# Algebra and Algebraic Geometry Seminar Spring 2023

The seminar normally meets 2:30-3:30pm on Fridays, in the room VV B223.

## Algebra and Algebraic Geometry Mailing List

- Please join the AGS mailing list by sending an email to ags+join@g-groups.wisc.edu to hear about upcoming seminars, lunches, and other algebraic geometry events in the department (it is possible you must be on a math department computer to use this link).

## Spring 2023 Schedule

date | speaker | title | host/link to talk |
---|---|---|---|

February 3 | Dima Arinkin | Integrating symplectic stacks | Local |

February 24 | Ruijie Yang | Higher multiplier ideals | Maxim |

March 10 | Joerg Schuermann | Equivariant motivic characteristic classes of Schubert cells | Maxim |

March 24 | Daniel Corey | Degenerations of flag varieties and subdivisions of generalized permutahedra | Rodriguez |

March 31 | Tudor Padurariu | TBA | Maxim |

April 14 | Lena Ji | Rationality of conic bundle threefolds over non-closed field | Rodriguez/Ellenberg |

April 21 | Christopher O'Neill | Numerical semigroups, minimal presentations, and posets | Sobieska |

April 28 | Ayah Almousa | TBA | Rodriguez |

May 5 | James Hotchkiss | TBA | Caldararu |

May 12 | Yash Deshmukh | TBA | Caldararu |

## Abstracts

### Dima Arinkin

#### Integrating symplectic stacks

Shifted symplectic stacks, introduced by Pantev, Toën, Vaquie, and Vezzosi, are a natural generalization of symplectic manifolds in derived algebraic geometry. The word `shifted' here refers to cohomological shift, which can naturally occur in the derived setting: after all, the tangent space is now not a vector space, but a complex. Several classes of interesting moduli stacks carry shifted simplectic structures.

In my talk (based on a joint project with T.Pantev and B.Toën), I will present a way to generate shifted symplectic stacks. Informally, it involves integration along a (compact oriented) topological manifold X: starting with a family of shifted symplectic stacks over X, we produce a new stack of sections of this family, and equip it with a symplectic structure via an appropriate version of the Poincaré duality.

### Ruijie Yang

#### Higher multiplier ideals

For any Q-divisor D on a complex manifold X, there is a multiplier ideal associated to the pair (X,D), which is an ideal sheaf measuring the singularity of the pair and has many important applications in algebraic geometry, complex analytic geometry and commutative algebra. It turns out that this is only a small piece of a larger picture. In this talk, I will discuss the construction of a family of ideal sheaves associated to (X,D), indexed by an integer indicating the Hodge level, such that the lowest level recovers the usual multiplier ideals. We study their local and global properties systematically: the local properties rely on Saito's theory of mixed Hodge modules and some results inspired by Sabbah's theory of twistor D-modules; while the global properties need Sabbah-Schnell's theory of complex mixed Hodge modules and Beilinson-Bernstein’s theory of twisted D-modules from geometric representation theory. I will also compare this with the theory of Hodge ideals recently developed by Mustata and Popa. If time permits, I will discuss some application to the Riemann-Schottky problem via the singularity of theta divisors on principally polarized abelian varieties. This is based on the joint work in progress with Christian Schnell. I may also discuss some application to a homological characterization of higher rational and higher Du Bois singularities, based on joint work with Laurentiu Maxim.

### Joerg Schuermann

#### Equivariant motivic characteristic classes of Schubert cells

We explain in the context of complete flag varieties X=G/B the inductive calculation of equivariant motivic characteristic classes of Schubert cells via suitable Demazure-Lusztig operators, fitting with convolution actions of corresponding Hecke-algebras and Weyl groups. This is joint work with P. Aluffi, L. Mihalcea and C. Su.

### Daniel Corey

#### Degenerations of flag varieties and subdivisions of generalized permutahedra

We study the initial degenerations of the type-A flag varieties, and show how they are related to flag matroid strata and subdivisions of flag matroid polytopes. As applications, we give a complete proof of a conjecture of Keel and Tevelev on log canonical compactifications of moduli spaces of hyperplanes in projective space in general position, and study the Chow quotient of the complete flag variety by the diagonal torus of the projective linear group. This is based on joint work with Olarte and Luber.

### Lena Ji

#### Rationality of conic bundle threefolds over non-closed field

The intermediate Jacobian obstruction to rationality for complex threefolds was introduced by Clemens--Griffiths in their proof of the irrationality of the cubic threefold. For conic bundles over P^2, this obstruction characterizes rationality over the complex numbers. Recently, over non-closed fields k, Hassett--Tschinkel and Benoist--Wittenberg refined this obstruction by defining torsors over the intermediate Jacobian. For Fano threefolds of Picard rank 1, this refined obstruction can be used to characterize k-rationality. In this talk, we study the IJ torsor obstruction for conic bundles and explain why it does not characterize k-rationality in this higher Picard rank setting. This talk is based on joint work with S. Frei--S. Sankar--B. Viray--I. Vogt and joint work with M. Ji.

### Christopher O'Neill

#### Numerical semigroups, minimal presentations, and posets

A numerical semigroup is a subset S of the natural numbers that is closed under addition. One of the primary attributes of interest in commutative algebra are the relations (or trades) between the generators of S; any particular choice of minimal trades is called a minimal presentation of S (this is equivalent to choosing a minimal binomial generating set for the defining toric ideal of S). In this talk, we present a method of constructing a minimal presentation of S from a portion of its divisibility poset. Time permitting, we will explore connections to polyhedral geometry.

No familiarity with numerical semigroups or toric ideals will be assumed for this talk.