(The seminar webpage is here.)

**Isabel Vogt (Stanford)**

September 27, 2019

**Stability of normal bundles of space curves**

The normal bundle controls the deformation theory of a curve embedded in projective space. In this talk we study the stability of normal bundles of curves in P^3 using degeneration. This is joint work with Izzet Coskun and Eric Larson.

**Jordan Ellenberg (Wisconsin)**

October 4, 2019

**Heights of rational points on (some) stacks**

The study of rational points on algebraic stacks over global fields is in many respects very similar to the familiar world of Diophantine geometry of schemes. But one key element of that world is missing; a theory of heights. I will propose such a theory and explain how it recovers many already-in-use notions of complexity for points on stacks, while also generating new ones. I talked about this project in the Stanford number theory seminar in spring 2018; I will explain the basic ideas again and then talk about some examples we understand better now than we did then, such as points on the moduli stack of abelian varieties, and discuss some questions that remain open for future work.

**Stefan Schreieder (Munich)**

October 11, 2019

**Stably irrational hypersurfaces of small slopes**

We show that over any uncountable field of characteristic different from two, a very general hypersurface of dimension n>2 and degree at least log_2(n)+2 is not stably rational. This improves earlier results of Koll\’ar and Totaro, who proved the same result under a linear bound on the degree.

**Jim Bryan (University of British Columbia)**

October 25, 2019, 2:30 pm

**K3 surfaces with symplectic group actions, enumerative geometry, and modular forms**

The Hilbert scheme parameterizing n points on a K3 surface X is a holomorphic symplectic manifold with rich properties. In the 90s it was discovered that the generating function for the Euler characteristics of the Hilbert schemes is related to both modular forms and the enumerative geometry of rational curves on X. We show how this beautiful story generalizes to K3 surfaces with a symplectic action of a group G. Namely, the Euler characteristics of the “G-fixed Hilbert schemes” parametrizing G-invariant collections of points on X are related to modular forms of level |G| and the enumerative geometry of rational curves on the stack quotient [X/G] . These ideas lead to some beautiful new product formulas for theta functions associated to root lattices. The picture also generalizes to refinements of the Euler characteristic such as chi_y genus and elliptic genus leading to connections with Jacobi forms and Siegel modular forms.

**Nathan Pflueger (Amherst)**

October 25, 2019, 4 pm

**Relative Richardson Varieties**

A Richardson variety is an intersection of two Schubert varieties defined by transverse flags in a vector space. Richardson varieties have many nice geometric properties; for example, a theorem of Knutson, Woo, and Yong shows that their singularities are completely determined by those of Schubert varieties. I will discuss a generalization of this theorem to a relative context, where the two transverse flags are replaced by a moving pair of flags in a vector bundle that become non-transverse at some points. I will also discuss a theorem describing the cohomology of the resulting relative Richardson variety. I will describe an application to Brill-Noether theory, and some related conjectures. This is joint work with Melody Chan.

**Rachel Pries (Colorado State)**

November 8, 2019

**The intersection of the Torelli locus with the non-ordinary locus in PEL-type Shimura varieties**

Around 1980, mathematicians developed several techniques to study the Newton polygon stratification of the moduli space of principally polarized abelian varieties in positive characteristic p. In 2004, Faber and Van der Geer used these techniques to prove that the Torelli locus of Jacobians of smooth curves intersects every p-rank stratum. In 2013, Viehmann and Wedhorn proved that every Newton polygon satisfying the Kottwitz conditions occurs on Shimura varieties of PEL-type. In most cases, it is still not known whether the Torelli locus intersects these Newton polygon strata. We provide a positive answer for the mu-ordinary and non-mu ordinary strata in infinitely many cases. As an application, we produce infinitely many new examples of unusual Newton polygons which occur for Jacobians of smooth curves. This is joint work with Li, Mantovan, and Tang.

**Eric Riedl (Notre Dame)**

November 15, 2019

**Linear subvarieties of hypersurfaces and unirationality**

The de Jong-Debarre Conjecture predicts that the space of lines on any smooth hypersurface of degree d <= n in P^n has dimension 2n-d-3. We prove this conjecture for n > 2d, improving on the previously-known exponential bounds. We prove an analogous result for k-planes, and use this generalization to prove that an arbitrary smooth hypersurface is unirational if n > 2^{d!}. This is joint work with Roya Beheshti.

**Maddie Weinstein (Berkeley)**

November 22, 2019

**Metric Algebraic Geometry**

Metric algebraic geometry is a term proposed for the study of properties of real algebraic varieties that depend on a distance metric. The distance metric can be the Euclidean metric in the ambient space or a metric intrinsic to the variety. In this talk, we introduce metric algebraic geometry through discussion of Voronoi cells, bottlenecks, offset hypersurfaces, and the reach of an algebraic variety. We also show applications to the computational study of the geometry of data with nonlinear models.