To volunteer to give a talk, or for anything else regarding the seminar, contact Claudiu Raicu or Eric Riedl.

Abstracts can be found below. The seminar will meet on Thursdays, 3:30–4:30pm, either on Zoom or in 258 Hurley, ** unless otherwise noted.** Related events are also listed below.

Date | Speaker | Title |
---|---|---|

Thursday, Feb. 8 | Matt Weaver (Notre Dame) | Complete intersections and duality, with applications to Rees rings and tangent algebras |

Thursday, Feb. 15 | François Greer (Michigan State) | Compactifying special cycles |

Thursday, Mar. 7 | Takumi Murayama (Purdue) | TBA |

Thursday, Mar. 14 | No seminar (Spring break) | |

Thursday, Mar. 21 | Ralph Kaufmann (Purdue) | TBA |

Thursday, Apr. 18 | Fan Qin (Beijing Normal University) | TBA |

The seminar will meet on Thursdays, 3:30–4:30pm, either on Zoom or in 258 Hurley, ** unless otherwise noted.** Related events are also listed below.

Date | Speaker | Title |
---|---|---|

Thursday, Aug. 31 | Alessandra Costantini (Oklahoma State) | Rees algebras of linearly presented ideals |

Wednesday, Sep. 6, 4-5pm 129 Hayes-Healy Hall Department Colloquium |
Eric Riedl (Notre Dame) | Geometric Manin's and Lang's Conjectures |

Thursday, Sep. 7 | Keller VandeBogert (Notre Dame) | The Total Rank Conjecture in Characteristic Two |

Thursday, Sep. 14 | Anand Patel (Oklahoma State) | Counting cubic surfaces |

Thursday, Oct. 5 | Wenbo Niu (Arkansas) | Fundamental forms of algebraic varieties |

Friday, Oct. 6, 4-5pm 129 Hayes-Healy Hall Department Colloquium |
Claudia Polini (Notre Dame) | Rees algebras |

Thursday, Oct. 12 | Ethan Reed (Notre Dame) | An Isomorphism Theorem of Arithmetic Complexes |

Thursday, Oct. 19 | No seminar (Fall break) | |

Thursday, Oct. 26 | Justin Campbell (Chicago) | Langlands duality on the Beilinson-Drinfeld Grassmannian |

Thursday, Nov. 2 | Max Weinreich (Harvard) | TBA |

Thursday, Nov. 9 | Karthik Ganapathy (Michigan) | GL-equivariant modules over the infinite variable polynomial ring in positive characteristic |

Thursday, Nov. 16 127 Hayes-Healy Hall, 2:30-3:30pm Note special time / location |
Tamanna Chatterjee (Notre Dame) | Parabolic induction and parity sheaves for classical groups |

Thursday, Nov. 23 | No seminar (Thanksgiving) | |

Thursday, Nov. 30 127 Hayes-Healy Hall, 2:30-3:30pm Note special time / location |
Mahrud Sayrafi (Minnesota) | Splitting of vector bundles on toric varieties |

Thursday, Dec. 7 127 Hayes-Healy Hall, 2:30-3:30pm Note special time / location |
Teresa Yu (Michigan) | Standard monomial theory modulo Frobenius in characteristic two |

**Speaker**- Alessandra Costantini (Oklahoma State)
**Title**- Rees algebras of linearly presented ideals
**Abstract**- The Rees algebra of an ideal I is an invaluable tool in the study of the algebraic properties of I, as it encodes information on the asymptotic growth of the powers of I. Moreover, as Proj(R(I)) is the blowup of an affine scheme along V(I), Rees algebras represent an essential tool in the study of singularities. As the blowup construction describes Proj(R(I)) via parametric equations, a fundamental problem is to find the implicit equations of blowups. This is a difficult problem in general, as a priori one would need to determine all possible relations among the generators of all powers of I. In this talk, I will restrict to the case when I is a codimension-two perfect ideal in a polynomial ring k[x_1,...,x_d], admitting a presentation matrix consisting of linear entries. Most of the existing literature in this setting assumes the so-called G_d condition that the Fitting ideals Fitt_i(I) have codimension at least i+1 for i=1,...,d-1. Moving away from this assumption, we determine the defining ideal of the Rees algebra of I by requiring only that this codimension constraint is satisfied for i=1,...,d-2. This is part of joint work with Edward Price and Matthew Weaver.

**Speaker**- Keller VandeBogert (Notre Dame)
**Title**- The Total Rank Conjecture in Characteristic Two
**Abstract**- The total rank conjecture is a coarser version of the Buchsbaum-Eisenbud-Horrocks conjecture which, loosely stated, predicts that modules with large annihilators must also have "large" syzygies. In 2017, Walker proved that the total rank conjecture holds over rings of odd characteristic, using techniques that heavily relied on the invertibility of 2. In this talk, I will speak on joint work with Mark Walker where we settle (and generalize) the total rank conjecture over k-algebras of arbitrary characteristic. Our techniques take advantage of the classical Dold-Kan correspondence and allow us to prove an even stronger version of the total rank conjecture when k has characteristic 2.

**Speaker**- Anand Patel (Oklahoma State)
**Title**- Counting cubic surfaces
**Abstract**- The classical Thom-Porteous formulas essentially tell us how many times a matrix of a particular rank arises in a family of matrices. In other words, it tells us how many times a matrix is equivalent to the particular rank $r$ matrix of the form $\begin{pmatrix}1 &0 &\dots& 0\\ 0 & 1 & \dots & 0\\ \vdots & \vdots & \ddots & 0 \end{pmatrix}$ after performing row and column operations. The formulas themselves are certain expressions in chern classes of the vector bundles involved. The main point of my talk is to advertise the simple observation that these sorts of formulas continue to exist beyond linear algebra. Unfortunately, the new formulas are mostly intractable right now. In the ultra-classical world of cubic surfaces, however, Anand Deopurkar, Dennis Tseng and I did find some success, and my talk will focus primarily on this case. Our key formula unlocks things like: a general cubic surface arises 96120 times in a general 4-dimensional linear system of cubic surfaces, or a general cubic surface arises 42120 times as a hyperplane section of a general cubic threefold.

**Speaker**- Wenbo Niu (Arkansas)
**Title**- Fundamental forms of algebraic varieties
**Abstract**- Fundamental forms can be thought of as linear systems attached to the projectivization of Zariski tangent space at a nonsingular point of a variety. It was developed by the method of moving frames in differential geometry. In 1979, Griffiths-Harris used fundamental forms to study geometry of algebraic varieties and observed some vanishing phenomena. In this talk, I discuss a purely algebraic approach to the theory of fundamental forms without using moving frames. Furthermore, I will extend the vanishing of fundamental obtained by Griffiths-Harris and Landsberg to arbitrary order of fundamental forms. This is a joint work with L. Ein.

**Speaker**- Ethan Reed (Notre Dame)
**Title**- An Isomorphism Theorem of Arithmetic Complexes
**Abstract**- We consider generalizations of certain arithmetic complexes appearing in work of Raicu and VandeBogert in connection with the study of stable sheaf cohomology on flag varieties. Defined over the ring of integer valued polynomials, we prove an isomorphism of these complexes as conjectured by Gao, Raicu, and VandeBogert. In particular, this gives a more conceptual proof of an identification between the stable sheaf cohomology of hook and two column partition Schur functors applied to the cotangent sheaf of projective space. This talk is based on joint work with Luca Fiorindo, Shahriyar Roshan-Zamir, and Hongmiao Yu.

**Speaker**- Justin Campbell (Chicago)
**Title**- Langlands duality on the Beilinson-Drinfeld Grassmannian
**Abstract**- In recent joint work with Sam Raskin, we describe various categories of equivariant sheaves on the Beilinson-Drinfeld affine Grassmannian in Langlands dual terms. I will give an overview of this story, emphasizing some of the interesting infinite-dimensional geometry which occurs.

**Speaker**- Karthik Ganapathy (Michigan)
**Title**- GL-equivariant modules over the infinite variable polynomial ring in positive characteristic
**Abstract**- In the presence of a large group action, even non-noetherian rings sometimes behave like noetherian rings. For example, Cohen proved that every symmetric ideal in the infinite variable polynomial ring is generated by the orbit of finitely many polynomials. In this talk, I will introduce a nice and tractable class of modules over the infinite variable polynomial ring motivated by Cohen's theorem and explain how their structure becomes more complicated in positive characteristic.

**Speaker**- Tamanna Chatterjee (Notre Dame)
**Title**- Parabolic induction and parity sheaves for classical groups
**Abstract**- Parity sheaves are some constructible complexes defined on some stratified space where the strata satisfies some parity vanishing conditions. They are introduced by Carl Mautner, Daniel Juteau and Geordie Williamson in 2014. In characteristic 0 they coincide with the intersection cohomology complexes but in positive characteristic they are new and important objects. On flag variety they can be used as the "p-canonical basis" for Hecke algebras. It was noticed that on finite flag variety as well as affine grassmannian the parity sheaves correspond to the tilting sheaves. One expectation was to find similar relation on nilpotent cone, which Achar and Mautner started exploring in 2012. Another expectation was to understand the modular Springer correspondence in terms of parity sheaves. To study that one important conjecture has to be solved was made by Mautner. It says the parabolic induction functor defined on the nilpotent cones must preserves parity complexes. We break down this conjecture in two pieces and first try to prove that the parabolic induction functor sends parity sheaves associated to a cuspidal pair to a parity complex for classical groups. This is a ongoing project with Pramod N. Achar.

**Speaker**- Mahrud Sayrafi (Minnesota)
**Title**- Splitting of vector bundles on toric varieties
**Abstract**- In 1964, Horrocks proved that a vector bundle on a projective space splits as a sum of line bundles if and only if it has no intermediate cohomology. Generalizations of this criterion, under additional hypotheses, have been proven for other toric varieties, for instance by Eisenbud-Erman-Schreyer for products of projective spaces, by Schreyer for Segre-Veronese varieties, and Ottaviani for Grassmannians and quadrics. This talk is about a splitting criterion for arbitrary smooth projective toric varieties.

**Speaker**- Teresa Yu (Michigan)
**Title**- Standard monomial theory modulo Frobenius in characteristic two
**Abstract**- Over a field of characteristic zero, standard monomial theory and determinantal ideals provide an explicit decomposition of polynomial rings into simple GL_n-representations, which have characters given by Schur polynomials. In this talk, we present work towards developing an analogous theory for polynomial rings over a field of characteristic two modulo a Frobenius power of the maximal ideal generated by all variables. In particular, we obtain a filtration by modular GL_n-representations whose characters are given by certain truncated Schur polynomials, thus proving a conjecture by Gao-Raicu-VandeBogert in the characteristic two case. This is joint work with Laura Casabella.

**Speaker**- Matt Weaver (Notre Dame)
**Title**- Complete intersections and duality, with applications to Rees rings and tangent algebras
**Abstract**- The Rees algebra plays a central role in both algebraic geometry and commutative algebra, as it allows one to encode the data on the powers of an ideal, along with their syzygies. In this talk however, we consider the more general notion of Rees algebras of modules, which tend to be much more enigmatic. Typically one attempts to approximate these rings by a more understood algebra, namely the symmetric algebra and hopes to measure how much these two rings differ. In this talk, we associate a module measuring this difference and we show it is dual to the symmetric algebra when this is a bigraded complete intersection ring. We then discuss applications to determining the equations defining Rees rings of modules over complete intersection rings and, in particular, modules of K\"ahler differentials and their associated tangent algebras.

**Speaker**- François Greer (Michigan State)
**Title**- Compactifying special cycles
**Abstract**- A classical theorem of Borcherds and Zhang states that the cycle classes of Hodge loci in a moduli space of K3-type Hodge structures form the coefficients of a modular form. We investigate how well this theorem survives upon passing to a smooth compactification. As an application, we sketch a counterexample to the Severi Problem for rational surfaces.