determinantal Schemes, Part 5
Abstract:

This is joint work with Susan M. Cooper of California Polytechnic State University. Eisenbud and Popescu showed that certain finite determinantal subschemes of projective spaces defined by maximal minors of adjoint matrices of homogeneous linear forms are related by Veronese embeddings and a Gale transform. We extend this result to adjoint matrices of multihomogeneous multilinear forms. The subschemes now lie in products of projective spaces and the Veronese embeddings are replaced with Segre embeddings.

### April 23

Steven P. Diaz, SU
Title: The Gale Transform and Multi-Graded Determinantal Schemes, Part 4

### April 16

Steven P. Diaz, SU
Title: The Gale Transform and Multi-Graded Determinantal Schemes, Part 3

### April 9

Steven P. Diaz, SU
Title: The Gale Transform and Multi-Graded Determinantal Schemes, Part 2

### April 2

Steven P. Diaz, SU
Title: The Gale Transform and Multi-Graded Determinantal Schemes, Part 1

### March 26

Graham Leuschke, SU
Title: The Most Preposterous Theorem I Know, part 2
Abstract:

I'll discuss the weirdest theorem I've ever heard of: Osofsky's result that projective dimension depends on your set theory.

### March 19

Graham Leuschke, SU
Title: The Most Preposterous Theorem I Know

### March 5

Claudia Miller, SU
Title: Intersection Numbers, part 3

### February 26

Title: Breaking inseparable

This talk describes a problem raised by Larry Levy and Lee Klingler on resolving which rings have extensions of Steinitz' theorem. They addressed all cases except when the residue field extension is characteristic two and inseparable. They set up, but do not solve, a matrix problem to answer the general case of Dedekind-like rings, but use other methods to handle all other cases. Klingler and I address this matrix problem.

### February 19

Title: New Intersection via Adams Operations

### February 12

Dan I. Zacharia, SU
Title: A quick introduction to Auslander-Reiten theory, part 2

### February 5

Dan I. Zacharia, SU
Title: A quick introduction to Auslander-Reiten theory

### January 29

Claudia Miller, SU
Title: Intersection Numbers, part 2

### January 22

Claudia Miller, SU
Title: Intersection Numbers

## Fall 2006 Seminars

### December 5 at 3:30 (note earlier starting time!)

Philip Lynn, SU
PhD Dissertation Defense: Deformations of Plane Curve Singularities of Constant Class

### November 28

Declan Quinn, SU
Title: Galois actions, part 3

### November 14

Declan Quinn, SU
Title: Galois actions, part 2

### November 7

Declan Quinn, SU
Title: Galois actions

### Thursday, October 26 at 3:30 (note unusual day and time!)

Jinjia Li, SU
Title: Homology of perfect complexes over Cohen-Macaulay local rings, part 2
Abstract:

A perfect complex is a bounded complex of finite rank free modules. If all the homology of such a complex have finite length, one defines the Euler characteristic to be the alternating sum of the lengths of the homology. We will first discuss a property enjoyed by the homology of perfect complexes over a Cohen-Macaulay local ring A. Then, we apply this property to the situations: (1) the trivial extension of A and its canonical module, (2) the Frobenius endomorphism (in positive characteristic case), to obtain interesting results regarding the behavior of the Euler characteristics.

### October 17

Jinjia Li, SU
Title: Homology of perfect complexes over Cohen-Macaulay local rings

### October 10

Steven P. Diaz, SU
Title: Veronese Subrings, continued

### Thursday, October 5 (note unusual day!)

Steven P. Diaz, SU
Title: Veronese Subrings, continued

### September 26

Steven P. Diaz, SU
Title: Veronese Subrings

### September 19

Faculty meeting; no seminar

### September 12

Allen Pelley, SU
Title: Representations of Quivers and Weyl groups of Kac-Moody Algebras (part 2)
Abstract:

In this series of talks, we will see a brief introduction to representations of quivers and how they relate to Weyl groups of Kac-Moody algebras. Specifically, we will use representations of quivers to prove "for any Coxeter element of the Weyl group associated to an indecomposable symmetrizable generalized Cartan matrix, the group is infinite if and only if the powers of the element are reduced words." This strengthens known results of Howlett, Fomin-Zelevinsky. These talks should be accessible to graduate students who have completed MAT631-632.

### September 5

Allen Pelley, SU
Title: Representations of Quivers and Weyl groups of Kac-Moody Algebras (part 1)

## Spring 2006 Seminars

### May 2

Mark Kleiner, SU
Title: Preprojective modules and reduced words in the Weyl group of a quiver
Abstract:

This is joint work with Allen Pelley. Given a finite quiver without oriented cycles, the indecomposable preprojective modules over the path algebra are classified by and computed in terms of special sequences of vertices of the quiver. To each such sequence one associate a word in the Weyl group of the quiver, and the goal is to study the resulting set of words. It turns out that these words have very nice properties. In particular, they are reduced, and they detect isomorpism classes of indecomposable preprojectives.

### April 25

Graham Leuschke, SU
Title: The McKay Correspondence III
Abstract:

I will describe a confluence of ideas from group representation theory, commutative algebra, non-commutative algebra, and algebraic geometry, which centers around two-dimensional rings of invariants. Conjectural extensions to higher dimension are a very active area of research, and apparently come up in mathematical physics, while the two-dimensional theory is just beautiful mathematics.

### April 18

Graham Leuschke, SU
Title: The McKay Correspondence II

### April 11

Graham Leuschke, SU
Title: The McKay Correspondence

### Special Seminar: Friday, April 7, 2006, 3–4 pm in Carnegie 300

Izuru Mori, SUNY Brockport
Title: Co-point Modules over Koszul Algebras
Abstract:

Let $A$ be a graded algebra finitely generated in degree 1 over a field $k$. Point modules over $A$ introduced by Artin, Tate and Van den Bergh play an important role in studying $A$ in noncommutative algebraic geometry. In this talk, we define a dual notion of point module in terms of Koszul duality, which we call a co-point module. Using co-point modules, we will construct counter-examples to the following condition due to Auslander: for every finitely generated right module $M$ over a ring $R$, there is a natural number $n_M$ such that, for any finitely generated right module $N$ over $R$, $Ext ^i_R(M, N)=0$ for all $i>>0$ implies $Ext ^i_R(M, N)=0$ for all $i>n_M$.

### March 21

Dan I. Zacharia, SU
Title: Representation Dimension IV

### February 28

Dan I. Zacharia, SU
Title: Representation Dimension III

### February 21

Dan I. Zacharia, SU
Title: Representation Dimension II

### February 14

Dan I. Zacharia, SU
Title: Representation Dimension

### February 7

Markus Reitenbach, SU
Title: Projectivization of Poset Representations
Abstract:

I will give a brief introduction into the theory of representations of partially ordered sets and their relationship to representations of quivers respectively finite dimensional algebras. In particular, I will indicate how an algebraic technique called projectivization leads to the notion of a certain "derivative" of a given poset, and how this derivative can be used to answer questions about the representations of the original poset.

### January 31

Susan Cooper, SU
Title: Geometric Consequences of the Eisenbud-Green-Harris Conjecture
Abstract:

The Eisenbud-Green-Harris conjecture gives rise to a charaterization of the Hilbert functions of point sets which are subsets of certain families of complete intersections. In this talk we will review the characterization of such functions and investigate geometric consequences. In particular, we will see that subsets with certain Hilbert functions are guaranteed to have the "Cayley-Bacharach Property". We will also show that certain collections of subsets must have a "maximal Hilbert function".

### January 24

Frédérick Latour, SU
Title: Central extensions of preprojective algebras

## Fall 2005 Seminars

Sorry, these listings have been lost.

## Spring 2005 Seminars

### April 5 (Tuesday) — 5:00 pm — Carnegie 114

Dave Jorgensen, University of Texas at Arlington
Title: Gorenstein rings: not so nice

### March 29 (Tuesday) — 5:00 pm — Carnegie 114

Steve Diaz, SU
Title: Almost Split Sequences, Preprojective Algebras, and Multiplication Maps of Maximal Rank, II

### March 22 (Tuesday) — 5:00 pm — Carnegie 114

Steve Diaz, SU
Title: Almost Split Sequences, Preprojective Algebras, and Multiplication Maps of Maximal Rank

### March 8 (Tuesday) — 5:00 pm — Carnegie 114

Lars Winther Christensen, University of Nebraska
Title: Stability of G-dimensions
Abstract:

Auslander and Bridger's 1966 definition of the so-called G-dimension for finitely generated modules over a commutative, noetherian ring marked the start of the study of Gorenstein homological dimensions. In commutative algebra, these homological invariants are used to characterize Gorenstein rings in much the same way the usual homological dimensions characterize regular rings.

This talk will center around some questions on stability of Gorenstein dimensions. It will be an introduction/survey illustrated by some tangible examples to shed light on these questions.

### February 17 (Thursday) — 5:15, Physics 233 (note time change!)

Jinjia Li, University of Illinois
Title: Intersection Multiplicity and Asymptotic Length of Homologies

### February 1 (Tuesday)

Mark Kleiner, SU
Title: Algebras with Smallest Resolutions of Simple Modules, II

### January 25 (Tuesday)

Mark Kleiner, SU
Title: Algebras with Smallest Resolutions of Simple Modules

## Fall 2004 Seminars

### December 6 (Monday)

Sebastian Burciu, SU
Title: Representations of the Drinfel'd double, II

### December 1 (Wednesday)

Sebastian Burciu, SU
Title: Representations of the Drinfel'd double

### November 15 (Monday)

Claudia Miller, SU
Title: Extremal Algebras, II

### November 8 (Monday)

Claudia Miller, SU
Title: Extremal Algebras

### November 1 (Monday)

Graham Leuschke, SU
Title: Matrix factorizations and factoring matrices, II
Abstract:

### October 25 (Monday)

Graham Leuschke, SU
Title: Matrix factorizations and factoring matrices: the generic adjoint and maximal Cohen-Macaulay modules
Abstract:

I'll talk about joint work with Ragnar-Olaf Buchweitz (Toronto) inspired by the following question of G. Bergman: "Can one factor the classical adjoint of a generic matrix?". This unassuming (and not terribly motivated) question has unexpected connections to topology, representation theory, and commutative algebra. I'll describe some of these connections, focusing on the connection with the theory of matrix factorizations (a.k.a. maximal Cohen-Macaulay modules).

### October 11 (Monday)

Title: Hilbert-Kunz Functions, II

### October 4 (Monday)

Title: Hilbert-Kunz Functions
Abstract:

This talk will give an introduction to Hilbert-Kunz functions, including basic facts and what is currently known about them. In addition, I will present a new result (joint with Craig Huneke and Paul Monsky) regarding the existence of a second coefficient in the Hilbert-Kunz function of an m-primary ideal in a normal local ring of positive characteristic.

### September 27 (Monday):

Sebastian Burciu, SU
Title: Representations of Degree Three for Semisimple Hopf Algebras, II

### September 22 (Wednesday):

Sebastian Burciu, SU
Title: Representations of Degree Three for Semisimple Hopf Algebras

### September 20 (Monday):

No seminar today due to the A&S Faculty Meeting.

### September 13 and 15 (Monday and Wednesday):

No seminars this week.

### September 8 (Wednesday):

Frédérick Latour, SU
Title: Representations of Cherednik algebras in positive characteristic

Back to math.syr.edu.