# Toric Geometry Seminar

We meet on Wednesdays at 4pm in Sloan 159.

## Contents

A toric variety is a normal variety $X$ with a dense open algebraic torus $T\subseteq X$ so that $T$ acts on $X$ (in a way that extends $T$'s action on itself). People often imagine fixing $T$ (i.e. fixing a dimension), and then considering various ways of "partially compactifying" it to a toric variety. $T$ can be used to "sniff out" properties of $X$. For example, a regular function on $X$ restricts to a regular function on $T$. We (will) understand regular functions on $T$ very well, so a natural question to ask is, "which regular functions on $T$ come from regular functions on $X$?" By asking and answering these types of questions, one can show that toric varieties are determined by a combinatorial wigit, called a fan.

Much of toric geometry is devoted to building the dictionary between the combinatorics of fans (which are easy to think about) and the geometry of the corresponding toric varieties (which, at least initially, are hard to think about). The advantage of having such a dictionary at your command is that it allows you to quickly generate examples and perform calculations. Even if you are thinking about problems on non-toric varieties, working a few toric examples often reveals key insights. This approach is used to crack hard problems in algebraic geometry, but can also be used when learning the basics of the field.

## Schedule

 Date Speaker Topic Additional Resources Oct. 5 Chris Perez Affine Toric Varieties Oct. 12 Zeb Brady Toric Varieties and Fans Oct. 19 Jeff Manning Orbit-Cone Correspondence Oct. 26 Brian Hwang Toric Morphisms Nov. 2 Dori Bejleri Divisors Nov. 9 Dori Bejleri Divisors on Toric Varieties Nov. 16 Michel van Garrel Line Bundles Nov. 23 Michel van Garrel More on Line Bundles Nov. 30 Gjergji Zaimi Polytopes Dec. 6 Gjergji Zaimi Polytopes Jan. 18 Dori Bejleri Blow-ups, resolution of singularities Jan. 25 Ryan Properness, Chow's lemma Feb. 1 Anton Cox construction Feb. 8 Gjergji Topology of toric varieties Feb. 22 Gjergji Moment map Feb. 29 Brian Hwang The Symplectic Viewpoint (Toric Manifolds) Future ? Tropical Geometry Future ? Differential forms, Canonical sheaf

## Possible Topics

• affine toric varieties
• building a fan out of a toric variety
• building a toric variety out of a fan
• toric morphisms
• the orbit-cone correspondence
• Weil divisors, computing the class group
• Cartier divisors, line bundles, and their polytopes
• How to tell if a linear system is big, nef, ample, base-point free, etc?
• the Cox construction
• cohomology
• relation between tropical geometry and toric geometry.
• toric geometry and Ehrhart polynomials
• the moment map
• smoothness
• simplicialness
• properness of a map
• fiber bundles
• blow-ups

A list of abstracts from a toric geometry seminar at UC Berkeley years ago: [1]