Algecom-XXI
Algebra, Geometry and Combinatorics Day (AlGeCom) is a one day, informal meeting of mathematicians from the University of Illinois, Purdue University, IUPUI, Washington University at St. Louis, Loyola University Chicago, DePaul University, University of Notre Dame, the University of Michigan and nearby universities, with interests in algebra, geometry and combinatorics (widely interpreted).
Algecom Committee:
Date: April 10
Location: Virtual, Hosted by University of Notre Dame.
Local Organizers (questions related to Algecom XXI):
Zhao Gao (zgao1@nd.edu)
Lizda Moncada (lmoncada@nd.edu)
Participants will have to register in order to receive the Zoom link for the talks!
Tentative Schedule:
(All times are in Eastern Time Zone units.)
9:30-10:30am Miruna-Ştefana Sorea (SISSA)
10:45-11:45am Nir Gadish (MIT)
12:45-1:45pm Nathan Pflueger (Amherst College)
2:00-3:00pm Patricia Klein (Minnesota)
Abstracts:
Abstract:
Möbius inversion is classically a procedure in number theory that inverts summation of functions over the divisors of an integer. A similar construction is possible for every locally finite poset, and is governed by a so called Möbius function encoding the combinatorics. In 1936 Hall observed that the values of the Möbius function are Euler characteristics of intervals in the poset, suggesting a homotopy theoretic context for the inversion.
In this talk we will discuss a functorial 'space-level' realization of Möbius inversion for diagrams taking values in a category with homotopy equivalences. The role of the Möbius function will be played by hömotopy types whose reduced Euler characteristics are the classical values, and inversion will hold up to extensions (think inclusion-exclusion but with the alternating signs replaced by even/odd spheres). This provides a uniform perspective to many constructions in topology and algebra. Notable examples that I hope to mention include handle decompositions, Koszul resolutions, and filtrations of hyperplane arrangements.
Abstract:
Geometric vertex decomposition and liaison are two frameworks that have been used to produce similar results about similar families of algebraic varieties. In this talk, we will describe an explicit connection between these approaches. In particular, we describe how each geometrically vertex decomposable ideal is linked by a sequence of elementary G-biliaisons of height 1 to an ideal of indeterminates and, conversely, how every G-biliaison of a certain type gives rise to a geometric vertex decomposition. As a consequence, we can immediately conclude that several well-known families of ideals are glicci, including Schubert determinantal ideals, defining ideals of varieties of complexes, and defining ideals of graded lower bound cluster algebras.
This connection also gives us a framework for implementing with relative ease Gorla, Migliore, and Nagel’s strategy of using liaison to establish Gr\"obner bases. We describe briefly, as an application of this work, a proof of a recent conjecture of Hamaker, Pechenik, and Weigandt on diagonal Gr\"obner bases of Schubert determinantal ideals.
This talk is based on joint work with Jenna Rajchgot.
Abstract:
Classical Brill-Noether theory concerns the following question: given a smooth curve C and two positive integers d and r, what is the geometry of the space of degree-d line bundles on C with at least r+1 linearly independent sections? These spaces, called Brill-Noether varieties, have interesting geometry that is closely linked to the combinatorics of Young tableaux. For example, for a general curve C, when a Brill-Noether variety is 0-dimensional, its degree is equal to the number of standard Young tableaux on a rectangular partition. For a twice-marked curve (C,p,q), one considers the space of degree-d line bundles L with a prescribed "rank function" r(a,b) = h^0(C, L(-ap-bq)). For (C,p,q) general, when this locus is 0-dimensional, its degree is equal to the number of reduced words for a permutation associated to the rank function. I will discuss these results and related generalizations and conjectures, emphasizing the interplay between the underlying geometry and combinatorics.
Abstract: