## Fall 2021, Minneapolis, MN

Our next meeting is October 30, 2021, at the University of Minnesota in Minneapolis, MN. We plan to hold this meeting in-person, but will decide by October 1, 2021, if we need to move the meeting to a virtual format. Below you'll find a schedule, titles and abstracts, and more, as they become available.

### Speakers

**Patricia Klein**(Minnesota)**Monica Lewis**(Minnesota)**Matthew Mastroeni**(Iowa State)**Victor Reiner**(Minnesota)**Botong Wang**(Wisconsin)

### Registration

Please register for the upcoming meeting here.

### Funding?

Funding is available for mileage and lodging. Please apply when you
register by September 27, 2021, to be considered for
funding.
We gratefully acknowledge NSF funding from
grant number DMS-2000390.

### Schedule

Please use the SOUTH entrance of Vincent Hall, as it is the only entrance that will be unlocked.

- Saturday:
- 8:30-9:00: Registration (Vincent 120)
- 9:00-9:50:
**Patricia Klein**Bumpless pipe dreams encode Gröbner geometry of Schubert polynomials (Vincent 16) - 10:00-10:30: Coffee and Refreshments (Vincent 120)
- 10:30-11:20:
**Matthew Mastroeni**Chow rings of matroids are Koszul (Vincent 16) - 11:30-12:20
**Botong Wang**Singular Hodge theory of matroids (Vincent 16) - 12:30-2:00: Catered Lunch and Organized Discussions (Vincent 120/Courtyard -- Bring appropriate outdoor clothing!)
- 2:00-2:50:
**Victor Reiner**Invariant theory of cyclic permutations: a test case (Vincent 16) - 3:00-3:50:
**Monica Lewis**The closed support problem for local cohomology (Vincent 16)

### Titles and Abstracts

**Bumpless pipe dreams encode Gröbner geometry of Schubert polynomials**

by Patricia Klein (Minnesota)

Knutson and Miller established a connection between the
anti-diagonal Gröbner degenerations of matrix Schubert
varieties and the pre-existing combinatorics of pipe dreams. They
used this correspondence to give a geometrically-natural
explanation for the appearance of the combinatorially-defined
Schubert polynomials as representatives of Schubert
classes. Recently, Hamaker, Pechenik, and Weigandt conjectured a
similar connection between diagonal degenerations of matrix
Schubert varieties and bumpless pipe dreams, newer combinatorial
objects introduced by Lam, Lee, and Shimozono.
We prove this conjecture in full generality. The proof provides
tools for assessing the Cohen--Macaulayness of equidimensional
unions of matrix Schubert varieties, of which alternating sign
matrix varieties are an important example. This talk will
include sufficient geometric and combinatorial background to
situate our main result as well as a description of open
problems in homological commutative algebra motivated by this
geometry and combinatorics.

**The closed support problem for local cohomology**

by Monica Lewis (Minnesota)

Local cohomology modules are (typically) very large algebraic objects that encode rich geometric information about the structure of a commutative ring. It is an open question whether the local cohomology of a Noetherian ring must have a finite set of minimal primes. A positive answer to this and to many stronger questions has been given for large classes of regular rings by endowing the local cohomology of R with the action of a suitable noncommutative ring of endomorphisms: either the ring of differential operators over R in characteristic 0, or the ring generated by R and its Frobenius homomorphism in characteristic p > 0. In this talk, we will review the classic finiteness theory over a regular ring, will discuss what is known to hold (and known to fail) in the singular setting, and will present recent results for certain classes of positive characteristic singularities.

**Chow rings of matroids are Koszul**

by Matthew Mastroeni (Iowa State)

The Chow ring of an algebraic variety is an algebro-geometric analog of the cohomology ring of a smooth manifold that encodes important information about the intersections between its subvarieties. Feichtner and Yuzvinsky computed a presentation for the Chow ring of a smooth toric variety associated to a matroid (and some other data) which is now called the Chow ring of the matroid. These rings have garnered significant attention in recent years thanks in part to their role in the resolution of Rota’s Conjecture by Adiprasito, Huh, and Katz and in the resolution of the Top-Heavy Conjecture by Braden, Huh, Matherne, Proudfoot, and Wang. From a commutative algebra standpoint, Chow rings of matroids are very nice graded Artinian Gorenstein rings defined by quadratic relations, and so, a natural conjecture posed by Dotsenko is that the Chow ring of a matroid is always Koszul. In this talk, we will discuss various techniques for determining when a ring is Koszul and how the combinatorics of a matroid influences algebraic properties of its Chow ring, culminating with very recent work joint with Jason McCullough giving an affirmative answer to Dotsenko’s conjecture.

**Invariant theory of cyclic permutations: a test case**

by Vic Reiner (Minnesota)

Invariant theory studies, for a group G acting on a polynomial ring S, both the internal structure of the G-invariant ring R, and the structure of S as an R-module. These are best understood when G is finite, and S has characteristic zero, but mysteries remain. This talk will review the very well-understood case where G is a reflection group, such as the symmetric group on n letters, and then pivot to a thornier test case, when G is generated by an n-cycle inside the symmetric group. Although these cyclic groups have several features in their favor (e.g., they are abelian and permutation groups), their invariant theory still contains mysteries. In particular, we will discuss some results and conjectures about them arising from work with summer 2021 REU students.

**Singular Hodge theory of matroids**

by Botong Wang (Wisconsin)

The de Bruijn--Erdos theorem states that the pairwise intersection of n lines in the projective plane is either a single point or consists of at least n points. As its higher dimensional generalization, Dowling and Wilson proposed the Top-heavy conjecture, which states that in a rank d matroid, the number of rank k flats is always less or equal to number of rank d-k flats for k ≤ d/2. I will give an overview of a proof of the conjecture in the realizable case, which is a joint work with June Huh. The proof uses the singular Hodge theory of certain algebraic varieteis called the Schubert varieties of hyperplane arrangements. I will also give a brief summary of a more recent work, further joint with Tom Braden, Jacob Matherne and Nick Proudfoot, which proves the Top-heavy conjecture in full generality.

### Lodging and Travel Info

Those participants who are granted funding will be housed at a hotel within walking distance of our meeting. The conference organizers will arrange rooms for funded participants.

Recommended parking is in the Church Street Garage and the Washington Avenue Ramp. Here is a campus map.

We expect that out-of-town participants will drive, arriving on Friday, October 29, and depart on the afternoon of Saturday, October 30.