RUTGERS ALGEBRA SEMINAR -Fall 2022
Wednesdays at 2:00-3:00 PM in H705

A more comprehensive listing of all Math Department seminars is available.
Here is a link to the algebra seminars in previous semesters


Fall 2022 Seminars (Wednesdays at 2:00 AM in H705)
14 Sept Laurent Vera (RU) "Super-equivalences and odd categorification of sl2"
21 Sept Tamar Blanks (RU)  "Trace forms and the Witt invariants of finite groups"
28 Sept Lauren Heller (Berkeley) "Characterizing multigraded regularity on products of projective spaces"
 5 Oct
12 Oct Yael Davidov (RU)  "Admissibility of Groups over Semi-Global Fields in the 'Bad Characteristic' Case"
19 Oct Mandi Schaeffer Fry (Metropolitan State U) TBA (group representation theory)
26 Oct
 2 Nov
 9 Nov
Fall 2022 classes begin Tuesday September 6 and end Wednesday Dec. 14.
Spring 2022 Seminars (Wednesdays at 2:00 PM in H705)
no seminar January 19 as we start in remote mode
26 Jan Weihong Xu (Rutgers) "Quantum $K$-theory of Incidence Varieties" (remote)
 2 Feb Rudradip Biswas (Manchester) "Cofibrant objects in representation theory" (remote) 
 9 Feb Chuck Weibel (RU) "An introduction to monoid schemes"
16 Feb Ian Coley  (RU)  "Hochster's description of Spec(R)"
23 Feb no   seminar           --------------------------------------------
 2 Mar Alexei Entin (Tel Aviv)  "The minimal ramification problem in inverse Galois theory" (remote)
 9 Mar Pham Tiep (RU)   "Representations and tensor product growth"
16 Mar   no seminar      ------------------- Spring Break ----------
30 Mar Tim Burness (Bristol, UK) "Fixed point ratios for primitive groups and applications"
 6 Apr Eugen Rogozinnikov (Strasbourg) "Hermitian Lie groups of tube type as symplectic groups over noncommutative algebras"
13 Apr Eilidh McKemmie (RU) "A survey of various random generation problems for finite groups"
20 Apr Yael Davidov (RU)     "Exploring the admissibility of Groups and an Application of Field Patching"
27 Apr Daniel Douglas (Yale) "Skein algebras and quantum trace maps" 
 4 May
Spring 2022 classes begin Tuesday January 18 and end Monday May 2.
Spring break is March 12-20, 2022

Fall 2021 Seminars (Wednesdays at 11:00 AM in H525)
15 Sept Yom Kippur
29 Sept Yoav Segev   "A characterization of the quaternions using commutators"
 6 Oct Lev Borisov (RU) "Explicit equations for fake projective planes"
13 Oct Ian Coley   (RU) "Introduction to topoi"
20 OctAnders Buch (RU) POSTPONED to 12/8 ("Tevelev Degrees")
27 Oct Eilidh McKemmie (RU) "The probability of generating invariably a finite simple group"
 3 Nov Ian Coley and Chuck Weibel  "Localization, and the K-theory of monoid schemes" 
10 Nov Max Peroux (Penn) "Equivariant variations of topological Hochschild homology"
17 Nov Shira Gilat (RU)    "The infinite Brauer group"
24 Nov --- no seminar ---   Thanksgiving is Nov. 25
29 Nov (Monday) Wednesday class schedule 
 1 Dec --- no seminar ---   
 8 Dec Anders Buch (RU) "Tevelev Degrees"
Fall 2021 classes begin Tuesday September 1 and end Monday Dec. 13.
Spring 2021 Seminars (Wednesdays at 2:00 PM, on-line)
27 Jan Ian Coley (Rutgers) "Tensor Triangulated Geometry?"
 3 Feb Aline Zanardini (U.Penn) "Stability of pencils of plane curves"
10 Feb Svetlana Makarova (U.Penn) "Moduli spaces of stable sheaves over quasipolarized K3 surfaces, and Strange Duality"
17 Feb no seminar
24 Feb Patrick McFaddin (Fordham) "Separable algebras and rationality of arithmetic toric varieties"
 3 Mar Christian Klevdal (U.Utah) "Integrality of G-local systems"
10 Mar Justin Lacini   (U.Kansas) "On log del Pezzo surfaces in positive characteristic"
17 Mar  no seminar      ------------------- Spring Break ----------
24 Mar John Kopper (Penn State)  "Ample stable vector bundles on rational surfaces"
14 Apr Allechar Serrano Lopez (U.Utah) "Counting elliptic curves with prescribed torsion over imaginary quadratic fields"
21 Apr Franco Rota (Rutgers)   "Motivic semiorthogonal decompositions for abelian varieties"
28 Apr Ben Wormleighton (Washington U./St.Louis) "Geometry of mutations: mirrors and McKay"
 5 May Morgan Opie (Harvard)      "Complex rank 3 vector bundles on CP5"
12 May Avery Wilson (N. Carolina) "Compactifications of moduli of G-bundles and conformal blocks"
Spring 2021 classes begin Tuesday January 19 and end Monday May 3.

Fall 2020 Seminars (Wednesdays at 2:00 PM, on-line)
 9 Sept Lev Borisov  Rutgers "A journey from the octonionic P2 to a fake P2"
16 Sept Stefano Filipazzi (UCLA) "On the boundedness of  n-folds of Kodaira dimension n-1"
23 Sept Yifeng Huang (U.Michigan) "Betti numbers of unordered configuration spaces of a punctured torus"
30 Sept Andrea Ricolfi (SISSA, Italy) "Moduli of semiorthogonal decompositions"
 7 Oct Giacomo Mezzedimi (Hannover) "The Kodaira dimension of some moduli spaces of elliptic K3 surfaces"
14 Oct Katrina Honigs  (Oregon)  "An obstruction to weak  approximation on some Calabi-Yau threefolds"
21 Oct Alex Wertheim (UCLA)  "Degree One Milnor K-Invariants of Groups of Multiplicative Type"
28 Oct Inna Zakharevich (Cornell) 3:30PM Colloquium "The Dehn complex: scissors congruence, K-theory, and regulators"
 4 Nov Michael Wemyss (Glasgow)   "Tits cone intersections and Applications"
11 Nov Pieter Belmans (Univ. Bonn) 12:00 noon "Graph potentials as mirrors to moduli of vector bundles on curves"
18 Nov David Hemminger (UCLA) "Lannes' T-functor and Chow rings of classifying spaces"
25 Nov --- no seminar ---   Thanksgiving is Nov. 26; Friday class schedule 
 2 Dec Clover May  (UCLA) "Classifying perfect complexes of Mackey functors"
 9 Dec Be'eri Greenfeld (UCSD) "Combinatorics of words, symbolic dynamics and growth of algebras"

Fall 2020 classes begin Tuesday September 1 and end Thursday December 10
Spring 2020 Seminars (Wednesdays at 2:00 in H425)
Until March 2020, the Algebra Seminar met on Wednesdays at 2:00-3:00PM in the Hill Center, on Busch Campus of Rutgers University. After Spring Break, the seminar moved on-line.
22 Jan Shira Gilat (Bar-Ilan U.) "Higher norm principles for norm varieties"
12 Feb Chuck Weibel (Rutgers) "The K'-theory of monoid sets"
19 Feb Linhui Shen (Michigan State) "Quantum geometry of moduli spaces of local system"
26 Feb Lev Borisov  Rutgers    "Six explicit pairs of fake projective planes"
 4 Mar Saurabh Gosavi (Rutgers) "Generalized Brauer dimension"
18 Mar  no seminar      ------------------- Spring Break ----------
25 Mar  CANCELLED  -- Future seminars moved on-line
 1 Apr Joaquin Moraga (Princeton) "On the Jordan property for local fundamental groups"
 8 Apr Ian Coley (Rutgers)      "Higher K-theory via generators and relations"
15 Apr Franco Rota (Rutgers)    "Moduli spaces on the Kuznetsov component of Fano threefolds of index 2"
22 Apr James Cameron (UCLA)     "Group cohomology rings via equivariant cohomology"
29 Apr Luca Schaffler (U. Mass) "Compactifications of moduli of points and lines in the projective plane"
 6 May Angela Gibney (Rutgers)    CANCELLED 
Spring 2020 classes begin Tuesday January 22 and end Monday May 4


Fall 2019 Seminars (Wednesdays at 2:00 in H525)
18 Sep Sándor Kovács (U.Washington) "Rational singularities and their cousins in arbitrary characteristics"
20 Sep Jacob Lurie (IAS) "On Makkai's Strong Conceptual Completeness Theorem"
25 Sep Saurabh Gosavi (Rutgers) "Generalized Brauer dimension and other arithmetic invariants of semi-global fields"
 2 Oct Danny Krashen (Rutgers)  "The arithmetic of semiglobal fields via combinatorial topology"
 9 Oct Ian Coley (Rutgers)      "What is a derivator?" 
16 Oct Sumit Chandra Mishra (Emory U.) "Local-global principle for norm over semi-global fields"
23 Oct Chengxi Wang (Rutgers)   "strong exceptional collections of line bundles" 
30 Oct Angela Gibney (Rutgers)  "Vertex algebras of CohFT-type"
 6 Nov Robert Laugwitz (Nottingham, UK) "dg categories and their actions"
13 Nov Volodia Retakh (Rutgers)  "Noncommutative Laurent Phenomenon: two examples"
20 Nov Carl Lian  (Columbia)     "Enumerating pencils with moving ramification on curves"
27 Nov --- no seminar ---        Thanksgiving is Nov. 28; Friday class schedule 
11 Dec Diane Mclagan (U.Warwick) "Tropical scheme theory"
Fall 2019 classes begin Tuesday September 3 and end Wednesday Dec.11. Finals are December 14-21, 2019


Spring 2019 Seminars (Wednesdays at 2:00 in SERC 206)
Note: First 3 seminars were in H-005; 4th in H705; all others in SERC-206
23 Jan Patrick Brosnan  U.Maryland "Palindromicity and the local invariant cycle theorem"
30 Jan Khashayar Sartipi  UIUC "Paschke Categories, K-homology and the Riemann-Roch Transformation"
 6 Feb Chuck Weibel   Rutgers  "The Real graded Brauer group"
13 Feb Volodia Retakh Rutgers "An analogue of mapping class groups and noncommutative triangulated surfaces"
20 Feb Dawei Chen Boston College&IAS "Volumes and intersection theory on moduli spaces of abelian differentials"
 6 Mar no seminar  
13 Mar Jeanne Duflot Colorado State U. "A Degree Formula for Equivariant Cohomology"
20 Mar  no seminar      ------------------- Spring Break ----------
27 Mar Louis Rowen  Bar-Ilan Univ  "The algebraic theory of systems"
 3 Apr Iulia Gheorghita Boston College  "Effective divisors in the Hodge bundle"
10 Apr Gabriel Navarro  U.Valencia "Character Tables and Sylow Subgroups of Finite Groups"
17 Apr John Sheridan Stony Brook "Continuous families of divisors on symmetric powers of curves"
24 Apr Yaim Cooper  IAS  "Severi degrees via representation theory"
 1 May Dave Anderson  Ohio State  "Schubert calculus and the Satake correspondence"
Classes end Monday May 6; Finals are May 9-15, 2019


Fall 2018 Seminars (Wednesdays at 2:00 in H525)
19 Sep Nicola Tarasca  Rutgers  "Geometry and Combinatorics of moduli spaces of curves" 
26 Sep Angela Gibney  Rutgers  "Basepoint free loci on $M_{0,n}$-bar from Gromov-Witten theory of smooth homogeneous varieties"
 5 Oct(FRI) Michael Larsen Indiana U  "Irrationality of Motivic Zeta Functions"
    *** Friday at 10:00 AM in Hill 005 ***
10 Oct Yotam Hendel  Weizmann Inst. "On singularity properties of convolutions of algebraic morphisms"
17 Oct Qixiao Ma    Columbia Univ.  "Brauer class over the Picard scheme of curves"
24 Oct Sandra Di Rocco KTH-Sweden   "Generalized Polar Geometry"
31 Oct Igor Rapinchuk  Michigan State "Algebraic groups with good reduction and unramified cohomology" 
 7 Nov Isabel Vogt     MIT          "Low degree points on curves"
14 Nov Bob Guralnick   USC         "Low Degree Cohomology" 
21 Nov --- no seminar ---        Thanksgiving is Nov. 22; Friday class schedule 
28 Nov Julie Bergner  U.Virginia  "2-Segal spaces and algebraic K-theory"
 5 Dec Chengxi Wang     Rutgers    "Quantum Cohomology of Grassmannians"
12 Dec Patrick Brosnan  U.Maryland  POSTPONED
Classes end Wednesday Dec. 12; Finals begin Dec. 15, 2018

Here is a link to the algebra seminars in previous semesters

Abstracts of seminar talks


Fall 2022


Admissibility of Groups over Semi-Global Fields in the “Bad Characteristic” Case (Yael Davidov, Oct.12, 2022)
We say a finite group, G, is admissible over a field, F, if there exists a division algebra with center F and a maximal subfield K such that K/F is Galois with group G. The question of which groups are admissible over a given field is generally difficult to answer but has been solved in the case that F is a transcendence degree 1 extension of a complete discretely valued field with algebraically closed residue field, so long as the characteristic of the residue field does not divide the order of the group. This result was obtained in a paper by Harbater, Hartmann and Krashen using field patching techniques in 2009.
In this talk we will be discussing progress towards generalizing this result and trying to answer the question, what happens when the characteristic of the residue field does divide the order of G? We will restrict our attention to a special case to make the discussion accessible.


Trace forms and the Witt invariants of finite groups (Tamar Blanks, Sept. 21, 2022):
A Witt invariant of an algebraic group G over a field k is a natural transformation from G-torsors to the Witt ring, that is, a rule that assigns quadratic forms to algebraic objects in a way that respects field extensions over k. An important example is the invariant sending each etale algebra to its trace form. Serre showed that the ring of Witt invariants of the symmetric group is generated by the trace form invariant and its exterior powers.
In this talk we will discuss work towards generalizing Serre's result to other Weyl groups, and more generally to other finite groups. We will also describe the connection between Witt invariants and cohomological invariants via the Milnor conjecture.


Super-equivalences and odd categorification of sl2 (Laurent Vera, Sept. 14, 2022):
In their seminal work on categorifications of quantum groups, Chuang and Rouquier showed that an action of sl2 on a category gives rise to derived equivalences. These equivalences can be used to prove Broué’s abelian defect group conjecture for symmetric groups.
In this talk, I will present a “super version” of these results. I will introduce the odd 2-category associated with sl2 and describe the properties of its 2-representation theory. I will then describe the super analogues of the Chuang-Rouquier complexes and explain how they give rise to derived equivalences on 2-representations. These derived equivalences lead to a proof of the abelian defect group conjecture for spin symmetric groups. This is joint work with Mark Ebert and Aaron Lauda.

Spring 2022


Skein algebras and quantum trace maps (Daniel Douglas, April 27, 2022):
Skein algebras are certain noncommutative algebras associated to surfaces, appearing at the interface of low-dimensional topology, representation theory, and combinatorics. They occur as quantum deformations of character varieties with respect to their natural Poisson structure, and in particular possess fascinating connections to quantum groups. In this talk, I will discuss the problem of embedding skein algebras into quantum tori, the latter of which have a relatively simple algebraic structure. One such embedding, called the quantum trace map, has been used to shed light on the representation theory of skein algebras, and is related to Fock and Goncharov’s quantum higher Teichmüller theory.


Exploring the admissibility of Groups and an Application of Field Patching (Yael Davidov, April 20, 2022):
Similarly to the inverse Galois problem, one can ask if a group G is admissible over a given field F. This is answered in the affirmative if there exists a division algebra with F as its center that contains a maximal subfield that is a Galois extension of F, with Galois group G.
We will review admissibility results over the rationals that have been proven by Schacher and Sonn. We will also give some idea as to how one might try to construct division algebras that prove the admissibility of a particular group. Finally, we will briefly outline how Harbater, Hartmann and Krashen were able to obtain admissibility criteria for groups over a particular class of fields using field patching techniques.


Hermitian Lie groups of tube type as symplectic groups over noncommutative algebras (Eugen Rogozinnikov, Aprli 6, 2022):
We introduce the symplectic group Sp_2(A,σ) over a noncommutative algebra A with an anti-involution σ. We realize several classical Lie groups as Sp2 over various noncommutative algebras, which provides new insights into their structure theory. We construct several geometric spaces, on which the groups Sp2(A,σ) act. We introduce the space of isotropic A-lines, which generalizes the projective line. We describe the action of Sp2(A,σ) on isotropic A-lines, generalize the Kashiwara-Maslov index of triples and the cross ratio of quadruples of isotropic A-lines as invariants of this action.
When the algebra A is Hermitian or the complexification of a Hermitian algebra, we introduce the symmetric space XSp2(A,σ), and construct different models of this space. Applying this to classical Hermitian Lie groups of tube type (realized as Sp2(A,σ)) and their complexifications, we obtain different models of the symmetric space as noncommutative generalizations of models of the hyperbolic plane and of the three-dimensional hyperbolic space.


Fixed point ratios for primitive groups and applications (Tim Burness, March 30, 2022):
Let G be a finite permutation group and recall that the fixed point ratio of an element x, denoted fpr(x), is the proportion of points fixed by x. Fixed point ratios for finite primitive groups have been studied for many decades, finding a wide range of applications.
In this talk, I will present some of the main results and applications, focussing on recent joint work with Bob Guralnick where we determine the triples (G,x,r) such that G is primitive, x has prime order r and fpr(x) > 1/(r+1). The latter result allows us to prove new results on the minimal degree and minimal index of primitive groups, and we have used it in joint work with Moreto and Navarro to study the commuting probability of p-elements in finite groups.


Representations and tensor product growth (Pham Tiep, March 9, 2022):
The deep theory of approximate subgroups establishes 3-step product growth for subsets of finite simple groups G of Lie type of bounded rank. We will discuss 2-step growth results for representations of such groups G (including those of unbounded rank), where products of subsets are replaced by tensor products of representations. This is joint work with M. Larsen and A. Shalev.


The minimal ramification problem in inverse Galois theory (Alexei Entin, March 2, 2022):
For a number field K and a finite group G the Boston-Markin Conjecture predicts the minimal number of ramified places (of K) in a Galois extension L/K with Galois group G. The conjecture is wide open even for the symmetric and alternating groups S_n, A_n over the field of rational numbers Q. We formulate a function field version of this conjecture, settle it for the rational function field K=F_q(T) and G=S_n with a mild restriction on q,n and make significant progress towards the G=A_n case. We also discuss some other groups and the connection between the minimal ramification problem and the Abhyankar conjectures on the etale fundamental group of the affine line in positive characteristic.


An introduction to monoid schemes (Chuck Weibel, Feb. 9, 2022):
If A is a pointed abelian monoid, its prime ideals make sense and form a topological space, analogous to Spec of a ring; the notion of a monoid scheme is analogous to the notion of a scheme in Algebraic Geometry. The monoid ring construction k[A] gives a link to geometry. In this talk I will give an introduction to the basic ideas, including toric monoid schemes, which model toric varieties.


Cofibrant objects in representation theory (Rudradip Biswas, Feb. 2, 2022):
Cofibrant modules, as defined by Benson, play important roles in many cohomology questions of infinite discrete groups. In this talk, I will (a) talk about my new work on the relation between the class of these modules and Gorenstein projectives where I'll build on Dembegioti-Talelli's work, and (b) highlight new results from one of my older papers on the behaviour of an invariant closely related to these modules. If time permits, I'll show a possible generalization of many of these results to certain classes of topological groups.


Quantum $K$-theory of Incidence Varieties (Weihong Xu, Jan. 26, 2022):
Buch, Mihalcea, Chaput, and Perrin proved that for cominuscule flag varieties, (T-equivariant) K-theoretic (3-pointed, genus 0) Gromov-Witten invariants can be computed in the (equivariant) ordinary K-theory ring. Buch and Mihalcea have a related conjecture for all type A flag varieties.
In this talk, I will discuss work that proves this conjecture in the first non-cominuscule case--the incidence variety Fl(1,n-1;n). The proof is based on showing that Gromov-Witten varieties of stable maps to Fl(1,n-1;n) with markings sent to a Schubert variety, a Schubert divisor, and a point are rationally connected. As applications, I will also discuss positive formulas that determine the equivariant quantum K-theory ring of Fl(1,n-1;n). The talk is based on the arxiv preprint at https://arxiv.org/abs/2112.13036.


Fall 2021


Tevelev degrees (Anders Buch, December 8, 2021):
Let X be a non-singular complex projective variety. The virtual Tevelev degree of X associated to (g,d,n) is the (virtual) degree of the forgetful map from theKontsevich moduli space Mg,n(X,d) of n-pointed stable maps to X of genus g and degree d, to the product Mg,n × Xn. Recent work of Lian and Pandharipande shows that this invariant is enumerative in many cases, that is, it is the number of degree-d maps from a fixed genus-g curve to X, that send n fixed points in the curve to n fixed points in X.
I will speak about a simple formula for this degree in terms of the (small) quantum cohomology ring of X. If X is a Grassmann variety (or more generally, a cominuscule flag variety) then the virtual Tevelev degrees of X can be expressed in terms of the (real) eigenvalues of a symmetric endomorphism of the quantum cohomology ring. If X is a complete intersection of low degree compared to its dimension, then the virtual Tevelev degrees of X are given by an explicit product formula. I will do my best to keep this talk student-friendly, so the most of it will be about explaining the ingredients of this abstract. The results are joint work with Rahul Pandharipande.


Equivariant variations of topological Hochschild homology (Maximilien Peroux, November 10, 2021):
Topological Hochschild homology (THH) is an important variant for rings and ring spectra. It is built as a geometric realization of a cyclic bar construction. It is endowed with an action of the circle, because it is a geometric realization of a cyclic object. The simplex category factors through Connes' category Λ. Similarly, real topological Hochschild homology (THR) for rings (and ring spectra) with anti-involution is endowed with a O(2)-action. Here instead of the cyclic category Λ, we use the dihedral category Ξ.
From work in progress with Gabe Angelini-Knoll and Mona Merling, I present a generalization of Λ and Ξ called crossed simplicial groups, introduced by Fiedorwicz and Loday. To each crossed simplical group G, I define THG, an equivariant analogue of THH. Its input is a ring spectrum with a twisted group action. THG is an algebraic invariant endowed with different action and cyclotomic structure, and generalizes THH and THR.


Localization, and the K-theory of monoid schemes (Ian Coley and Chuck Weibel, November 3, 2021):
We develop the K-theory of sets with an action of a pointed monoid (or monoid scheme), analogous to the $K$-theory of modules over a ring (or scheme).
In order to form localization sequences, we construct the quotient category of a nice regular category by a Serre subcategory. A special case is the localization of an abelian category by a Serre subcategory.


The probability of generating invariably a finite simple group (Eilidh McKemmie, October 27, 2021):
We say a group is invariably generated by a subset if every tuple in the product of conjugacy classes of elements in that subset is a generating tuple. We discuss the history of computational Galois theory and probabilistic generation problems to motivate some results about the probability of generating invariably a finite simple group, joint work with Daniele Garzoni. We also highlight some methods for studying probabilistic invariable generation.


Introduction to topoi (Ian Coley, October 13, 2021):
The theory of sheaves on a topological space or scheme admits a generalization to sheaves on a category equipped with a topology, which we call a site. This level of generality allows us access to interesting cohomology theories on schemes that don't make sense at the point-set level. We'll give the basic definitions, warm up by categorifying the notion of sheaves on a topological space, then get into these new topologies and their associated sheaf cohomologies.


Explicit equations for fake projective planes (Lev Borisov, October 6, 2021):
There are 50 complex conjugate pairs of fake projective planes, realized as quotients of the complex 2-ball. However, in most cases there are no known explicit embeddings into a projective space. In this talk I will describe my work over the past several years (with multiple co-authors) which resulted in explicit equations for 9 out of the 50 pairs. It is a wild ride in the field of computer assisted AG computations.


A characterization of the quaternions using commutators (Yoav Segev, September 29, 2021):
Let D be a quaternion division algebra over a field F. Thus D=F +F i +F j+ F k, with i^2, j^2 in F and k=ij=-ji. A pure quaternion is an element p in D such that p is in F i+F j+F k.
It is easy to check that p^2 is in F, for a pure quaternion p, and that given x,y in D, the commutator (x,y)=xy-yx is a pure quaternion.
We show that this characterizes quaternion division algebras, namely, any associative ring R with 1, such that the commutator (x,y) is not a zero divisor and satisfies (x,y)^2 is in the center of R, for all nonzero x,y in R, is a quaternion division algebra. The proof is elementary and self contained.
This is joint work with Erwin Kleinfeld


Spring 2021


Compactifications of moduli of G-bundles and conformal blocks (Avery Wilson, May 12, 2021):
I will talk about Schmitt and Munoz-Castaneda's compactification of the moduli space of G-bundles on a curve and its relation to conformal blocks. I use this compactification to prove finite generation of the conformal blocks algebra over the stack of stable curves of genus >1, which Belkale-Gibney had previously proven for G=SL(r). This yields a nice compactification for the relative moduli space of G-bundles.


Complex rank 3 vector bundles on CP5 (Morgan Opie, May 5,2021:
Given the ubiquity of vector bundles, it is perhaps surprising that there are so many open questions about them -- even on projective spaces. In this talk, I will outline my ongoing work on complex rank 3 topological vector bundles on CP5. I will describe a classification of such bundles, involving a connection to topological modular forms. I will also discuss a topological, rank-preserving additive structure which allows for the construction of new rank 3 bundles on CP^5 from "simple" ones. This construction is an analogue to an algebraic construction of Horrocks. As time allows, I will discuss future algebro-geometric directions related to this project.


Geometry of mutations: mirrors and McKay (Ben Wormleighton, April 28, 2021):
There are several notions of mutation that arise in different parts of algebra, geometry, and combinatorics. I will discuss some of these appearances in mirror symmetry and in the McKay correspondence with a view towards approaching classification problems for Fano varieties and for crepant resolutions of orbifold singularities.


Motivic semiorthogonal decompositions for abelian varieties (Fanco Rota, April 21, 2021):
A motivic semiorthogonal decomposition is the decomposition of the derived category of a quotient stack [X/G] into components related to the "fixed-point data". They represent a categorical analog of the Atiyah-Bott localization formula in equivariant cohomology, and their existence is conjectured for finite G (and an additional smoothess assumption) by Polishchuk and Van den Bergh.
I will present joint work with Bronson Lim, in which we construct a motivic semiorthogonal decomposition for a wide class of smooth quotients of abelian varieties by finite groups, using the recent classification by Auffarth, Lucchini Arteche, and Quezada.


Counting elliptic curves with prescribed torsion over imaginary quadratic fields (Allechar Serrano Lopez, April 14, 2021):
A generalization of Mazur's theorem states that there are 26 possibilities for the torsion subgroup of an elliptic curve over a quadratic extension of Q. If G is one of these groups, we count the number of elliptic curves of bounded naive height whose torsion subgroup is isomorphic to G in the case of imaginary quadratic fields.


Ample stable vector bundles on rational surfaces (John Kopper, March 24, 2021):
Ample vector bundles are among the most important "positive" vector bundles in algebraic geometry, but have resisted attempts at classification, especially in dimensions two and higher. In this talk, I will discuss a moduli-theoretic approach to this problem that dates to Le Potier and is particularly powerful on rational surfaces: study Chern characters for which the general stable bundle is ample.
After reviewing the ideas of stability and ampleness for vector bundles, I will discuss some new results in this direction for minimal rational surfaces. First, I will give a complete classification of Chern characters on these surfaces for which the general stable bundle is both ample and globally generated. Second, I will explain how this classification also holds in an asymptotic sense without the assumption of global generation. This is joint work with Jack Huizenga.


On log del Pezzo surfaces in positive characteristic (Justin Lacini, March 10, 2021):
A log del Pezzo surface is a normal surface with only Kawamata log terminal singularities and anti-ample canonical class. Over the complex numbers, Keel and McKernan have classified all but a bounded family of log del Pezzo surfaces of Picard number one.
In this talk we will extend their classification to positive characteristic. In particular, we will prove that for p>3 every log del Pezzo surface of Picard number one admits a log resolution that lifts to characteristic zero over a smooth base. As a consequence, we will see that Kawamata-Viehweg vanishing holds in this setting. Finally, we will conclude with some counterexamples in characteristic two, three and five.


Integrality of G-local systems (Christian Klevdal, March 3,2021):
Simpson conjectured that for a reductive group G, rigid G-local systems on a smooth projective complex variety are integral. I will discuss a proof of integrality for cohomologically rigid G-local systems. This generalizes and is inspired by work of Esnault and Groechenig for GL_n. Surprisingly, the main tools used in the proof (for general G and GL_n) are the work of L. Lafforgue on the Langlands program for curves over function fields, and work of Drinfeld on companions of \ell-adic sheaves. The major differences between general G and GL_n are first to make sense of companions for G-local systems, and second to show that the monodromy group of a rigid G-local system is semisimple.
All work is joint with Stefan Patrikis.


Separable algebras and rationality of arithmetic toric varieties (Patrick McFaddin, February 24, 2021):
The class of toric varieties defined over the complex numbers gives a robust testing ground for computing various invariants, e.g., algebraic K-theory and derived categories. To obtain a broader sense of the capabilities of these invariants, we look to the arithmetic setting and twisted forms of toric varieties. In this talk, I will discuss work on distinguishing forms of toric varieties using separable algebras and how this sheds light on the connection between derived categories and rationality questions. This is joint work with M. Ballard, A. Duncan, and A. Lamarche.


Moduli spaces of stable sheaves over quasipolarized K3 surfaces, and Strange Duality (Svetlana Makarova, February 10, 2021):
I will talk about a construction of relative moduli spaces of stable sheaves over the stack of quasipolarized surfaces. For this, I first retrace some of the classical results in the theory of moduli spaces of sheaves on surfaces to make them work over the nonample locus. Then I will recall the theory of good moduli spaces, whose study was initiated by Alper and concerns an intrinsic (stacky) reformulation of the notion of good quotients from GIT. Finally, I use a criterion by Alper-Heinloth-Halpern-Leistner, coupled with some categorical arguments, to prove existence of the good moduli space.


Stability of pencils of plane curves (Aline Zanardini, February 3, 2021):
I will discuss some recent results on the problem of classifying pencils of plane curves via geometric invariant theory. We will see how the stability of a pencil is related to the stability of its generators, to the log canonical threshold of its members, and to the multiplicities of its base points.


What is Tensor Triangulated Geometry? (Ian Coley, January 27, 2021):
Based on work of Thomason, Balmer defined a way to think about varieties from a purely category-theoretic point of view. By considering not only the triangulated structure of the derived category but also the tensor product, one can (nearly) do geometry within the category Db(X) itself. I will discuss the construction of the 'Balmer spectrum' and give some pertinent examples.

Fall 2020


Combinatorics of words, symbolic dynamics and growth of algebras (Be'eri Greenfeld, December 9, 2020):
The most important invariant of a finite dimensional algebra is its dimension. Let A be a finitely generated, infinite dimensional associative or Lie algebra over some base field F. A useful way to 'measure its infinitude' is to study its growth rate, namely, the asymptotic behavior of the dimensions of the spaces spanned by (at most n)-fold products of some fixed generators. Up to a natural asymptotic equivalence relation, this function becomes a well-defined invariant of the algebra itself, independent of the specification of generators.

The question of 'how do algebras grow?', or, which functions can be realized as growth rates of algebras (perhaps with additional algebraic properties, as grading, simplicity etc.) plays an important role in classifying infinite dimensional algebras of certain classes, and is thus connected to ring theory, noncommutative projective geometry, quantum algebra, arithmetic geometry, combinatorics of infinite words, symbolic dynamics and more.

We present new results on possible and impossible growth rates of important classes of associative and Lie algebras, thereby settling several open questions in this area. Among the tools we apply are novel techniques and recent constructions arising from noncommutative algebra, combinatorics of (infinite trees of) infinite words and convolution algebras of étale groupoids attached to them.


Classifying perfect complexes of Mackey functors (Clover May, December 2, 2020):
Mackey functors were introduced by Dress and Green to encode operations that behave like restriction and induction in representation theory. They play a central role in equivariant homotopy theory, where homotopy groups are replaced by homotopy Mackey functors. In this talk I will discuss joint work with Dan Dugger and Christy Hazel classifying perfect chain complexes of Mackey functors for G=Z/2. Our classification leads to a computation of the Balmer spectrum of the derived category. It has topological consequences as well, classifying all modules over the equivariant Eilenberg--MacLane spectrum HZ/2.


Lannes' T-functor and Chow rings of classifying spaces (David Hemminger, November 18, 2020):
Equivariant Chow rings, including Chow rings of classifying spaces of algebraic groups, appear often in nature but are difficult to compute. Like singular cohomology in topology, these Chow rings modulo a prime p carry the additional structure of unstable modules over the Steenrod algebra. We utilize this extra structure to refine estimates of equivariant Chow rings mod p. As a special case, we prove an analog of Quillen's stratification theorem, generalizing and recovering prior results of Yagita and Totaro.


Graph potentials as mirrors to moduli of vector bundles on curves (Pieter Belmans, November 11, 2020):
In a joint work with Sergey Galkin and Swarnava Mukhopadhyay we have introduced a class of Laurent polynomials associated to decorated trivalent graphs which we called graph potentials. These Laurent polynomials satisfy interesting symmetry and compatibility properties. Under mirror symmetry they are related to moduli spaces of rank 2 bundles (with fixed determinant of odd degree) on a curve of genus $g\geq 2$, which is a class of Fano varieties of dimension $3g-3$.
I will discuss (parts of) the (enumerative / homological) mirror symmetry picture for Fano varieties, and then explain what we understand for this class of varieties and what we can say about the (conjectural) semiorthogonal decomposition of the derived category.


Tits cone intersections and Applications (Michael Wemyss, November 4, 2020):
In the first half of the talk, I will give an overview of Tits cone intersections, which are structures that can be obtained from (possibly affine) ADE Dynkin diagrams, together with a choice of nodes. This is quite elementary, but visually very beautiful, and it has some really remarkable features and applications.
In the second half of the talk I will highlight some of the applications to algebraic geometry, mainly to 3-fold flopping contractions, through mutation and stability conditions. This should be viewed as a categorification of the first half of my talk. Parts are joint work with Yuki Hirano, parts with Osamu Iyama.


The Dehn complex: scissors congruence, K-theory, and regulators (Inn Zakharevich, October 28, 2020):
Hilbert's third problem asks: do there exist two polyhedra with the same volume which are not scissors congruent? In other words, if P and Qare polyhedra with the same volume, is it always possible to write P as the union of P_i, and Q as the union of Q_i, such that the P's and Q's intersect only on the boundaries and such that P_i is congruent to Q_i?

In 1901 Dehn answered this question in the negative by constructing a second scissors congruence invariant now called the "Dehn invariant," and showing that a cube and a regular tetrahedron never have equal Dehn invariants, regardless of their volumes. We can then restate Hilbert's third problem: do the volume and Dehn invariant separate the scissors congruence classes? In 1965 Sydler showed that the answer is yes; in 1968 Jessen showed that this result extends to dimension 4, and in 1982 Dupont and Sah constructed analogs of such results in spherical and hyperbolic geometries. However, the problem remains open past dimension 4. By iterating Dehn invariants Goncharov constructed a chain complex, and conjectured that the homology of this chain complex is related to certain graded portions of the algebraic K-theory of the complex numbers, with the volume appearing as a regulator.

In joint work with Jonathan Campbell, we have constructed a new analysis of this chain complex which illuminates the connection between the Dehn complex and algebraic K-theory, and which opens new routes for extending Dehn's results to higher dimensions. In this talk we will discuss this construction and its connections to both algebraic and Hermitian K-theory, and discuss the new avenues of attack that this presents for the generalized Hilbert's third problem.


Degree One Milnor K-Invariants of Groups of Multiplicative Type (Alex Wertheim, October 21, 2020):
Many important algebraic objects can be viewed as G-torsors over a field F, where G is an algebraic group over F. For example, there is a natural bijection between F-isomorphism classes of central simple F-algebras of degree n and PGL_n(F)-torsors over Spec(F). Much as one may study principal bundles on a manifold via characteristic classes, one may likewise study G-torsors over a field via certain associated Galois cohomology classes. This principle is made precise by the notion of a cohomological invariant, which was first introduced by Serre.

In this talk, we will determine the cohomological invariants for algebraic groups of multiplicative type with values in H^1(-, Q/Z(1). Our main technical analysis will center around a careful examination of mu_n-torsors over a smooth, connected, reductive algebraic group. Along the way, we will compute a related group of invariants for smooth, connected, reductive groups


An obstruction to weak approximation on some Calabi-Yau threefolds (Katrina Honigs, October 14, 2020):
The study of Q-rational points on algebraic varieties is fundamental to arithmetic geometry. One of the few methods available to show that a variety does not have any Q-points is to give a Brauer-Manin obstruction. Hosono and Takagi have constructed a class of Calabi-Yau threefolds that occur as a linear section of a double quintic symmetroid and given a detailed analysis of them as complex varieties in the context of mirror symmetry. This construction can be used to produce varieties over Q as well, and these threefolds come tantalizingly equipped with a natural Brauer class. In work with Hashimoto, Lamarche and Vogt, we analyze these threefolds and their Brauer class over Q and give a condition under which the Brauer class obstructs weak approximation, though it cannot obstruct the existence of Q-rational points.


The Kodaira dimension of some moduli spaces of elliptic K3 surfaces (Giacomo Mezzedimi, October 7, 2020):
Let $\mathcal{M}_{2k}$ denote the moduli space of $U\oplus \langle -2k\rangle$-polarized K3 surfaces. Geometrically, the K3 surfaces in $\mathcal{M}_{2k}$ are elliptic and contain an extra curve class, depending on $k\ge 1$. I will report on a joint work with M. Fortuna and M. Hoff, in which we compute the Kodaira dimension of $\mathcal{M}_{2k}$ for almost all $k$: more precisely, we show that it is of general type if $k\ge 220$ and unirational if $k\le 50$, $k\not\in \{11,35,42,48\}$. After introducing the general problem, I will compare the strategies used to obtain both results. If time permits, I will show some examples arising from explicit geometric constructions.


Moduli of semiorthogonal decompositions (Andrea Ricolfi, September 30, 2020):
We discuss the existence of a moduli space parametrising semiorthogonal decompositions on the fibres of a smooth projective morphism X/U. More precisely, we define a functor on (Sch/U) sending V/U to the set of semiorthogonal decompositions on Perf(X_V). We show this functor defines an etale algebraic space over U. As an application, we prove that if the generic fibre of X/U is indecomposable, then so are all fibres. We discuss some examples and applications. Joint work with Pieter Belmans and Shinnosuke Okawa.


Betti numbers of unordered configuration spaces of a punctured torus (Yifeng Huang, September 23, 2020):
Let X be a elliptic curve over C with one point removed, and consider the unordered configuration spaces Conf^n(X)={(x_1,...,x_n): x_i\ne x_j for i\ne j} / S_n. We present a rational function in two variables from whose coefficients we can read off the i-th Betti numbers of Conf^n(X) for all i and n. The key of the proof is a property called "purity", which was known to Kim for (ordered or unordered) configuration spaces of the complex plane with r >= 0 points removed. We show that the unordered configuration spaces of X also have purity (but with different weights). This is a joint work with G. Cheong.


On the boundedness of n-folds of Kodaira dimension n-1 (Stefano Filipazzi, September 16, 2020):
One of the main topics in the classification of algebraic varieties is boundedness. Loosely speaking, a set of varieties is called bounded if it can be parametrized by a scheme of finite type. In the literature, there is extensive work regarding the boundedness of varieties belonging to the three building blocks of the birational classificaiton of varieties: varieties of Fano type, Calabi--Yau type, and general type. Recently, work of Di Cerbo--Svaldi and Birkar introduced ideas to deduce boundedness statements for fibrations from boundedness results concerning these three classes of varieties. Following this philosophy, in this talk I will discuss some natural conditions for a set of n-folds of Kodaira dimension n-1 to be bounded.

Part of this talk is based on joint work with Roberto Svaldi.


A journey from the octonionic P2 to a fake P2 (Lev Borisov, September 9, 2020):
This is joint work with Anders Buch and Enrico Fatighenti. We discover a family of surfaces of general type with K2=3 and p=q=0 as free C13 quotients of special linear cuts of the octonionic projective plane OP2. A special member of the family has 3 singularities of type A2, and is a quotient of a fake projective plane, which we construct explicitly.

Spring 2020


Compactifications of moduli of points and lines in the projective plane (Luca Schaffler, April 29, 2020):
Projective duality identifies the moduli space Bn parametrizing configurations of n general points in projective plane with X(3,n), parametrizing configurations of n general lines in the dual plane. When considering degenerations of such objects, it is interesting to compare different compactifications of the above moduli spaces.
In this work, we consider Gerritzen-Piwek's compactification Bn and Kapranov's Chow quotient compactification X(3,n), and we show they have isomorphic normalizations.
We prove that Bn does not admit a modular interpretation claimed by Gerritzen and Piwek, namely a family of n-pointed central fibers of Mustafin joins associated to one-parameter degenerations of n points in the plane. We construct the correct compactification of Bn which admits such a family, and we describe it for n=5,6. This is joint work in progress with Jenia Tevelev.


Group cohomology rings via equivariant cohomology (James Cameron, April 22, 2020):
The cohomology rings of finite groups are typically very complicated, but their geometric properties are often tractable and retain representation theoretic information. These geometric properties become more clear once one considers group cohomology rings in the context of equivariant cohomology. In this talk I will discuss how to use techniques involving flag varieties dating back to Quillen and a filtration of equivariant cohomology rings due to Duflot to study the associated primes and local cohomology modules of group cohomology rings.
This talk will be online, using webex


Moduli spaces on the Kuznetsov component of Fano threefolds of index 2 (Franco Rota, April 15, 2020):
The derived category of a Fano threefold Y of Picard rank 1 and index 2 admits a semiorthogonal decomposition. This defines a non-trivial subcategory Ku(Y) called the Kuznetsov component, which encodes most of the geometry of Y.
I will present a joint work with M. Altavilla and M. Petkovic, in which we describe certain moduli spaces of Bridgeland-stable objects in Ku(Y), via the stability conditions constructed by Bayer, Macri, Lahoz and Stellari. Furthermore, in our work we study the behavior of the Abel-Jacobi map on these moduli space. As an application in the case of degree d=2, we prove a strengthening of a categorical Torelli Theorem by Bernardara and Tabuada.
This talk will be online, using webex


Higher K-theory via generators and relations (Ian Coley, April 8, 2020):
K0 (the Grothendieck group) of an exact category has a nice description in terms of generators and relations. Nenashev (after Quillen and Gillet-Grayson) proved that K1 can also be described in terms of generators and relations, and Grayson extended that argument to all higher K-groups. I will sketch Grayson's argument and (ideally) show some advantages of the generators and relations approach.
This talk will be online, using webex


On the Jordan property for local fundamental groups (Joaquin Moraga, April 1, 2020):
We discuss the Jordan property for the local fundamental group of klt singularities. We also show how the existence of a large Abelian subgroup of such a group reflects on the geometry of the singularity. Finally, we show a characterization theorem for klt 3-fold singularities with large local fundamental group.
This talk will be online, using webex


Six explicit pairs of fake projective planes (Lev Borisov, February 26, 2020):
I will briefly review the history of fake projective planes and will talk about my latest work on the subject, joint with Enrico Fatighenti.


Quantum geometry of moduli spaces of local system (Linhui Shen, February 19, 2020):
Let G be a split semi-simple algebraic group over Q. We introduce a natural cluster structure on moduli spaces of G-local systems over surfaces with marked points. As a consequence, the moduli spaces of G-local systems admit natural Poisson structures, and can be further quantized. We will study the principal series representations of such quantum spaces. It will recover many classical topics, such as the q-deformed Toda systems, quantum groups, and the modular functor conjecture for such representations. This talk will mainly be based on joint work with A.B. Goncharov.


The K'-theory of monoid sets (Chuck Weibel, February 5, 2020):
There are three flavors of K-theory for a pointed abelian monoid A; they depend on the A-sets one allows. This talk considers the well-behaved family of partially cancellative (pc) A-sets, and its K-theory. For example, if A is the natural numbers, then pc A-sets are just rooted trees.


Higher norm principles for norm varieties (Shira Gilat, January 22, 2020):
The norm principle for a division algebra states that the image of the reduced norm is an invariant of its Brauer-equivalence class. This can be generalized to symbols in the Milnor K-group KMn(F). We prove a generalized norm principle for symbols in KMn(F) for a prime-to-p closed field F of characteristic zero (for some prime p).  We also give a new proof for the norm principle for division algebras, using the decomposition theorem for (noncommutative) polynomials over the algebra.

Fall 2019


Tropical scheme theory (Diane Mclagan, December 11, 2019):
Tropical geometry can be viewed as algebraic geometry over the tropical semiring (R union infinity, with operations min and +). This perspective has proved surprisingly effective over the last decade, but has so far has mostly been restricted to the study of varieties and cycles. I will discuss a program to construct a scheme theory for tropical geometry. This builds on schemes over semirings, but also introduces concepts from matroid theory. This is joint work with Felipe Rincon, involving also work of Jeff and Noah Giansiracusa and others.


Enumerating pencils with moving ramification on curves, (Carl Lian, November 20, 2019):
We consider the general problem of enumerating branched covers of the projective line from a fixed general curve subject to ramification conditions at possibly moving points. Our main computations are in genus 1; the theory of limit linear series allows one to reduce to this case. We first obtain a simple formula for a weighted count of pencils on a fixed elliptic curve E, where base-points are allowed. We then deduce, using an inclusion-exclusion procedure, formulas for the numbers of maps E→ P1 with moving ramification conditions. A striking consequence is the invariance of these counts under a certain involution. Our results generalize work of Harris, Logan, Osserman, and Farkas-Moschetti-Naranjo-Pirola.


Noncommutative Laurent Phenomena: two examples (Volodia Retakh, November 13, 2019):
We discuss two examples when iterations of the noncommutative rational map are given by noncommutative Laurent polynomials. The first example is related to noncommutative triangulation of surfaces. The second example, which leads to a noncommutative version of the Catalan numbers, is related to solutions of determinant-like equations. The talk is based on joint papers with A. Berenstein from U. of Oregon.


Vertex algebras of CohFT-type (Angela Gibney, October 30, 2019):
Finitely generated admissible modules over "vertex algebras of CohFT-type" can be used to construct vector bundles of coinvariants and conformal blocks on moduli spaces of stable curves. In this talk I will say what vertex algebras of CohFT-type are, and explain how such bundles define semisimple cohomological field theories.
As an application, one can give an expression for their total Chern character in terms of the fusion rules. I'll give some examples.


Strong exceptional collections of line bundles (Chengxi Wang, October 23, 2019):
We study strong exceptional collections of line bundles on Fano toric Deligne-Mumford stacks with rank of Picard group at most two. We prove that any strong exceptional collection of line bundles generates the derived category of the stack, as long as the number of elements in the collection equals the rank of the (Grothendieck) K-theory group of the stack.
The problem reduces to an interesting combinatorial problem and is solved by combinatorial means.


Local-global principle for norm over semi-global fields, Sumit Chandra Mishra, Oct. 16, 2019):
Let K be a complete discretely valued field with residue field κ. Let F be a function field in one variable over K and X a regular proper model of F with reduced special fibre X a union of regular curves with normal crossings. Suppose that the graph associated to X is a tree (e.g. F = K(t). Let L/F be a Galois extension of degree n with Galois group Gand n coprime to char(κ). Suppose that κ is algebraically closed field or a finite field containing a primitive nth root of unity. Then we show that an element in F* is a norm from the extension L/F if it is a norm from the extensions L⊗FFν (i.e., $L\otimes_F F_\nu/F_\nu$) for all discrete valuations ν of F.


What is a derivator? (Ian Coley, October 9, 2019):
Derivators were introduced in the 90s by Grothendieck, Heller, and Franke (independently) to generalize triangulated categories and answer questions in homotopy theory and algebraic geometry using a more abstract framework. Since then, applications to modular representation theory, tensor triangulated geometry, tilting theory, K-theory, equivariant homotopy theory, and more have been developed by scores of mathematicians.
This talk will give the basic definition of a derivator, motivated by the initial question of enhancing a triangulated category, describe some of these useful applications to the "real world" away from category theory. We assume a priori the listener's interest in triangulated category theory and one or more of the above disciplines. In particular, no knowledge of infinity/quasicategories is required!


Generalized Brauer dimension and other arithmetic invariants of semi-global fields (Saurabh Gosavi, October 2, 19):
Given a finite set of Brauer classes B of a fixed period ℓ, we define ind(B) to be the gcd of degrees of field extensions L/F such that α⊗FL=0 for every α in B. We provide upper-bounds for ind(B) which depends upon arithmetic invariants of fields of lower arithmetic complexity. As a simple application of our result, we will obtain upper-bounds for the splitting index of quadratic forms and finiteness of symbol length for function fields of curves over higher-local fields.


Rational singularities and their cousins in arbitrary characteristics (Sándor Kovacs, Sept. 18, 2019):
I will discuss several results about rational and closely related singularities in arbitrary characteristics. The results concern various properties of these singularities including their behavior with respect to deformations and degenerations, and applications to moduli theory.

On Makkai's Strong Conceptual Completeness Theorem (Jacob Lurie, Sept. 20, 2019):
One of the most fundamental results of mathematical logic is the celebrated Godel completeness theorem, which asserts that every consistent first-order theory T admits a model. In the 1980s, Makkai proved a much sharper result: any first-order theory T can be recovered, up to a suitable notion of equivalence, from its category of models Mod(T) together with some additional structure (supplied by the theory of ultraproducts). In this talk, I'll explain the statement of Makkai's theorem and sketch a new proof of it, inspired by the theory of "pro-etale sheaves" studied by Scholze and Bhatt-Scholze.

Spring 2019


Severi degrees via representation theory (Dave Anderson, May 1, 2019):
As a vector space, the cohomology of the Grassmannian Gr(k,n) is isomorphic to the k-th exterior power of C^n. The geometric Satake correspondence explains how to naturally upgrade this isomorphism to one of $gl_n$-representations. Inspired by work of Golyshev and Manivel from 2011, we use these ideas to find new proofs of Giambelli formulas for ordinary and orthogonal Grassmannians, as well as rim-hook rules for quantum cohomology. This is joint with Antonio Nigro.


Severi degrees via representation theory (Yaim Cooper, April 24, 2019):
The Severi degrees of $P^1$ x $P^1$ can be computed in terms of an explicit operator on the Fock space $F[P^1]$. We will discuss this and variations on this theme. We will explain how to use this approach to compute the relative Gromov-Witten theory of other surfaces, such as Hirzebruch surfaces and Ex$P^1$. We will also discuss operators for calculating descendants. Joint with R. Pandharipande.


Continuous families of divisors on symmetric powers of curves (John Sheridan, April 17, 2019):
For X a smooth projective variety, we consider its set of effective divisors in a fixed cohomology class. This set naturally forms a projective scheme and if X is a curve, this scheme is a smooth, irreducible variety (fibered in linear systems over the Picard variety). However, when X is of higher dimension, this scheme can be singular and reducible. We study its structure explicitly when X is a symmetric power of a curve.


Character Tables and Sylow Subgroups of Finite Groups (Gabriel Navarro, April 10, 2019):
Brauer's Problem 12 asks which properties of Sylow subgroups can be detected in the character table of a finite group. We will talk about recent progress on this problem.


Effective divisors in the Hodge bundle (Iulia Gheorghita, April 3, 2019):
Computing effective divisor classes can reveal important information about the geometry of the underlying space. For example, in 1982 Harris and Mumford computed the Brill-Noether divisor class and used it to determine the Kodaira dimension of the moduli space of curves.
In this talk I will explain how to compute the divisor class of the locus of canonical divisors in the projectivized Hodge bundle over the moduli space of curves which have a zero at a Weierstrass point. I will also explain the extremality of the divisor class arising from the stratum of canonical divisors with a double zero.


The algebraic theory of systems (Louis Rowen, March 27, 2019):
The notion of ``system'' is introduced to unify classical algebra with tropical mathematics, hyperfields, and other related areas for which we can embed a partial algebraic structure into a fuller structure from which we can extract more information. The main ideas are a generalized negation map since our structures lack classical negatives, and a ``surpassing relation'' to replace equality.
We discuss this theory with emphasis on the main applications, which will be described from the beginning:
1. Classical algebra
2. Supertropical mathematics (used for valuations and tropicalization)
3. Symmetrized systems (used for embedding additively idempotent semi structures into systems)
4. Hyperfields


A Degree Formula for Equivariant Cohomology (Jeanne Duflot, March 13, 2019):
I will talk about a generalization of a result of Lynn on the "degree" of an equivariant cohomology ring $H^*_G(X)$. The degree of a graded module is a certain coefficient of its Poincaré series, expanded as a Laurent series about t=1. The main theorem, which is joint with Mark Blumstein, is an additivity formula for degree: $$\deg(H^*_G(X)) = \sum_{[A,c] \in \mathcal{Q'}_{max}(G,X)}\frac{1}{|W_G(A,c)|} \deg(H^*_{C_G(A,c)}(c)).$$


Volumes and intersection theory on moduli spaces of abelian differentials (Dawei Chen, February 20, 2019):
Computing volumes of moduli spaces has significance in many fields. For instance, the celebrated Witten's conjecture regarding intersection numbers on the Deligne-Mumford moduli space of stable curves has a fascinating connection to the Weil-Petersson volume, which motivated Mirzakhani to give a proof via Teichmueller theory, hyperbolic geometry, and symplectic geometry. The initial two other proofs of Witten's conjecture by Kontsevich and by Okounkov-Pandharipande also used various ideas in ribbon graphs, Gromov-Witten theory, and Hurwitz theory.
In this talk I will introduce an analogous formula of intersection numbers on the moduli spaces of abelian differentials that computes the Masur-Veech volumes. This is joint work with Moeller, Sauvaget, and Zagier (arXiv:1901.01785).


The Real graded Brauer group (Chuck Weibel, February 6, 2019):
We introduce a version of the Brauer--Wall group for Real vector bundles of algebras (in the sense of Atiyah), and compare it to the topological analogue of the Witt group. For varieties over the reals, these invariants capture the topological parts of the Brauer--Wall and Witt groups.


Paschke Categories, K-homology and the Riemann-Roch Transformation (Khashayar Sartipi, January 30, 2019):
For a separable C*-algebra A, we introduce an exact C*-category called the Paschke Category of A, which is completely functorial in A, and show that its K-theory groups are isomorphic to the topological K-homology groups of the C*-algebra A. Then we use the Dolbeault complex and ideas from the classical methods in Kasparov K-theory to construct an acyclic chain complex in this category, which in turn, induces a Riemann-Roch transformation in the homotopy category of spectra, from the algebraic K-theory spectrum of a complex manifold X, to its topological K-homology spectrum.


Palindromicity and the local invariant cycle theorem (Patrick Brosnan, January 23, 2019):
In its most basic form, the local invariant cycle theorem of Beilinson, Bernstein and Deligne (BBD) gives a surjection from the cohomology of the special fiber of a proper morphism of smooth varieties to the monodromy invariants of the general fiber. This result, which is one of the last theorems stated in the book by BBD, is a relatively easy consequence of their famous decomposition theorem.
In joint work with Tim Chow on a combinatorial problem, we needed a simple condition ensuring that the above surjection is actually an isomorphism. Our theorem is that this happens if and only if the special fiber has palindromic cohomology. I will explain the proof of this theorem and a generalization proved using the (now known) Kashiwara conjecture. I will also say a little bit about the combinatorial problem (the Shareshian-Wachs conjecture on Hessenberg varieties) which motivated our work.

Fall 2018


2-Segal spaces and algebraic K-theory (Julie Bergner, November 28, 2018):
The notion of a 2-Segal space was defined by Dyckerhoff and Kapranov and independently by Galvez-Carrillo, Kock, and Tonks under the name of decomposition space. Although these two sets of authors had different motivations for their work, they both saw that a key example is obtained by applying Waldhausen's S-construction to an exact category, showing that 2-Segal spaces are deeply connected to algebraic K-theory.
In joint work with Osorno, Ozornova, Rovelli, and Scheimbauer, we show that any 2-Segal space arises from a suitable generalization of this construction. Furthermore, our generalized input has a close relationship to the CGW categories of Campbell and Zakharevich. In this talk, I'll introduce 2-Segal structures and discuss what we know and would like to know about the role they play in algebraic K-theory.


Low Degree Cohomology (Bob Guralnick November 14, 2018):
Let G be a finite group with V an absolutely irreducible kG-module with k a field of positive characteristic. We are interested in bounds on the dimension of the first and second degree cohomology groups of G with coefficients in V. We will discuss some old and new bounds, conjectures and applications.


Low degree points on curves (Isabel Vogt, November 7, 2018):
We will discuss an arithmetic analogue of the gonality of a curve over a number field: the smallest positive integer $e$ such that the points of residue degree bounded by $e$ are infinite. By work of Faltings, Harris-Silverman and Abramovich-Harris, it is understood when this invariant is 1, 2, or 3; by work of Debarre-Fahlaoui these criteria do not generalize to $e$ at least 4. We will focus on scenarios under which we can guarantee that this invariant is actually equal to the gonality using the auxiliary geometry of a surface containing the curve. This is joint work with Geoffrey Smith.


Algebraic groups with good reduction and unramified cohomology (Igor Rapinchuk, October 31, 2018):
Let $G$ be an absolutely almost simple algebraic group over a field K, which we assume to be equipped with a natural set V of discrete valuations. In this talk, our focus will be on the K-forms of $G$ that have good reduction at all v in V . When K is the fraction field of a Dedekind domain, a similar question was considered by G. Harder; the case where $K=\mathbb{Q}$ and V is the set of all p-adic places was analyzed in detail by B.H. Gross and B. Conrad. I will discuss several emerging results in the higher-dimensional situation, where K is the function field $k(C)$ of a smooth geometrically irreducible curve $C$ over a number field k, or even an arbitrary finitely generated field.

These problems turn out to be closely related to finiteness properties of unramified cohomology, and I will present available results over various classes of fields. I will also highlight some connections with other questions involving the genus of $G$ (i.e., the set of isomorphism classes of K-forms of $G$ having the same isomorphism classes of maximal K-tori as $G$), Hasse principles, etc. The talk will be based in part on joint work with V. Chernousov and A. Rapinchuk


Generalized Polar Geometry (Sandra Di Rocco, October 24, 2018):
Polar classes are very classical objects in Algebraic Geometry. A brief introduction to the subject will be presented and ideas and preliminarily results towards generalizations will be explained. These ideas can be applied towards variety sampling and relevant applications in Kinematics and Biochemistry.


Brauer class over the Picard scheme of curves (Qixiao Ma, October 24, 2018):
We study the Brauer class rising from the obstruction to the existence of a tautological line bundle on the Picard scheme of curves. If we consider the universal totally degenerate curve with a fixed dual graph, then, using symmetries of the graph, we give bounds on the period and index of the Brauer classes. As a result, we provide some division algebra of prime degree, serving as candidates for the cyclicity problem.


On singularity properties of convolutions of algebraic morphisms (Yotam Hendel, October 10, 2018):
In analysis, the convolution of two functions results in a smoother, better behaved function. It is interesting to ask whether an analogue of this phenomenon exists in the setting of algebraic geometry. Let $f$ and $g$ be two morphisms from algebraic varieties X and Y to an algebraic group $G$. We define their convolution to be a morphism $f*g$ from $X\times Y$ to $G$ by first applying each morphism and then multiplying using the group structure of $G$.

In this talk, we present some properties of this convolution operation, as well as a recent result which states that after finitely many self convolutions every dominant morphism $f:X\to G$ from a smooth, absolutely irreducible variety X to an algebraic group G becomes flat with reduced fibers of rational singularities (this property is abbreviated FRS). The FRS property is of particular interest since by works of Aizenbud and Avni, FRS morphisms are characterized by having fibers whose point count over the finite rings $Z/p^kZ$ is well-behaved. This leads to applications in probability, group theory, representation growth and more. We will discuss some of these applications, and if time permits, the main ideas of the proof which utilize model-theoretic methods. Joint work with Itay Glazer.


Irrationality of Motivic Zeta Functions (Michael Larsen, October 5, 2018):
It is a remarkable fact that the Riemann zeta function extends to a meromorphic function on the whole complex plane. A conjecture of Weil, proved by Dwork, asserts that the zeta function of any variety over a finite field is likewise meromorphic, from which it follows that it can be expressed as a rational function. In the case of curves, Kapranov observed that this is true in a very strong sense, which continues to hold even in characteristic zero. He asked whether this remains true for higher dimensional varieties. Valery Lunts and I disproved his conjecture fifteen years ago, and recently disproved a weaker conjecture due to Denef and Loeser. This explains, in some sense, why Weil's conjecture was so much easier in dimension 1 than in higher dimension.



Charles Weibel / weibel @ math.rutgers.edu / November 1, 2021