*2017-2018 Academic Year*

Pretalks begin an hour earlier in LC 317R (unless otherwise indicated)

Date |
Room |
Speaker |
Title |
Host |

Aug 25 3:30pm |
LC 317R | Jesse Kass(University of South Carolina) |
How to count lines on the cubic surface arithmetically?(No pretalk) |
(local) |

Aug 28 3:30pm |
LC 407 |
Blake Farman(University of South Carolina) |
Kernels for Noncommutative Projective Schemes(No pretalk) |
(local) |

Labor Day | ||||

Oct 9 3:30pm |
LC 317R | Bastian Haase(Emory University) |
Gerbe patching over arithmetic curves with a view towards homogeneous spaces |
Duncan |

Oct 16 3:30pm |
LC 317R | Matthew Ballard(University of South Carolina) |
The derived geometry of birational maps |
(local) |

Fall Break | ||||

Oct 23 3:30pm |
LC 317R | Xiaofei Yi(University of South Carolina) |
Algebraic Linkage |
(local) |

Oct 30 3:30pm |
LC 317R | Alexander Duncan(University of South Carolina) |
Cubic surfaces in positive characteristic |
(local) |

Nov 6 3:30pm |
LC 317R | Dustin Cartwright(University of Tennessee) |
Dual complexes of uniruled varieties |
Duncan |

Nov 13 3:30pm |
LC 317R | Eloisa Grifo(University of Virginia) |
Symbolic powers and the Containment Problem |
Vraciu |

Nov 20 3:30pm |
LC 317R | Patrick McFaddin(University of South Carolina) |
Exceptional collections on toric varieties |
(local) |

Thanksgiving Break | ||||

Nov 27 3:30pm |
LC 317R | Andrew Kustin(University of South Carolina) |
Use a Macaulay inverse system to detect an embedded deformation |
(local) |

Dec 4 3:30pm |
LC 317R | - |
- |
- |

Abstracts

**Matthew Ballard** - **The derived geometry of birational maps**

We will discuss how to assign an integral transform of derived categories to any D-flip. This involves using a little homotopical machinery and yields a functor with pleasant homological properties. Some examples will be discussed.

**Pretalk:**
I will talk about flips and flops and expectations for how they change the derived category.

**Dustin Cartwright** - **Dual complexes of uniruled varieties**

The dual complex of a semistable degeneration records the combinatorics of the intersections in the special fiber. The dual complex is homotopy equivalent to the Berkovich analytification, which is a non-Archimedean analogue of the analytic topology on a complex algebraic variety. I will talk about the following relation between the geometry of the general fiber and the topology of the dual complex, and thus the analytification: in arbitrary characteristic, the dual complex of an n-dimensional uniruled variety has the homotopy type of an (n-1)-dimensional simplicial complex.

**Pretalk - Various properties of varieties containing the word "rational":**
A rational variety is on that contains a dense open set isomorphic to an
open subset of projective space. Although this definition is easy to
state, it turns out to be difficult to verify in examples. Instead,
algebraic geometers have noticed that it's easier to classify varieties
not in terms of their rationality, but in terms of the existence of
rational curves in the varieties. This leads to the notion of rationally
connected varieties, ruled varieties, and other variations on these
concepts.

**Alexander Duncan** - **Cubic surfaces in positive characteristic**

I describe the possible automorphism groups of a cubic surface over an algebraically closed field of arbitrary characteristic. We will see that, while the actual groups that appear in different characteristics can vary considerably, the differences can all be explained by a handful of simple geometric observations.

**Pretalk:**
Given two curves in the complex projective plane, Bezout's theorem
provides a simple formula for the number of intersection points
(counting multiplicities). The goal of intersection theory is to
generalize this idea to intersections of subvarieties of more general
varieties. I will discuss the intersection theory of surfaces with an
eye towards describing the configuration of 27 lines on a cubic
surface.

**Blake Farman** - **Noncommutative projective schemes and DG categories**

In their 1994 paper, Noncommutative Projective Schemes, Michael Artin and J.J. Zhang introduce a generalization of usual projective schemes to the setting of not necessarily commutative algebras over a commutative ring. Gonçalo Tabuada in 2005 endows the category of differential graded categories with the structure of a model category and in 2007 Toën shows that its homotopy category is symmetric monoidal closed. In this talk, we’ll give a brief overview of these results, adapting Artin and Zhang’s noncommutative projective schemes for the language of DG categories, and discuss a “geometric” description of this internal Hom for two noncommutative projective schemes.

**Eloisa Grifo** - **Symbolic powers and the Containment Problem**

Symbolic powers consist of the functions that vanish up to a certain
order at each point in a given variety. The Containment Problem for
symbolic and ordinary powers of ideals asks when the containment
I^{(a)} ⊆ I^{b} holds. If I is a radical ideal in a regular
ring, a famous result by Ein-Lazersfeld-Smith, Hochster-Huneke and
Ma-Schwede partially answers this question. Harbourne proposed an
improvement on this result, which unfortunately does not hold in full
generality. However, in this talk we will discuss versions of
Harbourne’s Conjecture that do hold. In particular, we will discuss
some joint work with Craig Huneke on the case when R/I is an F-pure
ring, and give evidence that Harbourne’s conjecture may always hold
*eventually*.

**Bastian Haase** - **Gerbe patching over arithmetic curves with a view towards homogeneous spaces**

We will discuss patching techniques and local-global principles for gerbes over arithmetic curves. The patching setup is the one introduced by Harbater, Hartmann and Krashen. The results obtained for gerbes can be viewed as a 2-categorical analogue on their results for torsors. Along the way, we also discuss bitorsor patching and local global principles for bitorsors. As an application of these results, we will study local-global principles for homogeneous spaces through their quotient stacks.

**Pretalk: Field Patching**

In this talk, we will give an introduction to the patching technique introduced by Harbater, Hartmann and Krashen. This technique allows to study algebraic objects such as quadratic forms or central simple algebras over a fixed field via studying the same objects over a finite system of overfields. After discussing the general framework, we will focus on the case of arithmetic curves and discuss a couple of recent results obtained via this technique.

**Jesse Kass** - **How to count lines on the cubic surface arithmetically?**

A celebrated 19th century result of Cayley and Salmon is that a smooth cubic surface over the complex numbers contains exactly 27 lines. Over the real numbers, the number of lines on a smooth cubic surface depends on the surface, but Segre showed that a certain signed count of lines is the same for such surfaces. In my talk, I will explain Segre’s result and then extend that result to an arbitrary field. This is an application of A1-homotopy theory.

All work is joint with Kirsten Wickelgren.

**Andrew Kustin** - **Use a Macaulay inverse system to detect an embedded deformation**

Let k be a field of characteristic not equal to 2, P be a standard-graded polynomial ring in four variables over k, and A=P/I be an Artinian Gorenstein k-algebra of embedding dimension four which is defined by six homogeneous forms and has socle degree three. We use the homogeneous Macaulay inverse system that corresponds to I to prove that A has an embedded deformation. In other words, we prove that there is an ideal J in P and an element f in P with f regular on P/J and I=(J,f). (It quickly follows that f is a quadratic form in P and that J is a 5-generated grade three Gorenstein ideal generated by the maximal order Pfaffians of a 5 by 5 alternating matrix of linear forms.)

The previous best results describing the structure of six-generated grade four Gorenstein ideals assume that P/I is a generic complete intersection. We treat the case when P/I is a graded Artinian algebra over a field at the expense of assuming that the socle degree of P/I is three.

The talk is about joint work with Sabine El Khoury from the American University of Beirut.

**Pretalk:**
If one is working in characteristic zero, then the Macaulay Inverse System for an Artinian Gorenstein ring P/I, with socle degree 3, is a homogeneous form of degree three in the polynomial ring generated by the four partial derivative operators. So, in some sense, the talk is about cubic surfaces in projective 3-space. In the pre-talk I will explain how to use Divided Powers to make the notion of Macaulay Inverse System both coordinate free and characteristic free.

**Patrick McFaddin** - **Exceptional collections on toric varieties**

Exceptional collections of a variety X are effectively decompositions of the derived category of X, analogous to an (semi-)orthonormal basis of a vector space with inner-product. The existence of exceptional collections for smooth projective toric varieties defined over the complex numbers was settled affirmatively by Kawamata using the framework afforded by the toric Minimal Model Program. More recently, Ballard, Favero, and Katzarkov have given another proof using Variation of GIT. The study of toric varieties defined over arbitrary fields (so-called arithmetic toric varieties) has been taken up by a number of authors, although much less is known about their derived geometry. In this talk, we will discuss an effective Galois descent result for such collections and provide applications to arithmetic toric varieties of dimension two, three, and four. This is joint work with M. Ballard and A. Duncan.

**Xiaofei Yi** - **Algebraic Linkage**

Two projective varieties in P^{3} are directly linked if their union is a complete intersection. Two projective varieties are linked if they can be connected by a sequence of direct links. Two ideals I and J are directly linked if there is a regular sequence α in the intersection such that I:(α)=J and J:(α)=I. And they are linked if they can be connected by a sequence of direct links. Some properties are invariant under linkage, but some properties require a "good" linkage class to be invariant. In the seminar, we will discuss some properties invariant under linkage class, and then discuss a technique to construct a good linkage class.
If time allows, we will also discuss the linkage in the homogeneous case.